Abstract
The purpose of this study was to evaluate the use of diverging‐cut aperture to minimize collimator contamination in proton therapy. Two sets of apertures with nondivergent and divergent edge were fabricated to produce a 10 cm×10 cm field at the radiation isocenter of a single‐room proton therapy unit. Transverse profiles were acquired in a scanning water tank with both aperture sets. Up to 9.5% extra dose was observed from aperture scattering near the field edges with the nondivergent aperture set at 2 cm above the water surface and remained 3.0% at depth of 10 cm. For the divergent set, the contamination was reduced to less than 3.5% and 1.3%, respectively. Our study demonstrated that scattering from apertures contaminated the dose distribution near the field edge at shallow depth. A diverging‐cut aperture was capable of reducing the contamination and is recommended for use in passive scattering proton therapy, especially when critical organs are lateral and proximal to the target at shallow depth.PACS numbers: 87.55.ne, 87.56.nk
Highlights
Aperture in proton therapy is modeled as an infinitesimally thin layer of beam stopper, the core algorithms of all commercially available treatment planning systems (TPS) has evolved from ray-tracing to pencil beam, which are more physically meaningful.[1]. It remains a convention to mill an aperture with nondiverging cut on the inner surface that encompasses the open field
One of the consequences from such a simplification is that it introduces regions where protons interacting with the aperture are not included in the TPS does calculation at all,(2,3) even though there is a nontrivial probability of these protons reentering patients
The full thickness of brass was provided by a group of slabs to reduce the weight of a single slab of aperture, they were treated as an unintegrated set in this study since all slabs shared the same divergence on the inner surfaces
Summary
Aperture in proton therapy is modeled as an infinitesimally thin layer of beam stopper, the core algorithms of all commercially available treatment planning systems (TPS) has evolved from ray-tracing to pencil beam, which are more physically meaningful.[1]. One of the consequences from such a simplification is that it introduces regions where protons interacting with the aperture are not included in the TPS does calculation at all,(2,3) even though there is a nontrivial probability of these protons reentering patients These edge-scattered protons from aperture add unexpected horns on beam profiles, perturbing dose distribution, increasing skin dose, and compromising the plan conformality. This perturbation in dose distribution is regarded as a contamination because of the deviation from the primary protons in terms of energy and direction of motion. The magnitude of this contamination has been underestimated or ignored clinically, as no commercial TPS takes it into consideration
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