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Multiple Energy Extraction Delivery Technique in Synchrotron-Based Intensity-Modulated Proton Therapy (IMPT) May Exacerbate Motion Interplay Effects for Lung Cancer Treatment

J E Younkin*, D Hernandez Morales, J Shen, X Ding, J B Stoker, M Bues, M Fatyga, T T Sio, S E Schild, W Liu, Mayo Clinic Arizona, Phoenix, AZ


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

Room: AAPM ePoster Library

Purpose: Multiple energy extraction (MEE) can reduce synchrotron-based spot-scanning proton treatment delivery time by 35% compared to single energy extraction (SEE). The aim of this study was to determine whether MEE may exacerbate motion interplay effects in terms of target dose homogeneity, coverage, and dose to organs-at-risk (OARs).

Methods: Interplay effects were measured for ten lung cancer clinical plans using 4D dynamic dose, which considers the temporal relationship between time-dependent spot delivery and patient breathing in the dose calculation. Averages of ten SEE and MEE dose distributions paired by randomly-selected initial phase angles were calculated for each patient in various fractionation schemes. Statistical significance of dose-volume histogram (DVH) index differences between SEE and MEE was determined using the Wilcoxon rank-sum test. For each treatment field, total MUs delivered to breathing phases for each energy layer were recorded. The average delivery time per spill for SEE and MEE was analyzed.

Results: Three patients (P3, P4, and P5) with respiratory periods ~4s had significantly worse target dose homogeneity and coverage using MEE delivered in single fraction. The worst-case patient (P4) had a 100% increase in D5% - D95% and 5% decrease in D95% on average using MEE compared to SEE. Delivery time per spill for ten patients were 4.07 ± 0.48s for MEE and 2.81 ± 0.47s for SEE; for patients with ~4s respiratory periods, synchronization between MEE delivery and patient respiration was observed, which seemed to result in worse interplay effects. Multiple fractionation did not eliminate significant differences in target dose homogeneity and coverage between MEE and SEE, but MEE still achieved clinically acceptable target coverage. No statistically significant differences were observed in OAR protection.

Conclusion: MEE can deliver clinically acceptable target dose coverage given a sufficient number of fractions with potentially reduced target dose homogeneity and coverage compared with SEE.

Funding Support, Disclosures, and Conflict of Interest: This research was supported by Arizona Biomedical Research Commission Investigator Award, by The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, and by The Kemper Marley Foundation.


Protons, Lung


TH- External Beam- Particle/high LET therapy: Proton therapy - Motion management - intrafraction

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