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Measurement Based Halo Characterization for Secondary MU Check in PBS Proton Therapy Plans

L Yin1*, C Ainsley1, E Garver1, L Dong1, (1) University of Pennsylvania, Philadelphia, PA


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

Room: AAPM ePoster Library

Purpose: this study we propose an independent approach to characterize the secondary Gaussians for calculating lateral dose deposition for secondary MU check of proton pencil beam scanning (PBS) treatment plans.

Methods: verification of treatment planning system (TPS) dose calculations, pencil beam convolution superposition algorithm was employed to calculate point dose in patient QA plans and compared to the TPS calculations. Lateral dose profile from each proton pencil beam was modeled with two Gaussian distributions. Dose deposition from multiple Coulomb scatter was approximated with a primary Gaussian distribution first with its sigma analytically calculated from Highland formula. Dose contributed by in-air tails of spot profile and nuclear interactions in the water was modeled approximately as a secondary Gaussian distribution (Halo). Sigma and weight of secondary Gaussian were derived from the scatter effect in field size factor measurements using uniformly spaced, mono-energetic proton beams at various energy and depths.

Results: models were characterized with the proposed method for all four treatment rooms. Based on these models, an in-house secondary MU check program was implemented in the clinic. A retrospective study was carried out to compare point doses calculated from the in-house MU check program versus TPS calculations and patient QA measurements for 1785 treatment fields in 474 patients treated in the last 12 months. An excellent agreement was found between in-house program and TPS calculations with average difference 0.20% ± 1.83%.

Conclusion: proposed and validated an effective approach for secondary Gaussian characterization which can be incorporated for secondary MU check of proton PBS plans.


Not Applicable / None Entered.


TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-deterministic

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