Out-of-field Dose for Scanned and Scattered Proton Beams

摘要

The dose delivered outside of the primary field to non-target tissues is of concern in all types of radiotherapy.For a given radiation type, this dose is dependent upon the delivery technique. Proton beams may be spread laterally tocover the target volume using scattering foils or magnetic scanning. Although scattered beams always use apertures ormulti-leaf collimators to limit the radiation to normal tissues, scanning beams may or may not use them. A direct com-parison of out-of-field dose for different proton delivery techniques has been thwarted because no institution had, as ofApril 2008, used multiple delivery techniques and comparisons have had to make extrapolations from measurements orcalculations made using different field sizes, ranges, modulations, collimator-to-skin distances, and collimator materials.In this study, the Monte Carlo program MCNPX was used to simulate the dose deposited outside of the primary field forscattered/collimated (STC), scanned/collimated (SNC), and scanned/uncollimated (SNU) beams that have the same fieldsize and penetration depth within a simulated patient. The out-of-field doses ranged from 10-2 to 10-7 of the prescribeddose. The ratio of STC to SNU dose varied from 2 to 420 depending upon the depth and distance off-axis while theSNC to SNU dose varied from 1.2 to 140. As the out-of-field doses are very small for all beam delivery techniques,decisions regarding which technique is appropriate for a given patient or facility should be based upon the magnitude ofthe out-of-field dose per unit prescribed dose and other issues rather than the ratio of doses for the different beam deliv-ery techniques.