Small Field Dosimetry Application Guide When

R A D I AT I O N T H E R A P Y
When small things matter.
Small Field Dosimetry
Application Guide
1 Introduction
Contents
2
1 Introduction 2
2 The Physics of Small Fields 3
3 Detector Types 10
4 Detector Selection Guide Overview:
Key Selection Criteria Overview:
Additional Selection Criteria 11
18
19
5 Absolute Dose Measurements
with PTW Small Field Detectors 21
6 Frequently Asked Questions 22
7 Detector Overview 25
8 References and Further Reading 34
Dose determination in small photon fields is an
important and challenging task. Small photon
fields are used in stereotactic radiosurgery as
well as in IMRT and IMAT, where mini or micro
MLCs create fields of 1 cm x 1 cm or smaller.
Current dosimetry protocols such as [IAEA
398, AAPM TG51, DIN 6800-2] describe
procedures for absolute dose measurements
based on ionization chambers at field sizes of
typically 10 cm x 10 cm. No advice is given as
to appropriate procedures and detectors for
field sizes of 1 cm x 1 cm. Presently, national
and international committees are working on
dedicated dosimetry protocols for small field
dosimetry, see e.g. [Alfonso2008].
2 The Physics of Small Fields
2.1 Under which conditions can you
consider a field as small?
} If the field is smaller than approximately
4 cm x 4 cm.
} If the focus is partially hidden by the
collimators.
} If lateral electron equilibrium is not given
in the center of the field.
2.2 The dose volume effect
When the dose changes noticeably across the
detector, the signal is subject to the volume
effect. As a consequence of the volume effect,
the dose in the field is underestimated and the
width of the penumbra is overestimated.
In Figure 1 you can see a size comparison of
some small field detectors against a Gaussian
shaped field of FWHM1 2 cm x 2 cm. From the
figure it is apparent that a diode is probably
small enough to characterize such a field but a
Semiflex 0.125 cm³ chamber is not. In Figure 2
the effect of a too large detector is described
in more detail, experimental results are shown
in Figure 3.
1
Dose [%]
Full width at half maximum, this is the same as the width of the 50 % isodose
Figure 1 Size comparison of a 2 cm x 2 cm FWHM Gaussian shaped field with some small field detectors.
3
a
b
4
c
Figure 2
Viewgraph showing the origin of the volume effect.
In part a) you can see the size of a Semiflex 0.125 cm³ chamber against a FWHM 2 cm x 2 cm
Gaussian field. Clearly, the chamber seems to be too big to characterize that field.
In part b) you can see what that chamber will actually do: it will average the dose across its
sensitive volume, depicted as a blue box. When you move the chamber through the field, it
will always average across its volume at every measurement position.
The result is shown in part c). The blue curve shows the signal after averaging. The CAX value
of the dose is underestimated, and the penumbra is broadened. Note that the FWHM field
size is still correct, the blue and red curves meet at 50 % height.
5
Dose normalized to Diamond CAX [%]
Output factor
a
b
6
c
Figure 3
Experimental verification of the volume effect.
In part a) the output factors2 for small square fields are shown. For the 1 cm x 1 cm field, the
reduction of the measured dose for the Semiflex and the PinPoint chamber is clearly visible.
Part b) shows a profile measurement in an 1 cm x 1 cm field. Again, the dose reduction
of the Semiflex chamber in the field center is apparent. Note that the field width
(50 % isodose) is measured correctly.
In part c) the penumbra broadening of the Semiflex chamber in a 10 cm x 10 cm field can
be seen. All measurements have been performed at 6 MV on an Elekta SLi18 Synergy
Digital Accelerator. Note that the field width (50 % isodose) is measured correctly.
2
Synonyms for output factor: relative dose factor and total scatter factor.
7
Additional effects due to CAX
normalization
Usually, profiles are evaluated after performing
a CAX3 normalization, i.e. all profiles are normalized such that their CAX-value corresponds
to 100 %. For the example in Figure 2, this corresponds to multiplying the entire blue curve
by 1.05. This includes the penumbra of the
measurement and the out-of-field part. Hence
if you combine the volume effect with a CAX
normalization, the out-of-field dose and the
penumbra dose will be slightly overestimated.
This can be seen in Figure 4 where the data
is taken from Figure 2 b) and normalized to
the respective CAX values of the curves. The
increase of the penumbra data leads to an
increase of the apparent field width (i.e. the
FWHM is broadened).
A similar effect can happen with depth dose
curves (PDDs) if there is a strong volume effect
present. As the volume effect depends on field
size and the field size depends on depth, the
volume effect at the normalization point (at
maximum dose) is different compared to positions deep in the water. This effect can distort
a PDD measurement.
Figure 4
Profiles of a 6 MV 1 cm x 1 cm field measured with a diode E (similar to diode SRS) and a Semiflex 0.125
chamber after CAX normalization. The data is the same as in Figure 2 b). In addition to penumbra
broadening two more effects are visible, indicated by arrows. (i) The FWHM of the Semiflex measurement seems larger than that of the other detectors. This is in contrast to the original measurement without CAX normalization shown in Figure 3 b). (ii) The dose in the out-of-field region is overestimated.
3
8
CAX stands for central axis.
2.3 Low energy response
2.4 Other effects in small fields
Low energy scattered radiation hardly
plays any role in small fields.
}
The alignment of beam and detector is
much more important compared to large
field sizes.
In large fields (roughly above 10 cm x 10 cm)
there is a large dose contribution due to lowenergy scattered radiation. In small fields,
the dose contribution by this radiation is very
small. Consequently, the low-energy response
(response to photons in the keV range) does
not play a role in small fields.
What about the out-of-field region? In the
out-of-field region, the radiation consists
only of scattered photons. For small fields this
radiation contains a low-energy part but it is
far less important than for large fields.
Hence, for small fields:
} Silicon diode detectors can be used.
}Shielding of the silicon diode is not
necessary.
}
Often, an irradiation is composed of many
small fields. To correctly add these up, the penumbras of the fields must be determined very precisely.
}
For small fields the field size must not equal
the set collimator value due to partial occlusion of the focus by the collimators and
penumbra overlap.
}
In small fields, lateral electron equilibrium
is often not given. Some of the common
assumptions used in radiation therapy cal­
culations might not be fully valid. For example, the density of the detector may play a
role [Fenwick2013].
}
Some small field systems are flattening filter
free linacs. Their spectrum is softer than
that of conventional linacs. This can have an
influence on kQ factors [IPEM 103].
Summary:
}If your detector is larger than 1/4th of the lateral field dimension, you should watch
out for the volume effect.
}keV scattered photon radiation does not play a role in small fields and is less
important in the out-of-field region compared to large fields.
}If the volume effect is present,
• The dose in the field center will be underestimated;
• The penumbra appears wider than it is.
}If in addition to the volume effect you perform a CAX normalization in a small field,
• The field (50 % isodose) will appear wider than it is;
• The dose in the out-of-field region will be overestimated;
• The dose of PDDs at large depths can be overestimated.
}[IPEM 103] recommends to use more than one detector to perform a high quality
characterization.
}[IPEM 103] recommends to use more than one detector to perform a high quality
characterization.
}For a thorough introduction see [Wuerfel2013]
9
3 Detector Types
The following section presents a quick introduction into the various types of single detectors
used for dose measurements in a water phantom.
