Abstract
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This study is aimed at identifying the potential benefits of using a patientspecific
aperture in proton beam scanning. For this purpose, an accurate Monte
Carlo model of the pencil beam scanning (PBS) proton therapy (PT) treatment
head at Massachusetts General Hospital (MGH) was developed based on
an existing model of the passive double-scattering (DS) system. The Monte
Carlo code specifies the treatment head at MGH with sub-millimeter accuracy.
The code was configured based on the results of experimental measurements
performed at MGH. This model was then used to compare out-of-field doses
in simulated DS treatments and PBS treatments. For the conditions explored,
the penumbra in PBS is wider than in DS, leading to higher absorbed doses
and equivalent doses adjacent to the primary field edge. For lateral distances
greater than 10 cm from the field edge, the doses in PBS appear to be lower
than those observed for DS. We found that placing a patient-specific aperture
at nozzle exit during PBS treatments can potentially reduce doses lateral to the
primary radiation field by over an order of magnitude. In conclusion, using a
patient-specific aperture has the potential to further improve the normal tissue
sparing capabilities of PBS.