results for neutrons

Neutron dose evaluation in radiotherapy
Francesco d’Errico
University of Pisa, Italy
Yale University, USA
Radiation therapy with a linear accelerator (LINAC)
Photoneutron production in accelerator head
• Photoneutrons produced by
interaction of photon beam with
accelerator components
• Produced mainly in the target,
primary collimator, flattener and
jaws/collimators
• Typical materials are
copper,tungsten, gold, lead and iron
• Neutron production in electron
mode is lower than in photon mode
• Direct production of neutrons by electrons
is at least 2 orders of magnitude lower
• Lower electron current
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production in accelerator head
• Photon must have energy greater than
binding energy of nucleus in atom
• Sn =Separation Energy
• Neutron production in primary
laminated barrier
• Lead has lower Sn than Iron
•
•
Pb-207 (22.1%): Sn =6.74 MeV (NCRP 79)
Pb-208 (52.4%) : Sn = 7.37 MeV
• Iron
•
•
Fe-57 (2.1%): Sn= 7.65 MeV
Fe-56 (91.7%): Sn =11.19 MeV
• Lead has a higher neutron yield than iron
• Steel is a better choice for reducing
neutron production
Nisy E. Ipe, 2007 AAPM Summer School
Schematic of Varian accelerator head
Photoneutron production in accelerator head
Photoneutron production
• Photoneutron spectrum from
accelerator head resembles a
fission spectrum
• Spectrum changes after
penetration through head
shielding
• Concrete room scattered
neutrons will further soften the
spectrum
• Spectrum outside the concrete
shielding resembles that of a
heavily shielded fission spectrum
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production
• Two Processes
–Direct Emission
•Average energy of direct
neutrons is ~ few MeV
•Spectra peak at energies > 2
MeV
•Have a sin2J angular
distribution, therefore
forward peaked
•Contributes about 10 to 20%
of neutron yield for
bremsstrahlung with upper
energies of 15 to 30 MV
Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production
• Two Processes
–Evaporation Neutrons
•Dominant process in heavy
nuclei
•Emitted isotropically
•Spectral distribution is
independent of photon energy
for energies that are a few
MeV above neutron production
threshold
•Average energy is ~ 1 -2 MeV
•Spectra peak at ~ 200-700 keV
Nisy E. Ipe, 2007 AAPM Summer School
Recent treatment modalities – IMRT
• SmartBeam IMRT "paints" a dose
to the tumor with pinpoint
precision, while sparing healthy
normal tissue.
– Minimizes hot spots
– Improves target inhomogeneity
– Provides detailed dose painting to the target
– Sculpts dose around critical structures more
effectively
– Allows treatments to occur in conventional
10 - 15 minute time slots
– Dose resolution, with up to 500 segments
per field
– Spatial resolution of 2.5 - 5mm
Recent treatment modalities – VMAT RapidArc™
• RapidArc uses a unique
algorithm that provides
excellent treatment delivery
control.
• Its single gantry rotation
speeds treatment delivery so
clinicians can develop
treatments that take one-half
• A RapidArc treatment may also
to one-eighth the time of
result in less radiation leakage
conventional IMRT
and scatter, so peripheral tissues
treatments—just two minutes
receive a lower overall dose.
in many cases.
Recent treatment modalities - Tomotherapy
Photoneutrons: NCI-WG 2001 recommendations
• Total-body dose for IMRT patients is higher, generally increasing
with the number of monitor units used for treatment. The
potential for complications related to this increased dose should
be recognized and considered.
• Increased neutron production for high-energy machines used for
IMRT should be considered.
• Increased workload values for IMRT (may be 2–5 times larger than
in conventional therapy) should be considered for the
leakage/transmission part of the treatment room shielding.
• Because IMRT is inherently less efficient (per MU) than
conventional RT, vendors should consider the use of more internal
shielding in the design of future IMRT machines.
Neutron dosimetry Conversion coefficients
Neutrons
– particle fluence to ambient or personal dose equivalent
– influence of phantom (multiple scattering) and quality factor
Neutron radiation protection dosimetry
• Utilization of physical phenomena
resembling dose (equivalent)
deposition in tissue.
• Measurements of LET spectra and
convolution over Q(L).
• Design of systems mimicking
the fluence to dose equivalent
conversion coefficient
Neutron detection by thermoluminescence
Neutrons are particles without charge, difficult to detect
TL induced by secondary particles from nuclear reactions:
(n,) (n,p) (n,d) etc
TLD for neutrons have high concentration of isotopes with
high cross section to neutrons
in LiF ~ 7,4% of Li is
n Radiation
LiF
6Li
6Li(n, )3H
Some neutron capture cross sections
3He(n,p)3H
10B(n,)7Li
6Li(n,)3H
Albedo-based neutron dosimeters
TL albedo neutron dosimeter response
Plastic Nuclear Track Detectors
• Particles of ionizing
radiation cause
molecular size damage
in solid material
• The damage can be
enlarged to microscopic
range by chemical
etching
How CR-39 PNTDs Work
How CR-39 PNTDs Work
How CR-39 PNTDs Work
Atomic force microscopy of latent tracks
•
•
Latent tracks 70-100 nm and must be enlarged to 5-30 µm to
be visible with optical microscopy
Tracks are enlarged by chemical or electrochemical etching
Atomic force microscopy of latent tracks
How CR-39 PNTDs Work
Result of Etching
Typical microscopic view-field with developed
tracks
Overlapping
Tracks
Identified
Tracks
• Track density is proportional to the exposure value [dose]
• With calibration the [equivalent] dose can be deducted
Neutron response mechanism
• Neutrons are non ionizing
radiation, but CR-39
(C12H18O7) is sensitivity to
fast neutron recois
• Neutrons interact with H
producing recoil protons
• Recoil protons create
latent tracks in the CR-39
material
• In practice, converter layers are utilized:
-
Converters modify (improve) the energy dependence of the reponse
-
Converters also protect the CR-39 chip against alpha particles from environmental
radon
Neutron energy dependent response
• Good energy dependence of response
• But, relatively low sensitivity
Sensitization by CO2 pretreatment
Superheated emulsions
• Fluorocarbon droplets kept in a steady superheated state by
emulsification in compliant gels.