3.1 Medium-size vented ionization
chambers
Gold standard for dose measurements are
vented ionization chambers as specified in IEC
60731. The sensitive volume of such chambers
is usually between 0.1 cm³ and 1.0 cm³. Their
only disadvantage is the relatively large size.
When used in small fields, large detectors
can be subject to the dose volume effect,
see chapter 2.2.
3.2 Small-size vented ionization
chambers
Small-size vented ion chambers (PinPoint
chambers) have a sensitive volume in the order
of 0.01 cm³. They can typically be used for dose
measurements in fields down to 2 cm x 2 cm.
Care must be taken if PinPoint chambers are
used in very large fields when stem and cable
effects become important. Make sure that the
chamber you use does not have a steel electrode.
10
3.3 Diamond detectors
Diamond detectors are solid state detectors
combining small size and high response. In
addition, their response is almost independent
upon energy, i.e. they are very much water
equivalent. They also feature a very good
directional response. Diamond detectors can
be constructed as solid state ionization chambers (TM60003 diamond) or as diodes
(T60019 microDiamond).
3.4 Silicon diodes
Silicon diode detectors feature the highest
response per volume of all common detector
types. Hence their sensitive volume is usually
small enough to avoid dose volume effects
down to very small fields. However, their directional response and their response to low-energy scattered photons is not ideal. To reduce the
effect of low-energy photons, diodes exist in a
shielded design where the shield reduces the
signal from these photons. In small fields the
low-energy scatter contribution is low, hence
diode shielding is not needed and unshielded
diodes are recommended for small fields
[IPEM 103].
4 Detector Selection Guide
Which detector
is best suited for
my application?
11
Detector Selection Tree
Minimum field size required 1 cm x 1 cm
MAXIMUM
field size (cm)
required:
10 x 10
Type of
measurement:
Suitable
detectors:
Recommended
detectors:
Remarks
Absolute dose 1 &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Diode E
Diode SRS
Diode P
microDiamond
Diode E
Diode SRS
Diode P
microDiamond
Diode P
microDiamond
Diode P
microDiamond
microDiamond
microDiamond
microDiamond
microDiamond
Diode E or SRS
Diode E or SRS
The influence of the density should be less
for the microDiamond because of its very thin
measurement volume. See [Fenwick2013],
page 2919.
1
12
20 x 20
In small fields absolute dose measurement
often requires cross calibration, see chapter
5 and Overview: Key selection criteria.
Profiles &
PDDs
30 x 30
40 x 40
Absolute dose &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Profiles &
PDDs
Diode P
microDiamond
Diode P
microDiamond
Diode P
microDiamond
Diode P
microDiamond
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Both microDiamond and Diode P are well
­suited for the entire field size range from
1 cm x 1 cm up to 40 cm x 40 cm. But if you
are aiming for utmost accuracy in large fields,
a medium sized air-filled ionization chamber
will be better than any solid state detector.
If you can choose between Diode P and
microDiamond, take the microDiamond.
13
Detector Selection Tree
Minimum field size required 2 cm x 2 cm
MAXIMUM
field size (cm)
required:
10 x 10
Type of
measurement:
20 x 20
Absolute dose 1 &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Suitable
detectors:
Diode E
Diode SRS
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Diode E
Diode SRS
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Recommended
detectors:
PinPoint 0.03
microDiamond
PinPoint 0.03
microDiamond
Remarks
The PinPoint 0.03 is best suited for absolute
dose measurements as it does not need to be
cross-calibrated.
1
14
In small fields absolute dose measurement
often requires cross calibration, see chapter
5 and Overview: Key selection criteria.
Profiles &
PDDs
The microDiamond detector is suitable for
absolute dose and output factor measurements.
However, whereas the microDiamond must be
cross-calibrated, the PinPoint 0.03 chamber can
be directly applied according to IAEA 398 and
DIN 6800-2.
30 x 30
40 x 40
Absolute dose &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Profiles &
PDDs
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Diode P
microDiamond
Diode P
microDiamond
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
microDiamond
for fields smaller
than 20 cm x 20 cm
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Semiflex 0.125
for larger fields
Both microDiamond and Diode P are well ­suited
for the entire field size range from
1 cm x 1 cm up to 40 cm x 40 cm. But if you
are aiming for utmost accuracy in large fields, a
medium sized air-filled ionization chamber will
be better than any solid state detector.
If you can choose between Diode P and
microDiamond, take the microDiamond.
15
Detector Selection Tree
Minimum field size required 3 cm x 3 cm
MAXIMUM
field size (cm)
required:
10 x 10
Type of
measurement:
20 x 20
Absolute dose 1 &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Suitable
detectors:
Diode E
Diode SRS
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Semiflex 0.125
Diode E
Diode SRS
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Semiflex 0.125
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Semiflex 0.125
Recommended
detectors:
Semiflex 0.125
microDiamond
Semiflex 0.125
microDiamond
Remarks
The Semiflex 0.125 is best suited for absolute
dose measurements as it does not need to be
cross-calibrated.
1
16
In small fields absolute dose measurement
often requires cross calibration, see chapter
5 and Overview: Key selection criteria.
Profiles &
PDDs
The microDiamond detector is suitable for
absolute dose and output factor measurements.
However, whereas the microDiamond must be
cross-calibrated, the Semiflex 0.125 chamber
can be directly applied according to IAEA 398
and DIN 6800-2.
30 x 30
40 x 40
Absolute dose &
output factors
Profiles &
PDDs
Absolute dose &
output factors
Profiles &
PDDs
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Semiflex 0.125
Diode P
microDiamond
PinPoint 0.015
PinPoint 0.03
PinPoint 3D
Semiflex 0.125
Diode P
microDiamond
Semiflex 0.125
Diode P
microDiamond
Semiflex 0.125
Semiflex 0.125
microDiamond
for fields less or equal
than 20 cm x 20 cm
Semiflex 0.125
microDiamond
for fields less or equal
than 20 cm x 20 cm
Semiflex 0.125
for fields larger than
20 cm x 20 cm
Semiflex 0.125
for fields larger than
20 cm x 20 cm
Though the PinPoint chambers, the
microDiamond and the Diode P are well
suited for measurements over the entire range
from 3 cm x 3 cm to 30 cm x 30 cm, we
recommend using a combination of two
detectors for the most precise profile and
PDD measurements.
Both microDiamond and Diode P are well suited
for the entire field size range from 1 cm x 1 cm
up to 40 cm x 40 cm. But if you are aiming for
utmost accuracy in large fields, a medium sized
air-filled ionization chamber will be better than
any solid state detector. If you can choose
between Diode P and microDiamond, take the
microDiamond.
For precise penumbra measurements in fields
smaller or equal than 20 cm x 20 cm, a detector
smaller than the Semiflex 0.125 should be used.
