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A 4D Monte Carlo Study Quantifying Changes in the Interplay Effect as a Function of Treatment Delivery Parameters in Proton Beam Scanning for Lung

S Dowdell

S Dowdell1*, C Grassberger1,2, GC Sharp1, H Paganetti1, (1) Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston MA, (2) Centre for Proton Radiotherapy, Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland

TU-A-108-2 Tuesday 8:00AM - 9:55AM Room: 108

Purpose: To assess the impact of delivery parameters of a pencil beam scanning proton therapy system on the dosimetric interplay effect during lung treatments.

Methods: 4D Monte Carlo simulations were performed using TOPAS for 5 lung cancer patients with varying tumor sizes (50.4-167.1cc) and superior-inferior (SI) motion amplitudes (2.9-30.1mm). Each patient was planned based on a 4DCT scan.
The complete time structure of the proton fields was incorporated into the simulation framework by including the time required to deposit dose, scan the beam laterally, change the beam energy and the time required for magnet settling.
Changes in the 4D dose distribution resulting from altering the spot size, energy switch time, magnet settling time, initial breathing phase, spot spacing, scanning direction, scanning speed, beam current and breathing period were determined using deformable image registration. The metrics used for analysis were mean dose, minimum dose, maximum dose, dose homogeneity, equivalent uniform dose (EUD), lung V₅, V₂₀ and mean lung dose.

Results: In general, longer delivery times led to less interplay and the interplay effect could not be predicted using motion amplitude alone.
Larger spot sizes (σ ~9-16mm) gave a EUD of 99.0% + 4.4% (1 standard deviation) of the prescription dose in 1 fraction compared to 86.1% + 13.1% for smaller spots (σ ~2-4mm). Smaller spot sizes gave EUD values as low as 65.3% in 1 fraction.
Target dose homogeneity improved with reduced spot spacing. The interplay effect can vary significantly with initial breathing phase. No definitive benefit was observed when scanning predominantly parallel or perpendicular to the SI axis.
Conventional fractionation reduced interplay, giving EUD values of at least 84.7% and 100.0% for the small and larger spot sizes respectively.

Conclusion: The interplay effect is highly patient specific and varies based on the motion amplitude, tumor location and various delivery parameters.

Funding Support, Disclosures, and Conflict of Interest: This project was supported by the National Cancer Institute Grant No. R01 CA111590

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