Uniform Scanning and Energy Stacking with Proton Beams AAPM - PowerPoint PPT Presentation

Uniform Scanning and Energy Stacking with Proton Beams AAPM Continuing Education Session 22-Jul-2010

Outline  Introduction to Technique - Moyers (15 min) » description of delivery techniques and terminology » radiobiology » lessons from scanning electron beam incidents » advantages and disadvantages of technique  Design and Implementation of Safe Delivery Systems - Anferov (15 min) » potential hazards » example hazard mitigations  Practical Aspects - Hsi (15 min) » optimization of scan and stacking patterns » multi-element detectors for measurements » scanning and stacking specific QA  Questions - All (10 min)

Scanning Terminology  scanning modes (as defined by DICOM-RT ion) » none » uniform scanning » modulated scanning  repainting  uniform scanning patterns » Lissajous » circular (single or multiple) » raster (rectilinear) » spiral » triangle

History of Uniform Scanning in the Clinic  Early  1955 » Michael Reese e patient scan » Uppsala p Lissajous beam scan 1957  1970 » Sagittaire e Lissajous beam scan » Berkeley He, Ne raster and circular beam  1985 scans  Recent  1995 » Mitsubishi p, C circular beam scan  2005 » IUCF/MPRI p raster beam scan  2007 » IBA p triangle beam scan  2011 » Mitsubishi p spiral beam scan  2011 » Sumitomo p spiral beam scan

Uniform Dose Coverage of Target in Depth Direction  energy stacking  rotating propellors  ridge filters  stepped cones

Energy Stacking Methods  direct extraction from accelerator  rangeshifter near accelerator  rangeshifter near gantry  rangeshifter in radiation head

Rangeshifter Types  binary slabs linear double wedges  circular double wedges   circular steps

Number of Requestable Energies Example Method Comments synchrotron interpolation 18,000 energies between 70 and 250 MeV synchrotron pre-programmed 256 energies (approximately 1 mm steps) cyclotron RS in SY 1 mm range steps  30 range steps cyclotron RS in head

Energy/Range Stability - During Treatment  accelerator energy stability  RS thickness stability  patient thickness stability

Energy/Range Reproducibility - Day-to-Day  accelerator energy reprod.  RS thickness reprod. note green area represents  0.18 mm

Energy Switching Time  variable energy synchrotron » could change energy multiple times during each cycle » typically only a single energy is extracted each cycle » verify energy before extraction » need to reconfigure SY  fixed energy cyclotron » move RS (  0.2 s) » verify energy before delivery » for SY RS, need to reconfigure SY » for head RS, do not need to reconfigure SY

MU Considerations  Uniform dose coverage concerns » flux non-uniformity during delivery » shifting scanning patterns » starting or stopping beam delivery in middle of scan pattern  Better dose uniformity with integral number of repaintings and larger number of repaintings.  Shallow layers use a small fraction of the total MU; difficult to repaint.  Flux rate, scan pattern, scan speed, and number of repaintings must be carefully balanced.  Typically the MU per portal is restricted to a minimum value. Das et al., (1994)

Interplay with Patient Motion  motion of beam versus motion of patient scattering » if a person walks back and sprinkler forth through a scattering  sprinkler, will get a little wet » if a person walks back and forth through a scanning sprinkler, may stay dry or may get soaked  fast uniform scanning of scanning each layer is typically sprinkler faster than respiration  but slow energy stacking may be an issue.

Radiobiology of Scanned Beams  thus far no direct comparison of uniform scanned proton beams to scattered proton beams  experiments with scanned electron beams showed RBE up to 1.29 depending upon scan pattern Meyn et al., (1991)

Lessons Learned from Previous Incidents with Scanned Electron Beams  Sagittaire example » bending magnet power supply stuck at wrong high energy (32 MeV) » energy feedback loop adjusted energy so beam would pass through energy analyzing slit in bending magnet stuck at wrong high energy » scanning magnet power supply set at correct low energy (13 MeV) » upstream dose monitor measured correct whole beam flux but fluence distribution downstream concentrated in middle of field » one patient had parallel opposed posterior cervical strip fields resulting in  800 cGy to spinal cord in one fraction » medical problems for patient within 45 minutes  Lessons for safety » energy interlocks for accidents - DAILY QA OF THE RANGE IS NOT SUFFICIENT » downstream fluence distribution detectors