17
Overview: Key Selection Criteria
Suitable
Detector
Key Selection Criteria
Field Size
Absolute Dose Measurements
max.
min.
cm x cm
cm x cm
down to… up to…
< 2 cm x 2 cm
2 cm x 2 cm
…
3 cm × 3 cm
3 cm x 3 cm
…
4 cm × 4 cm
Profile
Measurements
>
_ 4 cm × 4 cm <
_ 10 cm
PTW Detector
> 10 cm
1x1
10 x 10
✔
✔
✔
✔
✔
–
Diode E,
unshielded
1x1
10 x 10
✔
✔
✔
✔
✔
–
Diode SRS,
unshielded
1x1
40 x 40
✔
✔
✔
✔
✔
✔
Diode P,
shielded
1x1
20 x 20
✔
✔
✔
✔
✔
✔
microDiamond
Detector
2x2
30 x 30
–
✔
✔
✔
✔
✔
PinPoint
Chamber,
0.015 cm³
2x2
30 x 30
–
✔
✔
✔
✔
✔
PinPoint
Chamber,
0.03 cm³
2x2
30 x 30
–
✔
✔
✔
✔
✔
PinPoint
Chamber 3D,
0.016 cm³
3x3
40 x 40
–
–
✔
✔
–
✔
Semiflex
Chamber,
0.125 cm³
✔ Detector requires cross-calibration against Semiflex 0.125 cm³ chamber for absolute dose measurements.
– Not recommended or suitable
18
Overview: Additional Selection Criteria
Detectors
Additional Selection Criteria
Penumbra Out-Of-Field Dose
Dose Rate
Energy
Energy
Fast
Accuracy Dose
Stability Independence Response Response MeasureAccuracy
(MeV)
(keV)
ment 1
Diode E,
unshielded
++++
++
++
+++
+++
–
+
Diode SRS,
unshielded
++++
++
++ 2
++++
+++
–
+++
Diode P,
shielded
++++
++
++
+++
++
++
+
microDiamond
Detector
+++
+++
++++
+++
++++
+++
+
PinPoint Chamber,
0.015 cm³, axial
orientation
+++
++++
++++
++++ 4
++++ 5
++++
+++
PinPoint Chamber,
0.015 cm³, radial
orientation
++
++++
++++
++++ 4
++++ 5
++++
+++
PinPoint Chamber,
0.03 cm³, radial
orientation
++
++++
++++
++++ 4
++++ 6
++++
++++
PinPoint Chamber
3D, 0.016 cm³,
radial orientation
++
++++
++++
++++ 4
++++ 7
++++
+++
Semiflex Chamber,
0.125 cm³, radial
orientation
+
++++
++++
++++ 4
++++ 5
++++
++++
++++ excellent
1
2
3
4
5
6
7
+++ very good
++ good
+ OK
see “Fast measurement” on next page
_ 6MV
only <
this depends on the repetition frequency of the linac: below 180 Hz, basically no dose rate dependence.
Above 180 Hz correction by comparison against detector with known ks.
can be corrected, see e.g. [DIN6800-2]
can be corrected, kQ available in [DIN 6800-2] and [IAEA 398]
can be corrected, kQ available from PTW technical support
can be corrected, kQ available in [DETECTORS]
19
Why is it relevant?
Penumbra accuracy
Out-of-field dose accuracy
In IMRT and IMAT treatments, many small
fields are superimposed to get the full dose.
To make this work, the penumbra should be
known to a high accuracy.
In IMRT and IMAT treatments, many small fields
are superimposed to get the full dose. The
out-of-field dose can be several percent of the
central dose and will add up to a background
dose. In addition, it is a main contribution to
the dose in the surrounding healthy tissue.
Dose stability
When the dose stability is good, you seldom
have to recalibrate your detector. A bad dose
stability requires more frequent recalibrations.
Energy response (keV)
The keV energy response is important when
the beam contains a lot of scattered radiation. This is the case for large fields (more than
10 cm x 10 cm), especially in the out-of-field
region. In small fields (below 5 cm x 5 cm), the
effect is not important.
Energy response (MeV)
A good MeV energy response corresponds to
a quality correction factor kQ close to 1 for all
energies above 60Co. For air-filled ionization
chambers, kQ is known, for other detectors this
is not the case. Hence, the better the energy
response, the smaller is the induced uncertainty.
Note, the mean energy of a beam can slightly
change over a beam cross section or with depth
in the water.
20
Fast measurement
A good signal to noise ratio (SNR) is preferable
for profile and PDD measurements. The better
the SNR, the faster the measurement can be
performed.
Every detector is subject to quantum noise of
the radiation. The following quantities influence quantum noise: (i) the number of quanta
of the primary radiation, (ii) the material of the
detector (i.e. air, silicon, diamond, …), (iii) the
size of the detector (large detector: better SNR)
and (iv) the thickness of the detector (thick
detector: better SNR). Hence, depending on
your detector, the signal to noise ratio will be
different. We have classified detectors with a
high SNR as fast detectors. Note, the SNR is
mainly a material property of the detector, it is
not a function of the response of the detector.
Rule of thumb: if you use a high quality electrometer, the smallest air-filled ionization chamber (PinPoint 0.015 cm³) will have a better SNR
than any diode, even though the response is a
lot lower.
5 Absolute Dose Measurements with
PTW Small Field Detectors
}Fields < 2 cm × 2 cm
Cross-calibrate your small field detector
for each radiation quality in a 4 cm x 4 cm or 5 cm x 5 cm field against a Semiflex
0.125 cm³ ionization chamber.
}Fields 2 cm × 2 cm … 4 cm x 4 cm
Use a PinPoint ionization chamber directly or cross-calibrate your small field detector for each radiation quality in a 4 cm x 4 cm or 5 cm x 5 cm field against a Semiflex 0.125 cm³ ionization chamber.
_ 4 cm x 4 cm
}Fields >
Use a Semiflex 0.125 cm³ ionization
chamber.
}Detector orientation
Perform a new cross-calibration if you change the detector orientation.
Note: when you use an ionization chamber
directly, follow one of the international or
national dosimetry protocols, e.g. [IAEA 398,
AAPM TG51, DIN 6800 2]. Additional kQ
correction factors for the PinPoint chambers
are given in [Muir2011], [DETECTORS] or can
be obtained from PTW technical support.
5.1 How to perform the crosscalibration
For absolute dose measurements, all small field
detectors except air-filled ionization chambers
must be cross-calibrated against a medium-size
ionization chamber such as a Semiflex 0.125.
The cross-calibration is done in a phantom
for each radiation quality. It should be performed in two steps in a field of 4 cm x 4 cm
or 5 cm x 5 cm:
1.Use a medium-size vented ionization chamber, e.g. a Semiflex 0.125 chamber, to determine the dose Dref for the radiation quality and
depth of interest. Use one of the international
or national dosimetry protocols, e.g. [IAEA
398, AAPM TG51, DIN 6800-2].
2.Replace the medium-size ionization chamber
by the small-size detector to be cross-calibrated. Make sure the effective points of measurement are located at the same depth. Apply the
same number of monitor units as before and
determine the reading Dsmall of the small-size
detector. The cross-calibration factor for the
small-size detector is the ratio Dref/Dsmall.
After cross-calibration, the small-size detector
can be used in fields smaller than the cross-calibration field and at different depths, but always
at the same radiation quality and
detector orientation.
21
6 Frequently Asked Questions
How can I tell whether my detector is
too big for my field size?
Can I use any detector to perform
absolute dosimetry?
As a rule of thumb: if the dimension of your
detector is more than 25 % of the field width,
you might observe a volume effect. To make
sure, cross-calibrate a smaller detector against
yours in a 4 cm x 4 cm or 5 cm x 5 cm field and
compare their respective signals in the targeted
small field. If the measured doses clearly deviate, you are probably experiencing a volume
effect.
