May 31, 2002 36th PTCOG in Catania, Italy 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.

May 31, 2002 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…

For a spherical case,

1/3 of treated volume is out of the target.

beam

target

May 31, 2002 36th PTCOG in Catania, Italy 3

Idea for Variable SOBP

• The Layer-Stacking Irradiation MethodKanai et al., Med. Phys. 10, 344-346 (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, 2002 36th PTCOG in Catania, Italy 4

Layer-Stacking Irradiation System

Range Shifter and MLC synchronously controlled with delivered dose

May 31, 2002 36th PTCOG in Catania, Italy 5

Retention of Wobbling/Scattering Relationship for Uniform Field

range shifterfluence

wobblingto keepuniform field

instantaneousbeam size

May 31, 2002 36th PTCOG in Catania, Italy 6

Device Monitor/Control System

Monitorcounter

WobblerMagnets

RangeShifter

Multi-leafcollimator

last slice

ÅE

ÅE

ÅE

ÅE

ÅE

2nd slice

1st slice

Comparator Interlock

SEMMonitorcounter

last slice

ÅE

ÅE

ÅE

ÅE

ÅE

2nd slice

1st slice

SEM>Slice Count

beam on only when all devices are ready

“move”

“status”

May 31, 2002 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)

May 31, 2002 36th PTCOG in Catania, Italy 8

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 = 0.1irrelevant to prescribed dose or fractionation...

– Empirical clinical factor C=1.43 for continuity from n-therapy

€

RBE=C ⋅Dγ ⋅2 β

2

α2− 4 β

2lnS − α

For reasonable, practical, and traceable dose scale specific to HIMAC

May 31, 2002 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

May 31, 2002 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

May 31, 2002 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

May 31, 2002 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.

May 31, 2002 36th PTCOG in Catania, Italy 13

Dose Calculation

electron density dist.

beam dir/poscompensator

ray-tracing calc.

depth dist.

broad-beam model

dose dist.

MLCrange shifter

ray-tracing only once

accumulate stepwise calculation results

typically 1-2 min/beam

May 31, 2002 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.)

May 31, 2002 36th PTCOG in Catania, Italy 15

Verification of Variable SOBP

• Example:– T-shaped target

in water phantom– prescribing 2 GyE

– Physical dose

solid: calculated

circles: measured

May 31, 2002 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

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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

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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.