1 IMRT for Prostate Cancer Robert A. Price Jr., Ph.D. Philadelphia, PA AAPM Houston, July 30, 2008 alpha cradle foot holder All patients are simulated in the supine position. Reproducibility is achieved using a custom alpha cradle cast that extends from the mid-back to mid-thigh. The feet are positioned in a custom plexiglas foot-holder. The patient is told to have a 1/2- 3/4 bladder because during treatment a full bladder is difficult to maintain. Simulation (Positioning and Immobilization ) • The patient is asked to empty the rectum using an enema prior to simulation. Also, a low residue diet the night before simulation is recommended to reduce gas. If at simulation the rectum is >3 cm in width due to gas or stool, the patient is asked to try to expel the rectal contents. 4.5 cm 6 cm Bad rectum Good rectum 2.5 cm 3 cm CT Scans • Scans are acquired from approximately 2 cm above the top of the iliac crest to approximately mid-femur. This scan length will facilitate the use of non- coplanar beams when necessary. • Scans in the region beginning 2 cm above the femoral heads to the bottom of the ischial tuberosities are acquired using a 2.5 mm slice thickness and 2.5 mm table increment (Beacon patients: 1.25mm). All other regions may be scanned to result in a 1 cm slice thickness.
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IMRT for Prostate Cancer · · 2008-07-25IMRT for Prostate Cancer Robert A. Price Jr., Ph.D. Philadelphia, PA AAPM Houston, July 30, 2008 a lph cr d e foot holder All patients are
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IMRT for ProstateCancer
RobertA. Price Jr., Ph.D.Philadelphia,PA
AAPM Houston,July30,2008 alphacradle
footholder
All patientsaresimulatedin thesupineposition. Reproducibility isachievedusinga custom alphacradlecastthatextendsfrom the
mid-back to mid-thigh. Thefeetarepositionedin a customplexiglasfoot-holder. Thepatientis told to havea 1/2- 3/4 bladder
becauseduringtreatmenta full bladderis diff icult to maintain.
Simulation(Positioningand Immobilization)
• The patient is asked toempty the rectum using anenema prior to simulation.Also, a low residue diet thenight before simulation isrecommendedto reducegas.If at simulation therectumis>3 cm in width dueto gasorstool, the patient is askedtotry to expel the rectalcontents.
4.5cm
6 cm
Badrectum
Goodrectum
2.5cm
3 cm
CT Scans
• Scansare acquiredfrom approximately2 cm abovethetop of the iliac cresttoapproximatelymid-femur. This scanlength will facili tatetheuseof non-coplanarbeamswhennecessary.
• Scans in the region beginning 2 cmabove the femoral heads to the bottomof the ischial tuberosities are acquiredusing a 2.5 mm slice thickness and 2.5mm table increment (Beacon patients:1.25mm). Al l other regions may bescanned to result in a 1 cm slicethickness.
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MR Scans
• Al l prostatepatientsalsoundergoMRimaging within thedepartment,typically within onehalf hourbeforeorafter theCT scan. Scansareobtainedwithout contrast media.
• CT andMR imagesarefusedaccordingto bonyanatomyusingAll soft tissuestructuresarecontouredbasedon the MRinformation while the externalcontour and bony structures arebasedon CT.
•Retrogradeurethrogramsarenot performed.
1.5T, GE MedicalSystems
Extracapsular extension Seminalvesicles ?
CTMRI
Imaging modality mayaffecttreatmentregime
Prostate (CT)
Prostate(MR)
MRI CT
Imagingartifactsmayaffectcontouring
ContouredonCT
ContouredonMR
MR-CT fusionbasedon boneyanatomy
MR-basedprostate-rectum
interface
Mismatcharisesfromtime of scandifferences
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CT-basedprostate-rectum interface
Overlap (not includingPTV)
CTMRI
MR prior to beaconplacement (> 1 week)
Notethattheprostateis in adif ferentpositionrelative to thefemoralheads
MRICT
-Fusionbasedonboneyanatomy
-soft tissuebasedonMR
- plancalculatedonCT base(CT derived isocenter)
-Isodoselines generated basedonMR-defined target
-but the patient is aligned bybeacons(CT)
-may result in a geographical miss
MRICT
Solution: fusebasedonsoft tissue(prostate);alignmentwil l be
uneffected
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BladderRt FH
Lt FH
CTV
Rectum
A
P
S
I
PTV growth = 8mmin alldirectionsexcept
posteriorlywherea 5mmmarginis typically used
The “effective margin” is defined by thedistance betweenthe posterior aspect ofthe CTV and the prescription isodoselineand typically falls between3 and8 mm.