Usually absolute dosimetry is performed in
10 cm x 10 cm fields and according to international dosimetry protocols. Currently there
is no such protocol describing dose measurements in small fields. Hence, in small fields we
recommend to cross-calibrate the to-be-used
detector against a Semiflex 0.125 chamber in a
4 cm x 4 cm or 5 cm x 5 cm field. Any detector
can be used for cross-calibration as long as it
is stable during the entire measurement. We
recommend to perform the cross-calibration
before each measurement session to check for
detector dose stability (this is especially important when using diodes) and to check for the
reproducibility of the calibration procedure.
Do I need special detectors to perform
dosimetry in small fields?
Yes. There exists no detector that is suitable to
perform high accuracy measurements in very
small as well as very large fields. In larger fields
highest accuracy is reached with ionization
chambers, especially the semiflex 0.125 cm³.
In small fields, small field detectors should be
used.
Is film dosimetry the best solution for
small fields?
No. The main advantage of film dosimetry is
the very good spatial resolution. Unfortunately
this is the only advantage. Silver films exhibit
a very bad energy response in the keV-energy
range and their quality depends a lot on the
development process. [IPEM 103] recommends
not to use that type of film. Radiochromic films
require a high dose for development, their
result depends on handling, i.e. on staff, they
darken by a few percent after exposure, their
response can vary by several percent over the
area of the film, and there are batch-to-batch
variations [IPEM 103].
22
My field is smaller than 1 cm x 1 cm.
Which detector can I use?
If you need to measure smaller field sizes, we
recommend to use non-shielded detectors with
the smallest cross-section perpendicular to the
beam. These are the Diode E (T60017) for all
photon energies and the Diode SRS (T60018)
for photon energies of 6 MV and below. You
can also consider the microDiamond T60019
for such measurements.
For any detector we recomend to look up
correction factors for very small fields in the
scientific literature.
My field is not square. Which detector
is suitable?
There are formulas to calculate an equivalent
square field size for non-square field shapes.
The aim of these calculations is to predict the
output factor of an irregular field. To estimate
whether a detector will be prone to the volume
effect, these formulas cannot be used. Instead,
the smallest dimension of the field plays the
central role. For rectangular fields, this is the
small edge. For example, if your field size is 2
cm x 10 cm, take a detector that is suited for a
2 cm x 2 cm field.
For circular fields, the vendor of your irradiation unit will in most cases recommend
a detector for the measurements. As rule of
thumb: to measure output factors, i.e. when
measuring in the center of the field, take the
diameter of the field as smallest dimension.
For example, to measure the output factor in a
3 cm diameter field, take a detector that is suited
for a 3 cm x 3 cm field. For profile measurements,
it is difficult to give a precise recommendation.
If you are unsure which detector to use, take
the smaller one.
What is the advantage of silicon diodes
over air-filled ionization chambers?
Due to the higher density of atoms in silicon
compared to air, a diode detector can be
constructed very small and still have a good
response. Hence in high-gradient regions,
such as the penumbra, a diode detector will
be more precise. The microDiamond detector
combines the advantages of silicon diodes and
air-filled ionization chambers, but its crosssection in the beam is slightly larger than for
the PTW silicon diode detectors.
What is the advantage of air-filled
ionization chambers over silicon diodes?
When do I use a shielded diode?
In shielded diodes, the over-response to keVenergy scattered radiation – which is mainly
_ 10 cm x 10 cm – is compenpresent in fields >
sated by a metal shield absorbing that type of
radiation. Due to this combination, shielded
diodes can be used in the entire field size
range from 1 cm x 1 cm up to 40 cm x 40 cm.
Nevertheless one must keep in mind that this
large field size range does not come free of
costs. Shielded diodes are a compromise. They
can be used for very small and very large fields,
but if you want to increase the accuracy, we
recommend to use a microDiamond instead of
a Diode P. For highest accuracy use a small field
detector for small fields (e.g. an unshielded silicon diode or a microDiamond) and an air-filled
ionization chamber for large fields.
How can I check if my detector is
accurately positioned in the field?
The option CenterCheck of the MEPHYSTO
package allows you to check the positioning
and alignment of your detector in the beam.
In addition, you can improve reproducibility
and ease of use by mounting your detectors
using the TRUFIX system.
It is important to check the positioning at
shallow and large depths in the water. If you
use CenterCheck, this is done automatically.
In contrast to silicon diodes the response of
air-filled ionization chambers to low-energy
scattered radiation is excellent. For this reason,
they are suited to precisely deduce the dose
in large fields and in the out-of-field region.
In addition, air-filled ionization chambers are
perfectly suited to deduce the absolute dose
according to international dosimetry protocols.
Air-filled ionization chambers do not suffer any
response degradation due to irradiation.
23
How can I tell the effective point of
measurement and water equivalent
window thickness of PTW solid state
detectors?
Each PTW solid state detector has a colored
ring which is situated at the water equivalent
depth of the effective point of measurement of
the detector. To find the “zero” water position,
make the ring level with the water surface
and define this as zero water level. The detector should be used in axial orientation for this
procedure.
If you are using TRUFIX and the stop thimble
corresponding to your detector, the detector
will directly be positioned in the correct depth.
This, of course, requires that you first have
correctly set the zero position with TRUFIX.
Where do I place the reference detector
in a small field?
Placing a reference detector in a very small
field without disturbing the main detector
is not feasible. Simply placing the reference
detector outside the field border is not a very
good solution either, because the signal of the
reference will then be very noisy and will lead
to a noisy measurement (i.e. the curves will not
be flat). There are several options what you
could do:
}
I
f you are very sure that your linac is very stable, measure without reference
Y
ou can use a very large ionization cham}
ber, e.g. a Bragg-Peak chamber or a 100
mm CT-chamber as reference outside the
beam. The larger the chamber the better, a
Farmer chamber is still better than a semiflex
chamber. Do not use a diode as reference as
diodes exhibit strong quantum noise
}
Y
ou can increase your integration time. Four
times longer integration time leads to half
the noise
24
I
f possible, use the monitor chamber of the
}
linac as reference detector
Y
ou can measure the PDD, profile, etc.
}
several times. If several curves coincide, the
linac was stable
Y
ou can measure step by step irradiating a
}
fixed number of MUs for each data point
If you use a reference chamber outside of the
beam, remember to pre-irradiate it if it has not
been in the beam before. A more thorough
description including measured data can be
found in [Wuerfel2013].
How can I verify whether my reference
detector will reduce my signal quality
when situated outside of the field?
There is a relatively simple way to perform a
qualitative check. Set your linac to a large field
size, e.g. 30 cm x 30 cm and find a region
where the profile is flat. Set the step width to
very low values (i.e. many data points per cm).
Measure the profile twice, once with the reference in the beam and once with the reference
a few centimeters outside of the beam boundary. If you use a standard air-filled chamber
or a diode as reference, you will see the noise
increase (i.e. the measured curve will be less
flat) when the reference detector is placed outside of the field. Note, in large fields the dose
outside of the penumbra is much higher compared to small fields. Hence whatever effect
you observe, it will be enhanced in small field
measurements.
If you want a thorough noise characterization,
you can measure the signal of the TANDEM
using a terminal program. Then you can
deduce the standard deviation of the reference
and field detectors and their ratio. This is a
direct measure for the noise.
