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Breathing Interplay Effects During Proton Beam Spot Scanning: Simulation And Statistical Analysis

Student Name: Robertson, Daniel
Advisor Name: Campbell, Branton
Approval Date: 8/1/2008
Report Type: Capstone
Title: Breathing Interplay Effects During Proton Beam Spot Scanning: Simulation And Statistical Analysis
Abstract: The movement from passive scattering to active scanning in proton radiation therapy introduces the problem of interplay effects when elements of beam motion are on a similar time scale as intra-fractional tumor motion. This interplay can lead to significant deviation from the planned dose within a treatment fraction. Over the course of a treatment the total prescribed dose is delivered using several fields and in multiple fractions, which may lead to a homogeneous dose distribution with the dosimetric motion effect being just a wider penumbra. To account for situations where the typical 30 fractions may not provide enough repetition to satisfactorily wash out interplay effects, each field within a fraction can be ‘repainted’. The purpose of this work was to investigate the effectiveness of different repainting strategies. To assess the dosimetric impact of motion we performed a series of simulations considering a set of clinically relevant parameters. These were tumor motion amplitude, breathing period, and breathing trajectory for the target and time required to change the beam energy for the delivery system. Several repainting methods were developed and compared for the conditions of greatest dose delivery error. Breathing motion perpendicular to the beam direction was chiefly considered, using an asymmetric sine function. The results show that the broadening of the penumbra depends only on the amplitude of tumor motion, and is responsible for narrowing the full dose region and widening the low-dose region by as much as 2 cm for large tumor amplitudes. The SOBP exhibits a uniform shift of 10%-15% of the tumor amplitude (or 1-5 mm) in the caudal direction due to breathing asymmetry. Of the five re-painting techniques studied, so-called ‘Breath Sampling Repainting’ is most effective in reducing dose errors with a minimal addition to treatment time. In this method each energy level is repainted at several evenly spaced times within one breathing period. To keep dose delivery errors below 5% while minimizing treatment time, it is recommended that Breath Sampling Repainting be employed using 7 to 10 paintings per field.
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