The “effective margin” is defined as:
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1. The distance between the posterior aspectof the CTV and the anterior rectal wall
2. The distance between the posterior aspectof the CTV and the prescription isodoseline
3. The distance between the posterior aspectof the CTV and the posterior rectal wall
4. The average 3D PTV margin5. The difference between take-home pay and
what your better half allows you to spend
Numberof BeamDirections
In theinterestofdelivery time we
typically beginwith 6andprogressto ≤≤≤≤ 9
Simplerplanssuchasprostateonly or prostate
+ seminal vesiclestypically resultin fewer
beamdirectionsthanwith theadditionof
lymphatics
Typical Dose
Routine treatments• Prostate+ proximal sv
(80 Gy @ 2.0Gy/fx)
• Distal sv, lymphatics(56 Gy @ ~1.4Gy/fx)
Post Prostatectomy• Prostatebed
(64-68 Gy @ 2.0Gy/fx)
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Acceptance CriteriaWhatis a goodplan?
When canI stopplanning?
GoodDVH
PTV95 = 100%
R65 = 8.3%
R40 = 22.7%
B65 = 8.4%
R40 = 19%
DVH AcceptanceCriteri a
PTV 95 % ≥≥≥≥ 100% Rx
R65 Gy ≤≤≤≤ 17%V
R40 Gy ≤≤≤≤ 35%V
B65 Gy ≤≤≤≤ 25%V
B40 Gy ≤≤≤≤ 50%V
FH50 Gy ≤≤≤≤ 10%V
Good plan example(axial)
The50%isodoselineshouldfall within therectal contouron
any individual CT slice
The90%isodoseline shouldnot exceed½ thediameterof
therectalcontouronanyslice
“Effectivemargin”
100%
90%
80%
70%
60%
50%
CTV
DVH for badplan(meets DVH criteria)
R65 = 13.4%
R40 = 31.5%
B65 = 9%B40 = 21.3%
PTV95 = 100%
DVH AcceptanceCriteri a
PTV95 % ≥≥≥≥ 100% Rx
R65 Gy ≤≤≤≤ 17%V
R40 Gy ≤≤≤≤ 35%V
B65 Gy ≤≤≤≤ 25%V
B40 Gy ≤≤≤≤ 50%V
FH50 Gy ≤≤≤≤ 10%V
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100%
90%
80%
70%
60%
50%
Bad plan example (axial)
The50%isodoseline fallsoutsidetherectal contour
CTV
Routine prostate IMRT plan acceptance criteria at FCCCinclude all of the following except:
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0%
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1. The volume of either femoral head receiving50 Gy ≤≤≤≤ 20%
2. The volume of bladder receiving 65 Gy ≤≤≤≤ 25%3. The 50% isodose line should fall within the rectal
contour on each individual CT slice4. The 90% isodose line should not exceed ½ the
rectal diameter on any CT slice
5. The volume of rectum receiving 65 Gy ≤≤≤≤ 17%
Regionsfor doseconstraint
Price et al. IJROBP2003
Region Limit % volume↑ li mit Mi nimum Maximum1 90% of targetgoal 20 45%of targetgoal Target Max2 80% 20 40% 90%of targetgoal3 70% 20 35% 75%4 50% 1 25% 55%5 30% 1 15% 35%6 20% 1 10% 25%
Regions
• 26 previously treatedpatients (6 and10 MV)
• Theaveragenumber ofbeamdirections decreasedby 1.62with a standarderror (S.E.)of 0.12.
• The averagetime fordelivery decreased by28.6% with a S.E.of 2.0%decreasing from 17.5to 12.3minutes
• Theamountof non-target tissuereceivingD100 decreasedby15.7% with a S.E.of2.4%