7 Detector Overview
Dimensions, specs
Radiation Quality
T31010 0.125 cm³ Semiflex Chamber
radius of sensitive
volume 2.75 mm, length 6.5 mm
66 kV … 50 MV photons
(10 … 45) MeV electrons
(50 … 270) MeV protons
T31014 0.015 cm³
PinPoint Chamber
radius of sensitive volume 1 mm,
length 5 mm
60
T31015 0.03 cm³
PinPoint Chamber
radius of sensitive volume 1.45 mm,
length 5 mm
60
T31016 0.016 cm³ PinPoint 3D Chamber
radius of sensitive volume 1.45 mm,
length 2.9 mm
60
T60019 microDiamond
Detector
sensitive volume 100 kV … 25 MV photons
0.004 mm³, (6 … 25) MeV electrons
radius 1.1 mm,
thickness 0.001 mm
T60016 Dosimetry Diode P
60
sensitive volume Co… 25 MV photons
0.03 mm³, radius of
sensitive volume
0.6 mm, shielded
T60017 Dosimetry Diode E
60
sensitive volume Co… 25 MV photons
0.03 mm³, radius of
(6 … 25) MeV electrons
sensitive volume
0.6 mm, unshielded
T60018 Dosimetry Diode SRS sensitive volume 0.3 mm³, radius of
sensitive volume
0.6 mm, unshielded,
high response
60
T60020 Dosimetry Diode PR sensitive volume 0.02 mm³, radius of
sensitive volume
0.6 mm, unshielded
(50 … 270) MeV protons
Co… 50 MV photons
Co… 50 MV photons
Co… 50 MV photons
Co… 6 MV photons
25
0.125 cm3 Semiflex
Chamber
Type 31010
Standard therapy chamber for
scanning systems and for absolute
dosimetry
Features
Waterproof, semiflexible design for easy mounting in
scanning water phantoms
Minimized directional response
Sensitive volume 0.125 cm3, vented to air
Radioactive check device (option)
The 31010 semiflexible chamber is the ideal compromise
between small size for reasonable spatial resolution and
large sensitive volume for precise dose measurements.
This makes the 31010 chamber to one of the most commonly used chambers in scanning water phantom systems. The chamber volume of 0.125 cm3 gives enough
signal to use the chamber also for high precision
absolute dose measurements. The sensitive volume is
approximately spherical resulting in a flat angular
response and a uniform spatial resolution along all three
axes of a water phantom.
Specification
Type of product
Application
Measuring quantities
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Nominal response
Long-term stability
Chamber voltage
Polarity effect at 60Co
Photon energy response
Directional response in
water
Leakage current
Cable leakage
26
vented cylindrical
ionization chamber
absolute dosimetry in
radiotherapy beams
absorbed dose to water,
air kerma, exposure
60Co
Materials and measures:
Wall of sensitive volume 0.55 mm PMMA,
1.19 g/cm3
0.15 mm graphite,
0.82 g/cm3
Total wall area density
78 mg/cm2
Dimension of sensitive
radius 2.75 mm
volume
length 6.5 mm
Central electrode
Al 99.98, diameter 1.1 mm
Build-up cap
PMMA, thickness 3 mm
Ion collection efficiency
Ion collection time
Max. dose rate for
≥ 99.5 % saturation
≥ 99.0 % saturation
Max. dose per pulse for
≥ 99.5 % saturation
≥ 99.0 % saturation
Useful ranges:
Chamber voltage
Radiation quality
Field size
Temperature
0.125 cm3
waterproof, vented, fully
guarded
on chamber axis, 4.5 mm
from chamber tip
radial
3.3 nC/Gy
≤ 1 % per year
400 V nominal
± 500 V maximal
<1%
≤ ± 2 % (140 kV ... 280 kV)
≤ ± 4 % (140 kV ... 60Co)
≤ ± 0.5 % for rotation
around the chamber axis
and for tilting of the axis
up to ± 10°
≤ ± 4 fA
≤ 1 pC/(Gy·cm)
Humidity
Air pressure
at nominal voltage:
100 µs
6 Gy/s
12 Gy/s
0.5 mGy
1.0 mGy
± (100 ... 400) V
66 kV ... 50 MV photons
(10 ... 45) MeV electrons
(50 ... 270) MeV protons
(3 x 3) cm2 ... (40 x 40) cm2
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
(700 - 1060) hPa
Ordering Information
TN31010 Semiflex chamber 0.125 cm3,
connecting system BNT
TW31010 Semiflex chamber 0.125 cm3,
connecting system TNC
TM31010 Semiflex chamber 0.125 cm3,
connecting system M
Options
T48012 Radioactive check device 90Sr
T48002.1.004 Chamber holding device for check
device
PinPoint Chambers
Type 31014, 31015
Ultra small-sized therapy chambers
for dosimetry in high-energy photon
beams
Features
Small-sized sensitive volumes of only 0.015 cm3 and
0.03 cm3, 2 mm and 2.9 mm in diameter, vented to air
Very high spatial resolution when used for scans
perpendicular to the chamber axis
Aluminum central electrode
Radioactive check device (option)
The PinPoint chambers are ideal for dose measurements
in small fields as encountered e.g. in IORT, IMRT and
stereotactic beams. Relative dose distributions can be
measured with very high spatial resolution when the
chambers are moved perpendicular to the chamber axis.
The waterproof, fully guarded chambers can be used in
air, solid state phantoms and in water.
Specification
Type of products
Application
Measuring quantities
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Pre-irradiation dose
Nominal response
Long-term stability
Chamber voltage
Polarity effect
Directional response in
water
Leakage current
Cable leakage
vented cylindrical
ionization chambers
dosimetry in high-energy
photon beams with high
spatial resolution
absorbed dose to water,
air kerma, exposure
60Co
0.015 cm3, 0.03 cm3
waterproof, vented, fully
guarded
on chamber axis, 3.4 mm
from chamber tip
radial, axial (31014)
2 Gy
400 pC/Gy, 800 pC/Gy
≤ 1 % per year
400 V nominal
± 500 V maximal
≤±2%
≤ ± 0.5 % for rotation
around the chamber axis,
≤ ± 1 % for tilting of the
axis up to
± 20° (radial incidence)
± 15° (axial incidence)
≤ ± 4 fA
≤ 1 pC/(Gy·cm)
Materials and measures:
Wall of sensitive volume 0.57 mm PMMA,
1.19 g/cm3
0.09 mm graphite,
1.85 g/cm3
Total wall area density
85 mg/cm2
Dimensions of sensitive
radius 1 mm, 1.45 mm
volume
length 5 mm
Central electrode
Al 99.98, diameter 0.3 mm
Build-up cap
PMMA, thickness 3 mm
Ion collection efficiency
Ion collection time
Max. dose rate for
≥ 99.5 % saturation
≥ 99.0 % saturation
Max. dose per pulse for
≥ 99.5 % saturation
≥ 99.0 % saturation
Useful ranges:
Chamber voltage
Radiation quality
Field size
Temperature
Humidity
Air pressure
at nominal voltage:
20 µs, 50 µs
265 Gy/s, 29 Gy/s
580 Gy/s, 55 Gy/s
3.5 mGy, 1.2 mGy
7 mGy, 2.3 mGy
± (100 ... 400) V
... 50 MV photons
(2 x 2) cm2 ... (30 x 30) cm2
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
(700 ... 1060) hPa
60Co
Ordering Information
TN31014 PinPoint chamber 0.015 cm3,
connecting system BNT
TW31014 PinPoint chamber 0.015 cm3,
connecting system TNC
TM31014 PinPoint chamber 0.015 cm3,
connecting system M
TN31015 PinPoint chamber 0.03 cm3,
connecting system BNT
TW31015 PinPoint chamber 0.03 cm3,
connecting system TNC
TM31015 PinPoint chamber 0.03 cm3,
connecting system M
Options
T48012 Radioactive check device 90Sr
T48002.1.007 Chamber holding device for check
device
27
PinPoint 3D Chamber
Type 31016
Ultra small-sized therapy chamber
with 3D characteristics for dosimetry
in high-energy photon beams
Features
Small-sized sensitive volume 0.016 cm3, vented to air
Minimized directional response
Aluminum central electrode
Radioactive check device (option)
The 31016 PinPoint 3D chamber is ideal for dose measurements in small fields as encountered e.g. in IORT,
IMRT and stereotactic beams. Relative dose distributions
can be measured with high spatial resolution in any
direction. The waterproof, fully guarded chamber can be
used in air, solid state phantoms and in water.
