Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy

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.