PROTON BEAM THERAPYModel (PBT) Policies Page 2 PROTON BEAM THERAPY (PBT) radiation therapy, immobilization is critical. However, for PBT, the immobilization system can impact the dose distribution and therefore these devices must be carefully designed. Many photon-based patient immobilization devices are not appropriate for use in PBT. 2.This MContouring:odel Policy* Defining addresses the coverage target and for aPrvoidancoton Beame structures Therapy .is a multi-step process: DESCRIPTIONa. The radiation oncologist reviews the three-dimensional images and outlines the treatment target on Proton Beameach Therapy slice of(PB theT) isimage a technology set. The summationfor delivering of theseconformal contours external defines beam the radiation Gross Tumor with V positivelyolume (GT chargedV). atomic particlesFor m toultiple a well image-defined sets tr, eatmentthe physician volume may. PBT outline is approved separate by GTVs the Uon.S. each Food image and Drug set toAdministration. account for the effect of normal organ motion upon target location and shape. Some patients may not have GTVs if Due to its uniquethey ha doseve had deposition previous characteristics treatment with, PBT surgery can, inor certainchemotherapy situations, in, delivwhicher casethe prescribedtreatment targetplanning dose will while givingbe a lowbaseder dose on CT toVs normal as described tissues belowas compared. to photon-based forms of external beam radiotherapy. Photonb. beamsThe deposit radiation their oncolog greatestist drawsamount a mar of energygin around beneath the GTVthe patientto generat’s surfacee a Clinical with Tara grgetadual Volume reduction (CTV) in energy depositionwhich ealongncompasses the beam the path areas as at photons risk for passmicr oscopicthrough disease the target (i.e., and not then visible through on imaging an exit studies). point out of the body. In contrast,Other theCTVs physical may be profile creat edof baseda beam on of the proton estimat particlesed volume allows of f residualor the majority disease. of F orits multiple energy to image be sets, deposited ovtheer pha veryysician narrow may drawrange this of tissue margin at ar a ounddepth an largely aggregat determinede volume by containing the energy all of image the proton set GTVs beam. to A proton beamgenerat depositse an relatively organ-motion less radiation CTV, or Inenergyternal upon Target entering Volume the (ITV). body compared to a photon beam. The energy deposition of the proton beam then rapidly increases over a narrow range of tissue at a desired depth to producec. an Inintense X-ray dosetherapy distribution, to account pattern for unc calledertainties the Bragg in the peak. planning Beyond and thedeliv Braggery processes peak, energy, a final and margin dose is deposition rthenapidly added decrease to create, resulting a Planning in the Taabsencerget Volume of any (PTV). significant Analogous exit dose to thedeposited approach in normalused in tissueX-ray therapybeyond, the target. a lateral target expansion guards against under-dosing the target in the presence of daily setup variation and/or organ and patient motion. With PBT, however, the target expansion in the beam direction must TREATMENTalso ensure coverage for uncertainties in the range of the proton beam which may not perfectly match the radiologic depth of the target. The expansion in the beam direction may be different from the lateral PBT Treatment Planning expansion. Because the lateral and range expansions may differ for each beam, there is no longer a single PBT can allow for radiation treatment plans that are highly conformal to the target volume. PBT planning defines the PTV that is sufficient for a multi-field proton plan. Rather than prescribing a uniform dose to a PTV, in PBT necessary field sizes, gantry angles and beam energies needed to achieve the desired radiation dose distribution. the plan should be designed to cover the CTV in the presence of expected uncertainties. An assessment of patient suitability for PBT is an important step in the process of care. Changes in the density and compositiond. Near of tissuesby norma in thel stru pathctu resof thetha tbeam could have potentiall muchy gr beeat harmeer impactd by radiatioon the delivn (i.e.,ered “or gandoses atfor ris protonsk ”, o r O thanARs) are photons. Tissuealso intecontouredrfaces, .especially those with large differences in electron density, can lead to larger or unacceptable dosimetric uncertainties in PBT for certain patients. 