Specification
Type of product
Application
Measuring quantities
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Pre-irradiation dose
Nominal response
Long-term stability
Chamber voltage
Polarity effect
Directional response in
water
Leakage current
Cable leakage
28
vented cylindrical
ionization chamber
dosimetry in high-energy
photon beams
absorbed dose to water,
air kerma, exposure
60Co
0.016 cm3
waterproof, vented, fully
guarded
on chamber axis, 2.4 mm
from chamber tip
radial
2 Gy
400 pC/Gy
≤ 1 % per year
400 V nominal
± 500 V maximal
≤±2%
≤ ± 0.5 % for rotation
around the chamber axis,
≤ ± 1 % for tilting of the
axis up to ± 110°
≤ ± 4 fA
≤ 1 pC/(Gy·cm)
Materials and measures:
Wall of sensitive
0.57 mm PMMA,
volume
1.19 g/cm3
0.09 mm graphite,
1.85 g/cm3
Total wall area density
85 mg/cm2
Dimensions of sensitive
radius 1.45 mm
volume
length 2.9 mm
Central electrode
Al 99.98, diameter 0.3 mm
Build-up cap
PMMA, thickness 3 mm
Ion collection efficiency
Ion collection time
Max. dose rate for
≥ 99.5 % saturation
≥ 99.0 % saturation
Max. dose per pulse for
≥ 99.5 % saturation
≥ 99.0 % saturation
Useful ranges:
Chamber voltage
Radiation quality
Field size
Temperature
Humidity
Air pressure
at nominal voltage:
60 µs
19 Gy/s
38 Gy/s
1.0 mGy
1.9 mGy
± (100 ... 400) V
... 50 MV photons
(2 x 2) cm2 … (30 x 30) cm2
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
(700 ... 1060) hPa
60Co
Ordering Information
TN31016 PinPoint 3D chamber 0.016 cm3,
connecting system BNT
TW31016 PinPoint 3D chamber 0.016 cm3,
connecting system TNC
TM31016 PinPoint 3D chamber 0.016 cm3,
connecting system M
Options
T48012 Radioactive check device 90Sr
T48002.1.008 Chamber holding device for check
device
microDiamond
Type 60019
Diamond Detector for dosimetry in
high-energy photon and electron beams,
especially useful for small field dosimetry
Features
Small sensitive volume of 0.004 mm3
Excellent radiation hardness and temperature
independence
Near tissue-equivalence
Operates without high voltage
All connecting systems available (BNT, TNC, M)
The new microDiamond detector is a synthetic single
crystal diamond detector (SCDD), based on a unique
fabrication process [1, 2]. Significant advantages of the
synthetic production are standardised assembly and
consequently a high reproducibility of the dosimetric
properties and good availability of the detector.
Specification
Type of product
Application
Measuring
quantitiy
Reference
radiation quality
Nominal sensitive
volume
Design
Reference point
Direction of
incidence
Pre-irradiation dose
Nominal response
Long-term stability
Dose stability
Temperature
response
Energy response
Detector bias
Signal polarity
Directional
response in water
Leakage current1
Cable leakage
synthetic single crystal
diamond detector
dosimetry in radiotherapy
beams
absorbed dose to water
Materials and measures:
Entrance window
0.3 mm RW3
0.6 mm Epoxy
0.01 mm Al 99.5
Total window
101 mg/cm2
area density
Water-equivalent
1.0 mm
window thickness
Sensitive volume
circular, radius 1.1 mm,
thickness 1 µm
Outer dimensions
diameter 7 mm,
length 45.5 mm
Useful ranges:
Radiation quality
Field size2
Temperature
Humidity range
100 keV ... 25 MV photons
(6 ... 25) MeV electrons
(1 x 1) cm2 ... (40 x 40) cm2
(10 ... 35) °C, (50 ... 95) °F
(10 ... 80) %, max 20 g/m3
60Co
Ordering Information
0.004 mm3
TN60019 microDiamond Detector, connecting system BNT
TW60019 microDiamond Detector, connecting system TNC
TM60019 microDiamond Detector, connecting system M
waterproof, disk-shaped,
sensitive volume perpendicular to detector axis
on detector axis, 1 mm from
detector tip, marked by ring
axial
5 Gy
1 nC/Gy
≤ 0.5 % per year
< 0.25 % / kGy at 18 MV
≤ 0.08 % / K
± 8 % (100 keV ... 60Co)
0V
positive
≤ 1 % for tilting ≤ ± 40°
≤ 20 fA
≤ 200 fC / (Gy·cm)
The microDiamond detector is realized in collaboration with Marco Marinelli
and Gianluca Verona-Rinati and their team, Industrial Engineering
Department of Rome Tor Vergata University, Italy.
[1] I. Ciancaglioni, M. Marinelli, E. Milani, G. Prestopino, C. Verona,
G. Verona-Rinati, R. Consorti, A. Petrucci and F. De Notaristefani,
Dosimetric characterization of a synthetic single crystal diamond
detector in clinical radiation therapy small photon beams, Med. Phys. 39
(2012), 4493
[2] C. Di Venanzio, M. Marinelli, E. Milani, G. Prestopino, C. Verona,
G. Verona-Rinati, M. D. Falco, P. Bagalà, R. Santoni and M. Pimpinella,
Characterization of a synthetic single crystal diamond Schottky diode for
radiotherapy electron beam dosimetry, Med. Phys. 40 (2013), 021712
1
At the high end of the temperature range, higher leakage currents may
occur.
2
Measurements already at (0.5 x 0.5) cm2 possible. When using detectors
in extremely small field sizes, consult scientific literature prior to use.
29
Dosimetry Diode P
Type 60016
Waterproof silicon detector for
dosimetry in high-energy photon
beams up to field size 40 cm x 40 cm
Features
Useful for measurements in small and large photon
fields
Excellent spatial resolution
Minimized energy response for field size independent measurements up to 40 cm x 40 cm
The 60016 Dosimetry Diode P is ideal for dose measurements in small photon fields as encountered in IORT,
IMRT and stereotactic beams. The excellent spatial resolution makes it possible to measure very precisely beam
profiles even in the penumbra region of small fields. The
superior energy response enables the user to perform
accurate percentage depth dose measurements which are
field size independent up to field sizes of (40 x 40) cm2.
