Head & Neck IMRT Treatment Planning

Head & Neck IMRT
Treatment Planning
IMRT for H&N
• IMRT only – where all volumes are treated with
IMRT beams only
• IMRT and SCV static fields – where a split beam
approach is used and all volumes superior to the
match-line are treated with IMRT beams and the
supraclavicular areas/nodes are treated with
traditional static beams
Isocenter Placement
• For IMRT only - At simulation the “marked
isocenter” is generally placed at a stable location
somewhere near the superior-inferior center of the
area(s) of interest and just anterior to the vertebral
body
• IMRT & SCV split beams – the “marked
Isocenter” is generally placed just superior to the
larynx (if treatment volumes allow) so that the
larynx can be protected by a block in the SCV field
Contours
• The physician generally contours the
following structures:
– CTV volumes
– Parotids
– Cochlea
Contours
• CTV volumes
CTV1
CTV2
CTV3
• Parotids
Contours
•Cochlea
Contours
***These are possible physician or resident drawn structures. If contoured,
then be mindful to reduce ALARA
• Larynx avoid
• Arytenoids avoid
• Esophagus avoid
Contours
• Dosimetrist will also create the following:
–
–
–
–
–
–
–
–
Cord expanded 5mm
Brainstem expanded 5mm
Parotid subs
External & external contracted 3mm
CTV subs
PTVs
Total PTVs expanded
Normal tissue
Contours
External
• To create: Use the Autocontour, use the default threshold of
800 (lower) and 4096 (upper)
• Place an external contours at 5 slices above and below any
CTV contour. If the plan has static SCV fields, stop the external
on the isocenter slice
4096
Contours
External Contracted 3mm
• This contour will be used when creating CTVsubs and PTVs
to ensure that there is an adequate build-up region in place.
• To create: Set
“external” as source
& “external
contracted” as
destination ROI – set
variable margin at
0.3cm to Rt, Lt,
Anterior & Posterior,
and 0 to the Superior
& Inferior
Contours
CTV Subs
• A copy of the CTVs is required
because one should not edit the
original targets drawn by the
physician
• This copy is the original CTV
without the build-up region or
any CTVs of high prescription
doses.
• To create: Start with the CTV
going to the highest dose. Set
“CTV1” as source & “CTV1
sub” as destination ROI; set
“external contracted” to avoid
exterior; and set uniform margin
to 0cm
CTV1-60
Contours
CTV Subs
• Create the next CTVsub
for the next highest dose
• Set CTV2 as source &
CTV2 sub as destination
ROI; set external
contracted to avoid
exterior; set CTV1sub to
avoid interior; and set
uniform margin to 0cm
• Do this process for all
CTV volumes
Contours
Parotid Subs
• The is a copy of the
parotid without the CTV
targets
• To create: Set the “Rt
Parotid” (in pink) as source
structure and set all CTVs
to avoid interior -- create
new ROI named “Rt
Parotid sub” (in black)
•
Note: The term “sub” means that it is a copy of the physician drawn
volume that has been edited for planning purposes – so that pixels will
not share conflicting objectives -- “sub” stands for “subtracted”
Contours
PTVs
• These contours are 3 mm
expansions of the CTVsubs
• To create: Start with the
CTV going to the highest
dose; Set CTV1 sub as
source & PTV1 as
destination ROI – set
external contracted to avoid
exterior; set cord,
brainstem, and parotid subs
to avoid interior; set
uniform margin to 0.3cm
Contours
PTVs
• Create the next PTVs for
the next highest dose
• Set CTV2 sub as source
& PTV2 as destination
ROI; set external
contracted to avoid
exterior; set cord,
brainstem, and parotid subs
to avoid interior; set PTV1
to avoid interior; and set
uniform margin to 0.3cm
• Do this process for all
CTV volumes
Contours
PTVs
• “Shaving” or
“cleaning-up” PTVs
• It is sometimes
advantageous to take
the some of the
created PTV out of
abutting structures
(such as: Brain, Optic
nerves, larynx,
esophagus, or even
air)
Contours
Total PTVs expanded 5mm
• To Create: Set all PTV
volumes as source & Total
PTVs expanded 5mm as
destination ROI; and set
uniform margin to 0.5cm
Total PTVs expanded 5mm
0.5
Contours
Normal Tissue
• To Create: Set external
contracted as source &
Normal tissue as
destination ROI; set Total
PTVs expanded to avoid
interior; and set uniform
margin to 0cm
Beams
• The typical beam
arrangement for bilateral
cases consist of 9 beams
• See the “standard” gantry
angles to the right
•The field name is the
couch angle – gantry angle
and the area of treatment
Note: When there are static SCV fields with IMRT we rotate the collimator to
90 degrees so the collimators match on the SCV and IMRT fields
Beams
• The typical beam arrangement for unilateral cases
consist of 7 beams
• For left sided disease: 182, 350, 22, 54, 86, 118, and
150 degrees
• For right sided disease: 210, 242, 274, 306, 338, 10
and 178 degrees
Beams
• Using the “standard” 9-beam arrangement, Possible modification
for each beam’s gantry angle you could:
• Re-open the Blocks window for each beam:
– Evaluate for the following:
• gantry angle (can be done with full or half beam IMRT
technique): check shoulders (IF you have a CTV that
extended posterior)
• collimator angle (only use if the plan is full IMRT):
Minimize field width, possible less dose in shoulder
• whether or not the beam should be split (only use for full
IMRT): large flds (X1 and X2 Jaws) will need to be split
into two flds so the MLC leaf motion can be fully used to
achieve optimization
Beams
Gantry Angles
• If the targets are low in the inferior neck/scv area, some of the
lateral standard beams may go through more of the shoulder than
we would like -- In these cases we would edit the gantry angles
slightly, however, make sure that the beams are still well
distributed around the patient.