• Bubble nucleation triggered by neutrons above selectable
threshold energies.
• Can be totally insensitive to photons.
Some current technologies
Neutron energy dependence of emulsions
• Composition closely tissue equivalent
• Insensitive to photons
• Response resembling kerma equivalent coefficient
• The emulsions can measure dose equivalent in phantom without disrupting
charged particle equilibrium
Isocenter neutron doses
Neutron doses from various x-ray beams
18 MV
x-rays
15 MV
x-rays
10 MV
x-rays
In vivo photoneutron measurements
SDD fluence response vs quality-factor weighted
kerma coefficient and photoneutron spectrum
Bubble counting by scattered light
Photo
diodes
LEDs
• Instant read out
• Rate insentive
• Position sensitive
Study of a prostate treatment
Characteristics of various phantoms for out-of-field
measurements
Clinical simulations of prostate radiotherapy
BOMAB = BOttle MAnnikin ABsorber phantom,
industry standard (ANSI 1995) for calibrating
whole body counting systems
Irradiation channels (“pipes”)
BOMAB CT scan and simulated organs
Transverse
plane section view
Bladder
Prostate
PTV
Boost
volume
Rectum
Sagittal
Sagittal
plane section view
plane section view
≈5 cm
≈5 cm
≈5 cm
≈5 cm
Organs of interest: bladder, prostate, rectum
Organs of interest: bladder, prostate, rectum
Organs of interest: bladder, prostate, rectum
Treatment modalities in Pisa and Krakow
Modalities
Pisa
Varian Clinac 2100 C
MU/ Krakow
2 Gy Varian Clinac 2300 CD
6 MV single 10x10 cm2 field (ref) 251
6 MV single 10x10 cm2 field (ref)
Used for reference
15 MV single 10x10 cm2 field (ref) 218
18 MV single 10x10 cm2 field (ref)
6 MV 4-field MLC
15 MV 5-field MLC
266
18 MV 4-field MLC
6 MV IMRT
432
6 MV IMRT
18 MV IMRT
6 MV VMAT (RapidArc)
481
6 MV Tomotherapy
MU/
2 Gy
240
199
277
218
466
350
Treatment modalities – 5 fields with MCL
Treatment modalities – IMRT
Treatment modalities - VMAT
Dose profiles from different treatment plans
Dosimeter in-phantom placement
BTI sensitive detector length: 7.0 cm
SDD sensitive detector length: 2.5 cm
LEG
S
Measurement points  center of sensitive length
D’’ = 6.8
D’’ = 12
D’’ = 12
D’’ = 31.8
D’’ = 16.8
D’’ = 27
D’’ = 46.8
D’’ = 42
D’’ = 56.8
D’’ distance from outer flange – legs side.
Dimensions in cm
IMRT neutron dose as a function of nominal energy
1,E+03
μSv/Gy
1,E+02
1,E+01
18 MV - Krakow
6 MV - Pisa
1,E+00
0
5
10
15
Penetration distance along the sagittal plane (cm)
5 cm is bladder, 10 cm is prostate and 15 cm is rectum
20
MLC multiple field treatments
1,E+04
PTV Krakow 18 MV SDD
PTV Pisa 15 MV SDD
μSv/Gy
1,E+03
PTV Krakow 18 MV PADC
PTV Pisa 15 MV PADC
1,E+02
1,E+01
1,E+00
-10
0
10
20
30
Off-axis distance (cm)
40
50
Reference phantom literature comparison - SDD
6,E+03
This work (SDD, 20 MV)
This work (BDPND, 20 MV)
5,E+03
d'Errico 2000 (SDD, 18 MV)
d'Errico 2000 (SDD, 15 MV)
μSv/Gy
4,E+03
Awotwi-Pratt 2007 (SDD, 15 MV)
Lin 2007 (BDPND, 15 MV)
3,E+03
This work (SDD, 12 MV)
2,E+03
1,E+03
0,E+00
0
50
100
150
Off-axis distance (mm)
200
250
TrueBeam 10 cm x 10 cm field results
Conclusions and Prospects
• The “Pisa BOMAB” proved a viable approach
• The selected neutron dosimetry methods appear
reliable and reproducible
• But, measurement times are long and additional
campaigns are needed for a systematic analysis
• Tomotherapy appears to deliver the smallest
neutron doses
• A small neutron contamination is present already
at 6 MV and warrants further investigation