The waterproof detector can be used in air, solid state
phantoms and in water.
Specification
Type of product
Application
Measuring quantity
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Nominal response
Dose stability
Temperature response
Energy response
Detector bias voltage
Signal polarity
30
p-type silicon diode
dosimetry in radiotherapy
beams
absorbed dose to water
60Co
Directional response in
water
Leakage current
Cable leakage
≤ ± 0.5 % for rotation
around the chamber axis,
≤ ± 1 % for tilting ≤ ± 40°
≤ ± 50 fA
≤ 1 pC/(Gy·cm)
Materials and measures:
Entrance window
1 mm RW3,
1.045 g/cm3
1 mm epoxy
Total window area density 220 mg/cm2
Water-equivalent window 2.21 mm
thickness
Sensitive volume
1 mm2 circular
30 µm thick
Outer dimensions
diameter 7 mm,
length 47 mm
Useful ranges:
Radiation quality
Field size
Temperature
Humidity
60Co
... 25 MV photons
(1 x 1) cm2 ... (40 x 40) cm2
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
0.03 mm3
waterproof, disk-shaped
sensitive volume perpendicular to detector axis
on detector axis, 2 mm from
detector tip
axial
9 nC/Gy
≤ 0.5 %/kGy at 6 MV
≤ 1 %/kGy at 15 MV
≤ 0.5 %/kGy at 5 MeV
≤ 4 %/kGy at 21 MeV
≤ 0.4 %/K
at higher depths than dmax,
the percentage depth dose
curves match curves measured with ionization chambers within ± 0.5 %
0V
negative
Ordering Information
TN60016 Dosimetry Diode P, connecting system BNT
TW60016 Dosimetry Diode P, connecting system TNC
TM60016 Dosimetry Diode P, connecting system M
Dosimetry Diode E
Type 60017
Waterproof silicon detector for
dosimetry in high-energy electron and
photon beams
Features
Useful for measurements in all electron fields and for
small photon fields
Excellent spatial resolution
Minimized energy response
Thin entrance window for measurements in the
vicinity of surfaces and interfaces
The 60017 Dosimetry Diode E is ideal for dose measurements in small electron and photon fields as encountered in IORT, IMRT and stereotactic beams. The excellent spatial resolution makes it possible to measure very
precisely beam profiles even in the penumbra region of
small fields. The superior energy response enables the
user to perform accurate percentage depth dose measurements which are field size independent up to field
sizes of (40 x 40) cm2. The waterproof detector can be
used in air, solid state phantoms and in water.
Specification
Type of product
Application
Measuring quantity
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Nominal response
Dose stability
Temperature response
Energy response
Detector bias voltage
Signal polarity
p-type silicon diode
dosimetry in radiotherapy
beams
absorbed dose to water
60Co
0.03 mm3
waterproof, disk-shaped
sensitive volume perpendicular to detector axis
on detector axis, 0.77 mm
from detector tip
axial
9 nC/Gy
≤ 0.5 %/kGy at 6 MV
≤ 1 %/kGy at 15 MV
≤ 0.5 %/kGy at 5 MeV
≤ 4 %/kGy at 21 MeV
≤ 0.4 %/K
at higher depths than dmax,
the percentage depth dose
curves match curves measured with ionization chambers within ± 0.5 %
0V
negative
Directional response in
water
Leakage current
Cable leakage
≤ ± 0.5 % for rotation
around the chamber axis,
≤ ± 1 % for tilting ≤ ± 20°
≤ ± 50 fA
≤ 1 pC/(Gy·cm)
Materials and measures:
Entrance window
0.3 mm RW3,
1.045 g/cm3
0.4 mm epoxy
Total window area density 140 mg/cm2
Water-equivalent window 1.33 mm
thickness
Sensitive volume
1 mm2 circular
30 µm thick
Outer dimensions
diameter 7 mm,
length 45.5 mm
Useful ranges:
Radiation quality
Field size1
Temperature
Humidity
(6 ... 25) MeV electrons
... 25 MV photons
(1 x 1) cm2 ... (40 x 40) cm2
for electrons
(1 x 1) cm2 ... (10 x 10) cm2
for photons
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
60Co
Ordering Information
TN60017 Dosimetry Diode E, connecting system BNT
TW60017 Dosimetry Diode E, connecting system TNC
TM60017 Dosimetry Diode E, connecting system M
1
Measurements already at (0.5 x 0.5) cm2 possible. When using detectors
in extremely small field sizes, consult scientific literature prior to use.
31
Dosimetry Diode SRS
Type 60018
Waterproof silicon detector for
dosimetry in 6 MV photon beams up
to field size 10 cm x 10 cm
Features
Designed for measurements in small photon fields
with maximum 6 MV
Excellent spatial resolution
High response
Very low noise
Thin entrance window for measurements in the
vicinity of surfaces and interfaces
The 60018 Dosimetry Diode SRS is ideal for dose measurements in photon fields with a maximum field size
of 10 cm x 10 cm and with a maximum energy of 6 MV.
The very high response of this detector allows to measure beam profiles with a very high resolution and very
short dwell time. Typical use is beam profile measurement for stereotactic radio surgery (SRS).
Specification
Type of product
Application
Measuring quantity
Reference radiation
quality
Nominal sensitive
volume
Design
Reference point
Direction of incidence
Nominal response
Dose stability
Temperature response
Energy response
Detector bias voltage
Signal polarity
Directional response in
water
Leakage current
Cable leakage
32
p-type silicon diode
dosimetry in radiotherapy
beams
absorbed dose to water
60Co
Materials and measures:
Entrance window
0.3 mm RW3,
0.27 mm epoxy
Total window area density 140 mg/cm2
Water-equivalent window 1.31 mm
thickness
Sensitive volume
1 mm2 circular
250 µm thick
Outer dimensions
diameter 7 mm,
length 45.5 mm
Useful ranges:
Radiation quality
Field size1
Temperature
Humidity
60Co ... 6 MV photons
(1 x 1) cm2 ... (10 x 10) cm2
(10 ... 40) °C
(50 ... 104) °F
(10 ... 80) %, max 20 g/m3
Ordering Information
TN60018 Dosimetry Diode SRS, connecting system BNT
TW60018 Dosimetry Diode SRS, connecting system
TNC
TM60018 Dosimetry Diode SRS, connecting system M
0.3 mm3
waterproof, disk-shaped
sensitive volume perpendicular to detector axis
on detector axis, 0.74 mm
from detector tip
axial
175 nC/Gy
≤ 0.8 %/kGy at 6 MV
≤ (0.1 ± 0.05) %/K
at higher depths than dmax,
the percentage depth dose
curves match curves measured with ionization chambers within ± 0.5 %
0V
negative
≤ ± 0.5 % for rotation
around the chamber axis,
≤ ± 1 % for tilting ≤ ± 20°
≤ ± 50 fA
≤ 1 pC/(Gy·cm)
1
Measurements already at (0.5 x 0.5) cm2 possible. When using detectors
in extremely small field sizes, consult scientific literature prior to use.
