Treatment Planning for BroadBeam 3D Irradiation HeavyIon Radiotherapy

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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.

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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
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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
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Layer-Stacking Irradiation System
Range Shifter and MLC synchronously controlled
with delivered dose
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Retention of Wobbling/Scattering Relationship for
Uniform Field
fluence
range shifter
wobbling to keep uniform field
instantaneous beam size
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Device Monitor/Control System
beam on only when all devices are ready
move
status
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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)

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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 C1.43 for continuity
  from n-therapy

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

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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

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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

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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.

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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
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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.)

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Verification of Variable SOBP

• Example
• T-shaped targetin water phantom
• prescribing 2 GyE
• Physical dose
• solid calculated
• circles measured

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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 lt 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.