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Quantifying Lateral Penumbra with a Prototype Aperture for Pencil Beam Scanning Proton Beam Delivery

S. Ewart*1, S. Zimmerman1, E. Lee1,2, J. Speth1,2, A. Mascia1,2,(1) University of Cincinnati Department of Radiation Oncology, Cincinnati, OH, (2) Cincinnati Children's Hospital Medical Center, Cincinnati, OH


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Pencil Beam Scanning (PBS) proton therapy delivers superior dose distributions with increased flexibility compared to conventional proton therapy. However, this improved dose distribution may include the added cost of poorer lateral penumbra due in part to lack of patient-specific collimation, which is part of conventional proton therapy delivery systems. For this purpose, a prototype aperture has been developed. Assessing the usability and dosimetric characteristics of this prototype would inform future developments and potentially improve dose distributions, particularly for tumors near critical structures.

Methods: The proton therapy system is composed of a 250MeV cyclotron, beamline, PBS-only nozzle and 360-degree gantry. The prototype collimation system is composed of a brass aperture, mounted in a 2cm range shifter applicator. In the treatment planning software, PBS uniform fields for a given field size were created. At three depths, three types of fields were compared: open, collimated, and trimmed. These fields were measured, in middle of SOBP, in solid water at depths of 5 cm, 10 cm, and 15 cm at 300cGy using EBT3 gafchromic film. Field flatness, penumbra (80-20%) and field size (50-50%) were compared.

Results: The field sizes for each fields were deliberately matched for the sake of comparison. The lateral penumbra of the open, collimated, and trimmed fields at 5 cm depth are 9.1 mm, 3.0 mm and 3.4 mm, respectively. At 10 cm depth, the lateral penumbra of the open and trimmed fields are 8.6 mm and 4.4 mm; at 15 cm depth, the lateral penumbra are 9.4 mm and 6.2 mm respectively.

Conclusion: A PBS collimation system demonstrated sharpen penumbra, which could positively impact clinical dose distributions, particularly for tumors abutting critical structures. Further study is warranted on design, usability, and robustness of collimated PBS fields.


Penumbra, Collimation, Protons


TH- External Beam- Particle/high LET therapy: Proton therapy – Development (new technology and techniques)

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