Abstract
Proton therapy makes use of the favorable depth-dose distribution with its characteristic Bragg peak to spare normal tissue distal of the target volume. A steep dose gradient would be desired in lateral dimensions, too. The widespread spot scanning delivery technique is based, however, on pencil-beams with in-air spot full-widths-at-half-maximum of typically 1 cm or more. This hampers the sparing of organs-at-risk if small-scale structures adjacent to the target volume are concerned. The trimming of spot scanning fields with collimating apertures constitutes a simple measure to increase the transversal dose gradient. The current study describes the clinical implementation of brass apertures in conjunction with the pencil-beam scanning delivery mode at a horizontal, clinical treatment head based on commercial hardware and software components. Furthermore, clinical cases, which comprised craniopharyngiomas, re-irradiations and ocular tumors, were evaluated. The dosimetric benefits of 31 treatment plans using apertures were compared to the corresponding plans without aperture. Furthermore, an overview of the radiation protection aspects is given. Regarding the results, robust optimization considering range and setup uncertainties was combined with apertures. The treatment plan optimizations followed a single-field uniform dose or a restricted multi-field optimization approach. Robustness evaluation was expanded to account for possible deviations of the center of the pencil-beam delivery and the mechanical center of the aperture holder. Supplementary apertures improved the conformity index on average by 15.3%. The volume of the dose gradient surrounding the PTV (evaluated between 80 and 20% dose levels) was decreased on average by 17.6%. The mean dose of the hippocampi could be reduced on average by 2.9 GyRBE. In particular cases the apertures facilitated a sparing of an organ-at-risk, e.g. the eye lens or the brainstem. For six craniopharyngioma cases the inclusion of apertures led to a reduction of the mean dose of 1.5 GyRBE (13%) for the brain and 3.1 GyRBE (16%) for the hippocampi.
Highlights
The depth dose characteristics with its distinct Bragg Peak facilitate a conformation advantage of proton beam treatment fields over hard X-ray fields
The addition of field-specific apertures to pencil-beam scanning treatment fields was successfully introduced at a proton therapy center
which is based on commercial equipment
Summary
The depth dose characteristics with its distinct Bragg Peak facilitate a conformation advantage of proton beam treatment fields over hard X-ray fields. The lateral dose fall-off limits the options to cover the target volume while keeping the dose to organs at risk low This holds true for the pencil-beam scanning delivery method (PBS), which has gained importance over passive delivery techniques with collimating apertures in the last few years. The advantage of PBS with static apertures is the possibility to realize small air gaps in clinical treatment plans, which is beneficial for the lateral dose gradient [5, 11]. Dynamic adaptive collimators, which enable small air gaps, were clinically introduced [9, 10] These dynamic collimators are necessary to compensate for wide PBS spots from a gantrymounted cyclotron. Dedicated PBS nozzles can be equipped with an extra holder for apertures
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