Advantages and Disadvantages  advantages » uniform dose distribution for all energies and field sizes » smaller loss of range for large fields compared to scatterer technique » no need for electromechanical scatterer exchangers » higher particle use efficiency / less neutron production  disadvantages » requires additional time to switch energies » minimum MU constraint for portal » increased interplay with patient motion » requires diligent safety system

Part 2 Making Uniform Scanning safe

Safe Design Practices  If it can break – it will i.e consider all failure modes and look at the outcome  Failure Modes & Effects Analysis (FMEA) process: » Define failure modes and associated risks » Add mitigations that – Reduce probability of a failure mode – Detect failure and stop before any harm is done  Use KISS principle: » Keep It Simple Stupid !

Uniform Scanning Features 3 cm 3 Dose rate in a beam spot for average 2Gy/min to 10 1000 100 Dose Rate (Gy/min) 10 1 Double Uniform Spot Scattering Scanning Scanning 6 6 Insensitive to beam misalignment  5 5 High instantaneous dose rate  Overscan Ripple 4 4 Beam spot size can vary from 0.5 to 1.0  Overscan/ � Ripple [%] Line Spacing without perturbing uniformity 3 3 Scan pattern can be started and validated  2 2 prior to delivering dose to the patient ! 1 1 0 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 Line Spacing / Sigma

Hazard Ratings  No perceptible effect Insignificant  Small loss of performance MINOR  Loss of product function, but no damage to MODERATE user, patient, equipment.  Possible injury without irreversible damage HIGH  Possible injury with permanent damage CRITICAL  Death of user or patient Catastrophic

New Hazards due to Scanning System A. High dose rate in a beam spot can cause Critical large dose errors if scanning stops 5% dose error can accumulate in 5 msec . » » 100% dose error can accumulate in 100msec B. Non-uniform transverse dose distribution due High to errors in the scanning pattern. Accumulates over the course of the treatment . »

Sensitivity to Beam Failures Uniform Spot Beam Failures Scanning Scanning 1. Beam misalignment weak strong 2. Beam spot size error weak strong 3. Beam spot shape weak strong 4. Beam spot halo n/a moderate 5. Beam Intensity Rate weak moderate 6. Intensity Fluctuation weak strong 7. Beam Energy / per layer weak* strong * Only if using passive range modulation (ridge filters, range shifter in the nozzle)

Safety Mitigations  Start scanning verification prior to dose delivery » Apply checks that validate scan profile, scan amplitudes and scan accuracy.  Perform scanning system health validation at a fast rate (~1kHz) and interlock beam delivery. » Redundant hardware checking mitigates critical hazard of burning a hole through the target.  Monitor Field Flatness, Size and Symmetry throughout the treatment using segmented ion chamber. » This check validates accuracy of the dose delivery process.

Scanning System Health Checks Hardware Health checks Failure Mode Hazard Every 1 ms PS output change indicate beam Generator or A spot motion 1 cm or more Power Supply Every 1 ms PS output is within tolerance from Power Supply B Generator errors Every 0.1 ms must receive a trigger pulse Generator A indicating Generator updated its output Measure waveform parameters: Min and Max Generator, B values of Currents, Voltages, Frequencies waveform Waveform stability: waveform parameters do Generator, B not change during treatment Magnet Magnet health: analog circuit monitors Magnet A,B voltage from the pickup coils.

Scanning is only part of the picture Dose delivery system  Treatment energy setup validate beam penetration range  Lateral Beam spreading validate scanning safety  Dose modulation in depth validate ridge filters / range shifters  Dose conformation to target validate collimator & bolus  Measure the dose Redundant dose counters, MUs agreement Safety Checks

Safety Summary  Compared to a Double scattering system Uniform scanning adds two new hazards: » Stopped scanning » Incorrectly executed scanning  With dedicated safety electronics monitoring health of the scanning system uniform scanning can be safe and robust alternative to both double scattering and pencil beam scanning

Part 3 Practical aspects – utilize uniform scanning & discrete energy stacking protons for treatments Maglev train at China with maximum speed of 431 km/h (268 mph)