Note: Creating a quick contour for
the Rt humeral head and the Lt
humeral head can also assist in
setting up optimal beams
Beams
Gantry 280
Gantry 285
Beams
Collimator Angles
• Changing the collimator angle can also be a useful
technique for avoiding unwanted dose to the shoulders
(although we rarely have to worry about too much
dose to the shoulders)
• Collimation is also helpful in creating narrower flds
so that you may not have to split the beam
Beams
Gantry 240 / Coll 0
Gantry 235 / Coll 345
Beams
Split Beams
• If the field size is
approaching 20cm (or
larger) one will need to
split the beam into two
parts so the MLC leaves
can travel the width of the
fld freely to insure the best
optimization within each
control point.
23.7cm
Beams
Split Beams
IMRT Parameters
• These are the recommended
parameters for DMPO
• Max number of segments is
10-15 per field. This has been
found to produce the most
optimal plan without over doing
the amount of segments. In this
case 9 beams are used so max
number of segments is 135. A
segment is a “MLC control
point”
IMRT Objectives
Min Dose: applies to whole ROI volume. The objective is met
when the region of interest has a minimum dose that is greater
than or equal to the target dose. The icon appears at the 100%
volume level of the DVH.
Max Dose: applies to whole ROI volume. The objective is met
when the region of interest has a maximum dose that is greater
than or equal to the target dose. The icon appears at the 0%
volume level of the DVH.
Uniform Dose: Applies to the whole ROI volume. Can only be
used as an objective. The objective comes closer to being
satisfied as the dose throughout the ROI’s volume becomes
more uniform.
IMRT Objectives
Min DVH: Applies only to a percent volume of the ROI. The
objective is met when the specified percent of the volume of
the region of interest has a minimum dose that is greater than
or equal to the target dose.
Max DVH: Applies only to a percent volume of the ROI. The
objective is met when the specified percent of the volume of
the region of interest has a maximum dose that is less than or
equal to the target dose.
EUD (Equivalent Uniform Dose): Applies to the whole ROI
volume. Can only be used as an objective. Applies to the mean
dose.
IMRT starting objectives
Starting
objectives for a
BOT IMRT plan
Rt Parotid Sub
IMRT Objectives
Objectives
• These objectives are a good starting point. After the first
run through look at you objective values to see if you need
to adjust the objectives.
• On the first trial you want to get all of your target
coverage. Steadily increase the weighting on the targets
while keeping the unaffected tissues weighting at 1.
• Once target coverage has been achieved, copy your trial
and start pushing all of your unaffected tissues.
Planning Iterations
• Use the dose distributions, the DVHs and the objective values to
evaluate each plan iteration
Dose Painting
• “Painting” is when we create a contour with the
objective of either increasing or decreasing dose
to a certain limited area
– Cold or Hot spots in target volumes
– Pushing of dose off of a critical structure
Plan Evaluation
• Isodose line distributions
– PTV1 95% coverage
– CTVs (subs) 99-100% coverage
– Minimal 105% hotspots
• Dose Volume Histograms (DVH)
– Min. / Max. / Mean
• Tabular DVH
Final DVH
Include the following
ROIs on the Final
DVH:
• PTV1
• CTVsubs (all)
• Rt & Lt Parotids
•Brainstem, Brainstem
expanded
• Cord, Cord
expanded
• any other critical
structures as needed
Dose Constraints
• Cord: 45Gy max dose (<40Gy, if achievable)
• Cord Expanded 5mm: 50Gy max dose if possible
(depends on location of PTVs)
• Brainstem: 54Gy max dose (typically <40Gy)(try
for <30Gy to posterior portion)
• Brain: Tight 54Gy gradient
• Parotids: <26Gy mean dose
• Cochlea: <35Gy max dose (<30Gy, if possible)
Note: These are the maximum dose limits, you
should look to achieve below these constraints
Dose Constraints
•
•
•
•
•
•
•
Optic Chiasm & Optic Nerves: <54Gy max dose
Mandible: <70 max dose, no hot spots of > 105%
Oral cavity: ALARA
Larynx: ALARA
Esophagus: ALARA
Posterior Neck: <35Gy (try to control the 30 Gy)
Lens < 5Gy or <10 Gy if they are close to the target
Note: These are the maximum dose limits, you
should look to achieve below these constraints
Images of completed plan
- Tight gradient on the parotids with good coverage to
the targets.