Abstract
PurposeThe 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 MethodsThe 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.ResultsCollimation 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.ConclusionThe 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.
Highlights
The field of proton therapy has grown rapidly since its initial proposal, fueled by the prospects of increased local tumor control and reduction of treatment toxicities in comparison to conventional photon-based treatment modalities
Pencil beam scanning offers 2 clinical advantages over passive scattering proton beam therapy: pencil beam scanning (PBS) can significantly spare healthy tissues proximal to the planning target volume (PTV) owing to its ability to create variable width spread-out Bragg peaks across the target; and with the addition of computer-aided beamlet weight optimization, PBS gives rise to intensity-modulated proton beam therapy (IMPT), which is analogous to IMRT in external photon beam therapy
The introduction of PBS was thought to largely mitigate the need for external apertures [1,2,3, 6]. Both computational and clinical trial studies have recently emerged that paint an alternative picture: while proton therapy, in a theoretical sense, can provide superior healthy tissue sparing and increased target conformity over photon-based modalities, the conditions necessary to observe a significant clinical benefit are seldom achieved with PBS alone due in part to treatment planning uncertainties and increased lateral dose spread [7,8,9,10]
Summary
The field of proton therapy has grown rapidly since its initial proposal, fueled by the prospects of increased local tumor control and reduction of treatment toxicities in comparison to conventional photon-based treatment modalities. These advantages allow PBS to shape a dose distribution to closely match the target outline while minimizing the neutron production relative to its passive scattering counterpart [2,3,4,5] For these reasons, the introduction of PBS was thought to largely mitigate the need for external apertures [1,2,3, 6]. The introduction of PBS was thought to largely mitigate the need for external apertures [1,2,3, 6] Both computational and clinical trial studies have recently emerged that paint an alternative picture: while proton therapy, in a theoretical sense, can provide superior healthy tissue sparing and increased target conformity over photon-based modalities, the conditions necessary to observe a significant clinical benefit are seldom achieved with PBS alone due in part to treatment planning uncertainties and increased lateral dose spread [7,8,9,10]. The clinical importance of these parameters is not well understood, and the overall benefit of proton beam therapy is currently being tested in numerous clinical trials for various disease sites
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