IMRT workflow. Optimization and Inverse planning. Intensity distribution IMRT IMRT. Dose optimization for IMRT. Bram van Asselen

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1 IMRT workflow Otmzaton and Inverse lannng 69 Gy Bram van Asselen IMRT Intensty dstrbuton Webb 003: IMRT s the delvery of radaton to the atent va felds that have non-unform radaton fluence Purose: Fnd a set of modulated fluences that roduce a requred dose dstrbuton How do you get the fluence/ntensty dstrbuton? Forward Inverse Webb 003 IMRT Dose otmzaton for IMRT Forward aroach to dose otmzaton:. Derve a number of segments based on atent anatomy n beams-eye vew. Otmze the weght of these segments to acheve a requred dose dstrbuton Webb 003

2 MLC technque Beaumont Inverse lannng rocess Defnton of geometrcal arameters Selecton of beams Secfcaton of organ arameter Otmzaton Evaluaton Delverable lan Kestn et al, IJROB 48, , 000 Dose otmzaton for IMRT Ray tracng Inverse aroach to dose otmzaton:. Decomose a feld nto small fluence elements (bxels) and otmze ther weghts Dose dstrbuton Fluence ma Source From each fluence bxel a ray s traced through some voxels n the volume Dose otmzaton for IMRT Why nverse lannng? Inverse aroach to dose otmzaton:. The fluence s rearranged nto a seres of oen felds that aroxmate the otmzed fluence as closely as ossble Too many ossbltes to exlore smle 0x0 feld: 00 bxels of cm ) gantry, collmator, table orentatons Segments Fluence lttle chance of arrvng at otmum treatment by traland-error

3 Otmzaton What consttutes a good lan? Summarze the qualty of a lan n a sngle number: the cost functon Try to fnd the treatment lan wth the lowest value of the cost functon Verfy, f ths ndeed reresents the best lan How do we recognze f an otmal / accetable lan s found, what crtera secfy t s qualty? (.e. ts dstance from the otmum lan) Dose rescrton dose dstrbuton DVH bologcal arameters Physcal lmtatons, delverablty Comlexty of lan Tme Mathematcal framework: Beamlets Beamlets Beamlet : d (r), w D( r ) w d ( r ) w d ( r ) D( r ) w d ( r ) w d ( r ) 3 3 Beamlet : d (r), w Dose voxels Beamlet : d (r), w Beamlet : d (r), w Beamlet 3: d 3 (r), w 3 Beamlet 3: d 3 (r), w 3 r r D ( r) d ( r) w Mathematcal framework What s Inverse lannng? A dose dstrbuton D A set of weghts w for each of the fluence bxels M reresents the fractonal dose er unt beamweght (the dose model) The dose dstrbuton s lnked to the fluence weghts by D M w Assume D s the desred rescrbed dose: D M w Invert M to fnd the soluton for the bxel weghts w : " " D w M D M Problems: M - can t always be found w may have negatve elements, because of scatter contrbuton Recever Sender 3

4 Soluton What s a cost functon? Assume D s the desred rescrbed dose and w the requred bxel weghts : D M w A costfuncton s a mathematcal evaluaton of the treatment dose dstrbuton (wrt the desred dose dstrbuton) Costfunct on f D D What we want s to derve D b the best achevable dose and w b corresondng beamlet weghts: D M b w b How do we know D b s good enough comared wth D? Ideally, s should nclude all of our knowledge of radotheray: hyscal as well as bologcal dosmetrc requerements C-M Charle Ma, Fox Chase Cancer Center USA Cost functons Costfuncton - examle In rncle the cost functon C can have any form. Convergence s only guaranteed f C has only one mnmum If the cost functon sn t well behaved, the otmzaton may become nstable In each ont the dfference between calculated dose D and rescrbed dose D s weghted by a cost functon C D D D D C( w ) Target D M w Crtcal organ M w D M w D C( w ) Target Crtcal organ Otmzaton of the treatment lan Smle workflow teratve otmzaton Many dfferent costfunctons Many ways to otmze the treatment lan for a gven objectve functon Each lannng system has one or more otmzaton algorthms. Start wth emty beams. Offer grans of beam weght to each beam element n random order 3. cost s calculated 4. when cost s lower acceted, else rejected 4

5 Inverse otmzaton methods Gradent decent method Comuter smulated annealng Fltered back-rojecton Iteratve methods based on gradent descent algorthms Standard n commercal lannng systems Mnmum of the cost functon: best achevable dose dstrbuton Gradent descent algorthm Dose otmzaton The gradent of C s C( w) D D M w D C( w ) Target D D Target: 70 Gy OAR: 30 Gy modulate ntensty for sarng OAR Smlest form s: w k w k Target M D D k Dose otmzaton Dose otmzaton Two beams: modulate ntensty for sarng OAR modulate ntensty to mrove homogenety NB: AP s not fully blocked n center 5

6 Conventonal otmzaton rocess Qualty of dose model n otmzaton Fluence otmzaton Create leaf sequence Create delverable ntenstes Delverable dose calculaton Delverable lan No leaf ostons Leaf sequencer Accurate dose algorthm How to reconcle a fast and stable otmzaton wth accuracy? Use smle dose algorthm n otmzaton Recalculate wth correct model What f fnal dose calculaton shows an unaccetable lan? Drect aerture otmzaton Qualty of dose model n otmzaton Adjust fluence Intal Fluence Create leaf sequence Create delverable ntenstes Dose calculaton Evaluate lan objectve OK Leaf sequencer Soluton (RaySearch module n Pnnacle and Masterlan) Use smle dose algorthm n otmzaton Calculate after N teratons (tycally 5 to 0) dose wth accurate dose model Save the fluence Subtract the dose from the rescrton Otmze remanng fluence to match dose dfference wth smle model Otmzed fluence Qualty of dose model n otmzaton Remarks Soluton (CMS Monaco) Use reasonable dose algorthm n otmzaton Recalculate the otmal lan wth accurate dose model Allow for or otmzaton stes to mrove the lan, usng the accurate dose model Dscretzaton Smooth beams 6

7 Dscretzaton Dscretzaton The radaton oncologst draws ths contour But the otmzaton rogram sees ths: Smooth beams Smooth beams The ntensty dstrbuton tend to have local fluctuaton, more modulaton results n more comlex delvery Increase of delvery tme More MU needed More leaf travel More suscetble to treatment uncertantes Summary The dfference between actual and rescrbed dose s rojected onto the fluence ma A costfuncton s used to evaluated the otmzed dose dstrbuton relatve to the desred dose dstrbuton The costfuncton s used to otmze the dose dstrbuton Drect aerture otmzaton ncludes machne arameters n the otmzaton 7