May 31, 200236th PTCOG in Catania, Italy1 Treatment Planning for Broad-Beam 3D Irradiation Heavy-Ion...
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Transcript of May 31, 200236th PTCOG in Catania, Italy1 Treatment Planning for Broad-Beam 3D Irradiation Heavy-Ion...
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
May 31, 2002 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
May 31, 2002 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.