Treatment Planning for Broad-Beam 3D Irradiation Heavy-Ion Radiotherapy

1. 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. 36th PTCOG in Catania, Italy

2. 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… target For a spherical case, 1/3 of treated volume is out of the target. beam 36th PTCOG in Catania, Italy

3. treated volume target volume Idea for Variable SOBP • The Layer-Stacking Irradiation Method Kanai et al., Med. Phys. 10, 344-346 (1983) • Longitudinally divide the target  slices • Conform thin layer of SOBP (minipeak)to each slice… variable SOBP target beam 36th PTCOG in Catania, Italy

4. Layer-Stacking Irradiation System Range Shifter and MLC synchronously controlled with delivered dose 36th PTCOG in Catania, Italy

5. Retention of Wobbling/Scattering Relationship for Uniform Field fluence range shifter wobbling to keep uniform field instantaneous beam size 36th PTCOG in Catania, Italy

6. Device Monitor/Control System beam on only when all devices are ready “move” “status” 36th PTCOG in Catania, Italy

7. 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) 36th PTCOG in Catania, Italy

8. Biophysical Model • RBE based on HSG cell responses at fixed survival level, plus rescaling for historical reason • LQ a and b parameterized as a function of LET • Dose-averaged a and b for mixed-LET beam by ridge filter • Cobalt dose Dg = 4.04 Gy at survival level S = 0.1irrelevant 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 36th PTCOG in Catania, Italy

9. Depth-Dose for Minipeak Beam • Use measured data (+) for physical dose • RBE by model calculation • (clinical dose)= (RBE)  (physical);a scalar parameteri.e. 1 GyE + 1 GyE = 2 GyE • RBE gives concurrent enhancement to the minipeak 36th PTCOG in Catania, Italy

10. 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 36th PTCOG in Catania, Italy

11. 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 36th PTCOG in Catania, Italy

12. 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. 36th PTCOG in Catania, Italy

13. Dose Calculation electron density dist. MLCrange shifter accumulate stepwise calculation results ray-tracing calc. broad-beam model beam dir/pos compensator depth dist. dose dist. ray-tracing only once typically 1-2 min/beam 36th PTCOG in Catania, Italy

14. 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.) 36th PTCOG in Catania, Italy

15. Verification of Variable SOBP • Example: • T-shaped targetin water phantom • prescribing 2 GyE • Physical dose solid: calculated circles: measured 36th PTCOG in Catania, Italy

16. 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 36th PTCOG in Catania, Italy

17. 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 36th PTCOG in Catania, Italy

18. 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. 36th PTCOG in Catania, Italy