Dosimetry Diode PR
Type 60020
Waterproof silicon detector for
dosimetry in high-energy proton
beams
Features
Materials and measures:
Useful for measurements in proton fields
Excellent spatial resolution
Thin entrance window for measurements in the
vicinity of surfaces and interfaces
The 60020 Dosimetry Diode PR is ideal for dose
measurements in proton beams. The excellent spatial
resolution makes it possible to measure very precisely
beam profiles even in the penumbra region of small
fields The waterproof detector can be used in air, solid
state phantoms and in water.
Specification
Type of product
Application
Measuring
quantity
Reference
radiation quality
Nominal sensitive
volume
Design
Reference point
Direction of
incidence
Nominal response
Dose stability
Temperature
response
Detector bias
voltage
Signal polarity
Directional
response
Leakage current
Cable leakage
p-type silicon diode
dosimetry in radiotherapy
absorbed dose to water
60Co
0.02 mm3
waterproof, disk-shaped
sensitive volume perpendicular to detector axis
on detector axis, 0.77 mm
from detector tip
axial
Entrance window
Total window
area density
Water-equivalent
window thickness
Sensitive volume
Outer dimensions
0.3 mm RW3
0.4 mm epoxy
140 mg/cm2
1.33 mm
1 mm2 circular
20 µm thick
diameter 7 mm,
length 45.5 mm
Useful ranges:
Radiation quality
Field Size1
Temperature
Humidity
(50 … 270) MeV protons
(1 x 1) cm2... (40 x 40) cm2
(10 … 40) °C
(50 … 104) °F
(10 … 80) %, max 20 g/m3
Ordering Information
TN60020 Dosimetry Diode PR, connecting system BNT
TW60020 Dosimetry Diode PR, connecting system TNC
TM60020 Dosimetry Diode PR, connecting system M
6 nC/Gy
≤ 9 %/kGy
after initial preirradiation
≤ 0.4 %/K
0V
negative
≤ ± 0.5 % for rotation
around the chamber axis
≤ ± 1 % when tilting ± 20°
≤ ± 50 fA
≤ 1 pC/(Gy·cm)
1
Measurements already at (0.5 x 0.5) cm2 possible. When using detectors
in extremely small field sizes, consult scientific literature prior to use.
33
8 References and Further Reading
[AAPM TG51]AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy
photon and electron beams. Med. Phys. 26 (9), September 1999, 1847-1870
[Alfonso2008]A new formalism for reference dosimetry of small and nonstandard fields,
Med. Phys. 35 (2008), 5179-5186
[Crop2009] F. Crop et al., The influence of small field sizes, penumbra, spot size and
measurement depth on perturbation factors for microionization chambers,
Phys. Med. Biol. 54 (2009) 2951
[Cyarnecki2013]D. Cyarnecki and K. Zink, Monte Carlo calculated correction factors for diodes
and ion chambers in small photon fields, Phys. Med. Biol. 58 (2013) 2431–2444
[DETECTORS]PTW DETECTORS catalog, available at www.ptw.de
[DIN 6800-2]Dosismessverfahren nach der Sondenmethode für Photonen- und Elektronenstrahlung. Teil 2: Dosimetrie hochenergetischer Photonen- und Elektronenstrahlung mit Ionisationskammern, März 2008
[Fenwick2013] J.D. Fenwick et al., Using cavity theory to describe the dependence on detector
density of dosimeter response in non-equilibrium small fields, Phys. Med. Biol.
58 (2013), 2901
[Francescon2012]P. Francescon et al., Monte Carlo simulated correction factors for machine
specific reference field dose calibration and output factor measurement using
fixed and iris collimators on the CyberKnife system, Phys. Med. Biol. 57 (2012),
3741
[IAEA 398]Absorbed Dose Determination in External Beam Radiotherapy.
Technical Report Series No 398. International Atomic Energy Agency,
Vienna, 2000
[IPEM 103]Report Number 103, Small Field MV Photon Dosimetry, Institute of Physics
and Engineering in Medicine, 2010, ISBN 978 1 903613 45 0
[Muir2011]Muir et al., Measured and Monte Carlo calculated kQ factors: Accuracy and
comparison, Med. Phys. 38 (2011), 4600
[Scott2012] A.J.D. Scott et al., Characterizing the influence of detector density on dosi­
meter response in non-equilibrium small photon fields, Phys. Med. Biol. 57
(2012) 4461–4476
34
[Sterpin2012]E. Sterpin et al., Monte Carlo computed machine-specific correction factors
for reference dosimetry of TomoTherapy static beam for several ion chambers,
Med. Phys. 39 (2012), 4066
[Pantelis 2012]E. Pantelis et al., On the output factor measurements of the CyberKnife iris
collimator small fields: Experimental determination of the k[..] correction
factors for microchamber and diode detectors, Med. Phys. 39 (2012), 4875
[Wuerfel2013]
J .U. Wuerfel, Dose measurements in small fields, Medical Physics International
1 (2013), 81. You can download this article from the PTW website:
http://www.ptw.de/. Go to Literature > small field
8.1 PTW Small Field Detectors in Use
[Bruggmoser2007]G. Bruggmoser et al., Determination of the recombination correction
factor kS for some specific plane-parallel and cylindrical ionization chambers
in pulsed photon and electron beams, Phys. Med. Biol. 52 (2007), N35
[Ciancaglioni2012]I. Ciancaglioni et al., Dosimetric characterization of a synthetic single crystal
diamond detector in clinical radiation therapy small photon beams,
Med. Phys. 39 (2012), 4493
[Di Venanzio2013]C. Di Venanzio et al., Characterization of a synthetic single crystal diamond
Schottky diode for radiotherapy electron beam dosimetry, Med. Phys. 40
(2013), 021712
[Dzierma2012]Y. Dzierma et al., Beam properties and stability of a flattening-filter free 7 MV
beam – An overview, Med. Phys. 39 (2012), p. 2595
[Francescon2011]P. Francescon et al., Calculation of k_Qclin,Qmsr_fclin,fmsr for several small
detectors and for two linear accelerators using Monte Carlo simulations,
Med. Phys. 38 (2011), 6513
[Gago-Arias2013]A. Gago-Arias et al., Correction factors for ionization chamber dosimetry in
CyberKnife: Machine-specific, plan-class, and clinical fields, Med. Phys. 40
(2013) 011721
[Zhu2013]J. Zhu et al., A comparison of VMAT dosimetric verifications between fixed
and rotating gantry positions, Phys. Med. Biol. 58 (2013), 1315
H E A LT H P H Y S I C S
NUCLEAR MEDICINE
DIAGNOSTIC RADIOLOGY
R A D I AT I O N T H E R A P Y
Dosimetry Pioneers since 1922.
It all started with a brilliant invention - the revolutionary Hammer dosemeter in 1922. Ingenuity coupled
with German engineering know-how shaped the company’s history, leading to innovative dosimetry
products that later became an industry standard. Over the years, PTW has maintained its pioneering spirit,
growing into a global market leader of dosimetry applications, well known for its product excellence and
innovative strength. Today, PTW dosimetry is one of the first choices for healthcare professionals in
radiation therapy, diagnostic radiology, nuclear medicine and health physics.
For more information on PTW products visit www.ptw.de
or contact your local PTW representative:
©PTW. All Rights Reserved. Due to continuous innovation, product specifications are subject to
change without prior notice. Printing errors and omissions excepted. D920.200.00/04 2013-09.
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