Innovations and the Use of Collimators in the Delivery of Pencil Beam Scanning Proton Therapy

Abstract

Purpose The development of collimating technologies has become a recent focus in pencil beam scanning (PBS) proton therapy to improve the target conformity and healthy tissue sparing through field-specific or energy-layer–specific collimation. Given the growing popularity of collimators for low-energy treatments, the purpose of this work was to summarize the recent literature that has focused on the efficacy of collimators for PBS and highlight the development of clinical and preclinical collimators. Materials and Methods The collimators presented in this work were organized into 3 categories: per-field apertures, multileaf collimators (MLCs), and sliding-bar collimators. For each case, the system design and planning methodologies are summarized and intercompared from their existing literature. Energy-specific collimation is still a new paradigm in PBS and the 2 specific collimators tailored toward PBS are presented including the dynamic collimation system (DCS) and the Mevion Adaptive Aperture. Results Collimation during PBS can improve the target conformity and associated healthy tissue and critical structure avoidance. Between energy-specific collimators and static apertures, static apertures have the poorest dose conformity owing to collimating only the largest projection of a target in the beam's eye view but still provide an improvement over uncollimated treatments. While an external collimator increases secondary neutron production, the benefit of collimating the primary beam appears to outweigh the risk. The greatest benefit has been observed for low- energy treatment sites. Conclusion The consensus from current literature supports the use of external collimators in PBS under certain conditions, namely low-energy treatments or where the nominal spot size is large. While many recent studies paint a supportive picture, it is also important to understand the limitations of collimation in PBS that are specific to each collimator type. The emergence and paradigm of energy-specific collimation holds many promises for PBS proton therapy.

Technical note: Evaluation and second check of a commercial Monte Carlo dose engine for small‐field apertures in pencil beam scanning proton therapy

“…The enhanced MCsquare did not compare as favorably to the water measurements in terms of absolute dose for the smallest apertures; however the enhanced MCsquare results were very accurate for small apertures in terms of spatial dose distributions (film). Examining the patient-geometry results, the only significant discrepancy found between RayStation MC and MCsquare was for patient 5, which had a CTV of only 0.41 cm 3 (1.8 cm effective aperture opening diameter), an extremely challenging case.…”

“…When range shifters are used to treat shallow tumors, the beamlets are further scattered. Consequently, the lateral dose fall‐off is often inadequate for shallow and complex tumors 1–3 and can increase the possibility of complications such as brain necrosis 2 or the damaging of salivary glands 4…”

“…When range shifters are used to treat shallow tumors, the beamlets are further scattered. Consequently, the lateral dose fall-off is often inadequate for shallow and complex tumors [1][2][3] and can increase the possibility of complications such as brain necrosis 2 or the damaging of salivary glands. 4 Ideally, for pencil beam scanning, the proton beam acceleration and transport technologies would be improved with an emphasis on reducing spot sizes; however, reducing spot sizes at the level of the accelerator and transport technologies may not be a cost-effective approach.…”

Collimating individual beamlets in pencil beam scanning proton therapy, a dosimetric investigation

“…In general, there is always a cost associated with increasing the lateral dose resolution. Each approach has advantages and disadvantages (30). One such approach is to use a patientspecific aperture.…”

A novel optimization algorithm for enabling dynamically collimated proton arc therapy

“…External collimation has been shown as an effective technique to improve dose conformity for low-energy (less than approximately 160MeV) pencil beam scanning (PBS) proton therapy treatments, and it is expected to play an increasing role in the treatment of brain and head and neck cancers 7 . Low-energy beams have larger lateral spot sizes that increase the lateral penumbra, especially for the cases where external range shifters are necessary to treat superficial targets.…”