SU-F-T-161: The Effect of Pencil Beam Scanning Gantry Angle Dependent Spot Size Variations On Plan Quality

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.

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An adaptive spot placement method on Cartesian grid for pencil beam scanning proton therapy

Physics in Medicine and Biology ◽

10.1088/1361-6560/ac3b65 ◽

2021 ◽

Author(s):

Bowen Lin ◽

Shujun Fu ◽

Yuting Lin ◽

Ronny Rotondo ◽

Weizhang Huang ◽

...

Keyword(s):

Adaptive Sampling ◽

Organs At Risk ◽

Cartesian Grid ◽

Pencil Beam ◽

Plan Quality ◽

Beam Scanning ◽

Fine Grid ◽

Placement Method ◽

Pencil Beam Scanning ◽

Delivery Efficiency

Abstract Pencil beam scanning (PBS) proton radiotherapy (RT) offers flexible proton spot placement near treatment targets for delivering tumoricidal radiation dose to tumor targets while sparing organs-at-risk (OAR). Currently the spot placement is mostly based on a non-adaptive sampling (NS) strategy on a Cartesian grid. However, the spot density or spacing during NS is a constant for the Cartesian grid that is independent of the geometry of tumor targets, and thus can be suboptimal in terms of plan quality (e.g., target dose conformality) and delivery efficiency (e.g., number of spots). This work develops an adaptive sampling (AS) spot placement method on the Cartesian grid that fully accounts for the geometry of tumor targets. Compared with NS, AS places (1) a relatively fine grid of spots at the boundary of tumor targets to account for the geometry of tumor targets and treatment uncertainties (setup and range uncertainty) for improving dose conformality, and (2) a relatively coarse grid of spots in the interior of tumor targets to reduce the number of spots for improving delivery efficiency and robustness to the minimum-minitor-unit (MMU) constraint. The results demonstrate that (1) AS achieved comparable plan quality with NS for regular MMU and substantially improved plan quality from NS for large MMU, using merely about 10% of spots from NS, where AS was derived from the same Cartesian grid as NS; (2) on the other hand, with similar number of spots, AS had better plan quality than NS consistently for regular and large MMU.

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Impact of errors in spot size and spot position in robustly optimized pencil beam scanning proton‐based stereotactic body radiation therapy (SBRT) lung plans

Journal of Applied Clinical Medical Physics ◽

10.1002/acm2.13293 ◽

2021 ◽

Author(s):

Suresh Rana ◽

Anatoly B. Rosenfeld

Keyword(s):

Radiation Therapy ◽

Stereotactic Body Radiation Therapy ◽

Spot Size ◽

Pencil Beam ◽

Beam Scanning ◽

Stereotactic Body Radiation ◽

Pencil Beam Scanning

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Applications of various range shifters for proton pencil beam scanning radiotherapy

Radiation Oncology ◽

10.1186/s13014-021-01873-8 ◽

2021 ◽

Vol 16 (1) ◽

Author(s):

Haibo Lin ◽

Chengyu Shi ◽

Sheng Huang ◽

Jiajian Shen ◽

Minglei Kang ◽

...

Keyword(s):

Head And Neck ◽

Spot Size ◽

The Body ◽

Planning System ◽

Treatment Planning System ◽

Pencil Beam ◽

Beam Scanning ◽

Air Gaps ◽

Ion Chamber ◽

Pencil Beam Scanning

Abstract Background A range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors. Methods Three UPRS made by QFix (Avondale, PA, USA) allow treating targets across the body: U-shaped bolus (UB), anterior lateral bolus (ALB), and couch top bolus. Head-and-neck (HN) patients who used the UPRS were tested. The in-air spot sizes were measured and compared in this study at air gaps: 6 cm, 16 cm, and 26 cm. Measurements were performed in a solid water phantom using a single-field optimization pencil beam scanning field with the ALB placed at 0, 10, and 20 cm air gaps. The two-dimensional dose maps at the middle of the spread-out Bragg peak were measured using ion chamber array MatriXX PT (IBA-Dosimetry, Schwarzenbruck, Germany) located at isocenter and compared with the treatment planning system. Results A UPRS can be consistently placed close to the patient and maintains a relatively small spot size resulting in improved dose distributions. However, when a UPRS is non-removable (e.g. thick couch top), the quality of volumetric imaging is degraded due to their high Z material construction, hindering the value of Image-Guided Radiation Therapy (IGRT). Limitations of using UPRS with small air gaps include reduced couch weight limit, potential collision with patient or immobilization devices, and challenges using non-coplanar fields with certain UPRS. Our experience showed the combination of a U-shaped bolus exclusively for an HN target and an MRS as the complimentary device for head-and-neck targets as well as for all other treatment sites may be ideal to preserve the dosimetric advantages of pencil beam scanning proton treatments across the body. Conclusion We have described how to implement UPRS and MRS for various clinical indications using the PBS technique, and comprehensively reviewed the advantage and disadvantages of UPRS and MRS. We recommend the removable UB only to be employed for the brain and HN treatments while an automated MRS is used for all proton beams that require RS but not convenient or feasible to use UB.

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