3. Radiation Dose Prescribing: The radiation oncologist assigns specific dose coverage requirements for the CTV PBT trwhicheatment will planning be met evenis a multi in the-step presence process of and expected shares positional functions andcommon range to unc otherertainties forms. ofA externaltypical prescription beam radiotherapymay define planning: a dose that will be delivered to at least 99% of the CTV. This coverage requirement is often accompanied by a minimum acceptable point dose delivered within the CTV in the presence of expected 1. uncSimulationertainties and and Imaging:a constraint Thr eedescribing-dimensional an acceptable image acquisition range of ofdose the homogeneity target region. Additionally by simulation, PBT employing prescriptionCT, CT/ PET r and/orequirements MR scanning routinely equipment include dose is an constraints essential prforer theequisite OARs to (e PBT.g., uppertreatment limit planning of mean. doseIf , maximumrespiratory allowable or other point normal dose organ, and/or motion a critical is expected volume to of produce the OAR significant that must movement not receive of a thedose target above r egiona specifiedduring r adiotherapylimit). Doses delivto normalery, the structures radiation must oncologist also be may evaluated additionally in the elect presence to order of deliv multier-yphasic and range treatment uncplanningertainties image. A trsetseatment to account plan that for motionsatisfies when these r enderingrequirements target and volumes constraints. As in should all forms maximize of external the potential beam for disease control and minimize the risk of radiation injury to normal tissue. 4. Dosimetric Planning and Calculations: The qualified clinical medical physicist or a supervised dosimetrist *ASTRO model policies were developed as a means to efficiently communicate what ASTRO believes to be correct coverage policies Approved for radiationcalculates oncology a services. treatment The ASTRO plan Model to deliv Policieser dothe not prescribed serve as clinical radia guidelinestion doseand they to ar thee subject CTV to and periodic simultaneously review satisfyJune 2017 the and revisionnormal without tissue notice dose. The ASTROconstraints Model Policies by delivering may be reproduced significantly and distributed lower, without doses modification, to nearby for organs noncommercial. Delivery mechanisms purposesvary. , but through the use of scanning magnets or scattering devices PBT plans spread protons laterally over CPT Copyrightthe extent 2013 American of a target Medical volume Association.. In All some rights rcaseseserved, . the lateral spread of the protons is controlled through the use of an aperture. Additionally, multiple proton energies are combined, through the use of mechanical absorbers or accelerator energy changes, to deliver the planned dose distribution over the longitudinal extent of the target. Range compensation devices are sometimes used to match the range of the proton beam to the distal edge of the PROTON BEAM THERAPY (PBT) Page 2 radiation therapy, immobilization is critical. However, for PBT, the immobilization system can impact the dose distribution and therefore these devices must be carefully designed. Many photon-based patient immobilization devices are not appropriate for use in PBT. 2. Contouring: Defining the target and avoidance structures is a multi-step process: a. The radiation oncologist reviews the three-dimensional images and outlines the treatment target on each slice of the image set. The summation of these contours defines the Gross Tumor Volume (GTV). For multiple image sets, the physician may outline separate GTVs on each image set to account for the effect of normal organ motion upon target location and shape. Some patients may not have GTVs if they have had previous treatment with surgery or chemotherapy, in which case treatment planning will be based on CTVs as described below. b. The radiation oncologist draws a margin around the GTV to generate a Clinical Target Volume (CTV) which encompasses the areas at risk for microscopic disease (i.e., not visible on imaging studies). Other CTVs may be created based on the estimated volume of residual disease. For multiple image sets, the physician may draw this margin around an aggregate volume containing all image set GTVs to generate an organ-motion CTV, or Internal Target Volume (ITV). c. In X-ray therapy, to account for uncertainties in the planning and delivery processes, a final margin is then added to create a Planning Target Volume (PTV). Analogous to the approach used in X-ray therapy, a lateral target expansion guards against under-dosing the target in the presence of daily setup variation and/or organ and patient motion. With PBT, however, the target expansion in the beam direction must also ensure coverage for uncertainties in the range of the proton beam which may not perfectly match the radiologic depth of the target. The expansion in the beam direction may be different from the lateral expansion. Because the lateral