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Four-Dimensional Monte Carlo Simulations of Lung Cancer Patients Treated with Proton Beam Scanning to Assess Interplay Effects

C Grassberger

C Grassberger1 2*, J Shackleford1, G Sharp1, H Paganetti1, (1) Massachusetts General Hospital, Boston, MA, (2) Centre for Proton Radiotherapy, Paul Scherrer Institute, Villigen-PSI, Switzerland

TH-C-BRB-8 Thursday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

Purpose: To study the effects of breathing motion on lung cancer patients treated with active scanning proton therapy using four-dimensional Monte Carlo simulations. Beam data and machine parameters from two proton therapy centers are used to study the effect of varying spot-sizes and time to switch between energy layers.

Methods: For Monte Carlo simulations we employ TOPAS (TOol for PArticle Simulation), a toolkit based on Geant4. We developed methods to convert the output of our scanning treatment planning system into input suitable for Monte Carlo simulations. Furthermore we take into account the exact timeline of delivery, from the momentum-dependent spot movement to the time to switch between energy layers, enabling our Monte Carlo code to deliver the right spots to each of the 10 breathing phases of the patient's 4DCT dataset. Deformable image registration is employed to translate the dose delivered to each phase back to a reference phase for analysis.

Results: Patients show signs of interplay effect already at peak-to-peak movement amplitudes of 7.5mm. The spot size seems to have a significant impact on the dose homogeneity in the target: in one patient case the D95 of the CTV drops by only 2.7% for the large spot-size, while it is reduced by 8.9% for the smaller one. In this patient the mean dose in the CTV is reduced as well for small spots (-4.7%), while remaining stable for the broader peaks. The time to switch energy layers has also a significant impact, though the changes seem to be more related to the absolute length of the treatment compared to the breathing cycle.

Conclusions: This is the first time Monte Carlo is employed to study the interplay effect in proton beam scanning. The results show a clear variation of the effect for varying machine parameters in different patient cases.

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