The MSKCC Approach to IMRT Spiridon V. Spirou, PhD Department of Medical Physics Memorial Sloan-Kettering Cancer Center New York, NY Outline Optimization Field splitting Delivery Independent verification for IMRT Examples Prostate Nasopharynx Whole Abdomen 1
Optimization Quadratic objective function Gradient minimization methods Smoothing Multiple loops Other features: Optimization over a pre-existing distribution The Objective Function N pts ( Di Dpresc ) 2 i= 1 N pts 1 F = + w Di D 2 target min ( min ) N pts i= 1 N pts + w Di D 2 max ( max ) i= 1 Prescription dose Homogeneity terms included only if constraint violated F organ = max dose constraint + dv constraint Volume V dv D dv Dose 2
Smoothing of Intensity Profiles Local fluctuations in intensity profile due to numerical artifacts. Little or no dosimetric advantage. Difficult to deliver, requires longer beam-on time. Susceptible to treatment uncertainties. without smoothing Smoothing of Intensity Profiles Savitzky-Golay (least squares fit) before smoothing after smoothing 3
Smoothing included as part of the Objective Function Fobj = wi i target + wi i OAR + w j j beam ( D D ) i 2 ( D D ) i 2 ( x x ) ' j p c j 2 For over & under dose in the targets For over dose in critical organs, may include dose-volume conditions For smoothness in intensity profile x j : value before smoothing x j : value after smoothing Comparison of Smoothing Approaches Smoothing outside the objective function Gentler gradients, fewer MU. Cannot distinguish between noise and dosimetrically relevant fluctuations. Smoothing within the objective function Dosimetric effect of smoothing taken into account: heavier smoothing for noise, lighter for dosimetrically relevant fluctuations. Higher profile modulation, more MU 4
Example 1: Phantom OAR 5 Smoothing at the end of each iteration Smoothing within the objective function Example 2: Paraspinal ~ 5mm separation Cord Goal: Given that 95% of the must receive the prescription dose, what is the best Cord protection that can be achieved? 5
Paraspinal 90 40 30 Cord Smoothing at the end of each iteration Smoothing within the objective function Paraspinal 120 Smoothing within the objective function Smoothing at the end of each iteration Volume (%) 80 60 40 95 90 85 70 80 90 110 20 Cord 0 0 20 40 60 80 120 140 Dose (%) 6
Multiple loops More accurate incorporation of scattered dose in optimization For efficiency, scattered dose is only partially accounted for in optimization. Discrepancy with full-scatter dose calculation. Repeated cycles of optimizations and full-scatter dose calculations required to achieve optimal results. Volume (%) 80 60 40 20 Cord 0 0 20 40 60 80 120 Dose (%) Optimization Full scatter dose calc. More accurate incorporation of scattered dose in optimization: Iterative correction process Correction factor C i = 0 Optimization based on partial scattered dose calculation P i Intensity profiles Total dose D i = P i + C i Update C i = C i + δ i N δ i = F i -D i δ i < tolerance? F i calculated with full scattered dose Y done 7
More accurate incorporation of scattered dose in optimization: Iterative correction process Volume (%) 80 60 40 Cord Volume (%) 80 60 40 Cord 20 20 0 0 20 40 60 80 120 Dose (%) No correction 0 0 20 40 60 80 120 Dose (%) With correction Optimization Full scatter dose calc. Splitting Large IM Fields Considerations for automatic splitting Smooth subfields - no discontinuities. Split along a straight line or at a region of low intensity. Use feathering to reduce the effects of positioning uncertainties. Sub-fields should be designed to minimize beam-on time (MU). 8
Splitting Large IM Fields Example: nasopharynx PA field Before splitting After splitting Splitting Large IM Fields Example: nasopharynx PA field Leaf Pair 18 15 15 Intensity (MU) 10 5 Intensity (MU) 10 5 0-4 -2 0 2 4 X (cm) 0-4 -2 0 2 4 X (cm) Before splitting After splitting 9
Leaf Sequencer DMLC - Sliding window Accounts for: Mechanical limitations (e.g. leaf speed) Direct exposure Transmission through the leaves Rounded leaf-ends Scatter source Intensity under the leaves beam I(x) 1 P 1 : direct exposure P 2 : leaf rounded end P 3 : leaf transmission ε P 1 P 2 P 3 x 10
Variation of Output with Field Size Backprojection to the source plane Primary Source Scatter Source Source plane left leaf right leaf MLC opening MLC plane P P Isocenter plane Iterative process to generate leaf paths Desired fluence profile F(x) Assign F work (x) = F(x) Calculate leaf paths using F work (x) Modify working profile F work (x) = F work (x) - E(x) Calculate generated profile F g (x) taking all factors into account Calculate error E(x) = F g (x) - F(x) N error acceptable? Y finish 11
Independent verification for IMRT (a regulatory requirement) A separate program Input: Leaf sequencing file Beam-on time Jaw settings Beam data Factors accounted for: Rounded leaf ends Leaf transmission Tongue-and-groove Scatter source MLC scatter Output: Dose to points or planes in phantom 10 20 30 40 50 60 70 80 Calculation Measurement Prostate Similar anatomically and geometrically Prescription dose: 8640 or 8 cgy Fixed beam angles - 5 fields Template of optimization parameters and constraints 12
Prostate 8 cgy 109 90 70 50 Rectum Volume (%) 80 60 40 Rectum Bladder 20 0 0 20 40 60 80 120 Dose (%) Nasopharynx Prescription dose: 5400 cgy to microscopic disease 7000 cgy to gross disease Fixed beam angles - 7 fields 9 or 10 fields after splitting Template of optimization parameters and constraints 13
Nasopharynx 7000 cgy 80 Volume (%) 60 40 20 54 70 Bstem Cord R. parotid 0 0 2000 4000 6000 8000 Dose (cgy) 8400 7000 5400 4500 3500 70 54 Cord Parotid Whole Abdomen Conventional treatment: AP/PA, 6MV, extended distance Prescription dose: 3000 cgy Blocks to keep kidneys at 1800 cgy IMRT 5 fields, 15MV, isocentric Large fields: 2 isocenters, split fields in 3 Large volumes: little scatter accounted for in optimization (use multiple loops ) 14
Whole Abdomen 325 Ant 35 255 105 x Abdominal ISO Lt PA 15 MV 180 x Pelvic ISO Abdominal PA field Superior Left kidney Right kidney Right Junction 15
Abdominal PA Field: Beam Splitting Middle Part Left Part Right Part Whole Abdomen: Sagittal plane 115 110 90 70 50 Abdominal ISO Homogeneous Dose Distribution in Junction Pelvic ISO 16
Whole Abdomen: Coronal plane 115 110 90 70 50 Abdominal ISO Homogeneous Dose Distribution in Junction Pelvic ISO Whole Abdomen Pelvic bones IMRT Conventional 17
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