May 31, th PTCOG in Catania, Italy1 Treatment Planning for Broad-Beam 3D Irradiation Heavy-Ion Radiotherapy N. Kanematsu, M. Endo, and T. Kanai, Dept. of Med. Phys., NIRS H. Asakura, Accel. Eng. Corp. Y. Futami, Shizuoka Pref. H. Oka, AJS Co., Ltd. K. Yusa, Japan Sci. & Tech. Corp.

May 31, th PTCOG in Catania, Italy2 Problem of Fixed SOBP In the conventional particle therapy, –Field  projected target contour (by MLC) –Range  target distal surface (by compensator) –SOBP  max target thickness (by ridge filter) However, a target has variable thickness … For a spherical case, 1/3 of treated volume is out of the target. beam target

May 31, th PTCOG in Catania, Italy3 Idea for Variable SOBP The Layer-Stacking Irradiation Method Kanai et al., Med. Phys. 10, (1983) –Longitudinally divide the target  slices –Conform thin layer of SOBP (minipeak) to each slice …  variable SOBP beam target treated volume  target volume

May 31, th PTCOG in Catania, Italy4 Layer-Stacking Irradiation System Range Shifter and MLC synchronously controlled with delivered dose

May 31, th PTCOG in Catania, Italy5 Retention of Wobbling/Scattering Relationship for Uniform Field range shifter fluence wobbling to keep uniform field instantaneous beam size

May 31, th PTCOG in Catania, Italy6 Device Monitor/Control System beam on only when all devices are ready “ move ” “ status ”

May 31, th PTCOG in Catania, Italy7 Treatment Planning System Original system HIPLAN: –In-house RTP system for HIMAC since 1994 –Base of the planning procedures and clinical protocols System integration strategy: –Consistency with the ongoing treatments Same planning procedure Same biophysical model for C-therapy Same “ parallel broad-beam ” physical model though too primitive in the 2002 standard... –Practical performance (calculation speed, ease of use)

May 31, th PTCOG in Catania, Italy8 Biophysical Model RBE based on HSG cell responses at fixed survival level, plus rescaling for historical reason –LQ  and  parameterized as a function of LET –Dose-averaged  and  for mixed-LET beam by ridge filter –Cobalt dose D  = 4.04 Gy at survival level S = 0.1 irrelevant to prescribed dose or fractionation... –Empirical clinical factor C=1.43 for continuity from n-therapy For reasonable, practical, and traceable dose scale specific to HIMAC

May 31, th PTCOG in Catania, Italy9 Depth-Dose for Minipeak Beam Use measured data (+) for physical dose RBE by model calculation (clinical dose) = (RBE)  (physical); a scalar parameter i.e. 1 GyE + 1 GyE = 2 GyE RBE gives concurrent enhancement to the minipeak

May 31, th PTCOG in Catania, Italy10 Planning for Layer-Stacking Common to the conventional method –beam selection logic (energy, wobbler, scatterer) –range compensator design Newly integrated features –slice-by-slice range shifter setup –slice-by-slice MLC setup –step-dose optimization with RBE –stepwise dose calculations and dose accumulation

May 31, th PTCOG in Catania, Italy11 Range Shifter and MLC Setup Handled as a series of conventional irradiations Example: Range- compensated spherical 8-cm target Conform minipeak to each slice with range shifter and MLC

May 31, th PTCOG in Catania, Italy12 Step-Dose Optimization Equivalent to ridge-filter design. MLC partially blocks fragmentation tails.  dose non-uniformity. Fast iterative optimization to maximize dose uniformity in the target.

May 31, th PTCOG in Catania, Italy13 Dose Calculation electron density dist. beam dir/pos compensator ray-tracing calc. depth dist. broad-beam model dose dist. MLC range shifter ray-tracing only once accumulate stepwise calculation results typically 1-2 min/beam

May 31, th PTCOG in Catania, Italy14 Verification of RBE Consistency Both layer-stacking and conventional methods should have same RBE. Example: –cubic (8 cm) 3 target in water phantom –prescribing 1 GyE –dashed: conventional –solid: stacking (calc.) –circles: stacking (meas.)

May 31, th PTCOG in Catania, Italy15 Verification of Variable SOBP Example: –T-shaped target in water phantom –prescribing 2 GyE –Physical dose solid: calculated circles: measured

May 31, th PTCOG in Catania, Italy16 Study on Clinical Effectiveness Example: –actual patient image –tumor (yellow contour) in bone & soft tissue region Generally effective for –large target volume –single or a few ports –small organ motion layer-stacking conventional

May 31, th PTCOG in Catania, Italy17 Dose Distribution Analysis (a) CTV dose –non-uniformity < a few % –clinically little difference (b) Skin dose –100% area disappears –will reduce skin reactions solid: layer-stacking dashed: conventional

May 31, th PTCOG in Catania, Italy18 Conclusions The layer-stacking irradiation system for HIMAC is finally complete. RTP has been adapted to this method, achieving; –perfect continuity with ongoing C-therapy at HIMAC, –sufficient speed, and ease of use. This will provide an option for improved particle radiotherapy while coexisting with the conventional method on the same system. First treatment will be sometime in this summer. Obsolete parallel broad-beam model is subject to future refinement in a consistent manner.