Technical Note: Multiple energy extraction techniques for synchrotron‐based proton delivery systems may exacerbate motion interplay effects in lung cancer treatments

Abstract

Purpose The multiple energy extraction (MEE) delivery technique for synchrotron‐based proton delivery systems reduces beam delivery time by decelerating the beam multiple times during one accelerator spill, but this might cause additional plan quality degradation due to intrafractional motion. We seek to determine whether MEE causes significantly different plan quality degradation compared to single energy extraction (SEE) for lung cancer treatments due to the interplay effect. Methods Ten lung cancer patients treated with IMPT at our institution were nonrandomly sampled based on a representative range of tumor motion amplitudes, tumor volumes, and respiratory periods. Dose‐volume histogram (DVH) indices from single‐fraction SEE and MEE four‐dimensional (4D) dynamic dose distributions were compared using the Wilcoxon signed‐rank test. Distributions of monitor units (MU) to breathing phases were investigated for features associated with plan quality degradation. SEE and MEE DVH indices were compared in fractionated deliveries of the worst‐case patient treatment scenario to evaluate the impact of fractionation. Results There were no clinically significant differences in target mean dose, target dose conformity, or dose to organs‐at‐risk between SEE and MEE in single‐fraction delivery. Three patients had significantly worse dose homogeneity with MEE compared to SEE (single‐fraction mean D5%‐D95% increased by up to 9.6% of prescription dose), and plots of MU distribution to breathing phases showed synchronization patterns with MEE but not SEE. However, after 30 fractions the patient in the worst‐case scenario had clinically acceptable target dose homogeneity and coverage with MEE (mean D5%–D95% increased by 1% compared to SEE). Conclusions For some patients with breathing periods close to the mean spill duration, MEE resulted in significantly worse single‐fraction target dose homogeneity compared to SEE due to the interplay effect. However, this was mitigated by fractionation, and target dose homogeneity and coverage were clinically acceptable after 30 fractions with MEE.

Cardiopulmonary Toxicity Following Intensity-Modulated Proton Therapy (IMPT) Versus Intensity-Modulated Radiation Therapy (IMRT) for Stage III Non-Small Cell Lung Cancer

“…In particular, the impact of interplay effect was assessed by our in-house developed treatment planning system to ensure adequate target coverage (D 95% of the target received 95% of the prescription dose). [20][21][22][35][36][37] If a plan failed the interplay effect evaluation, the plan was re-optimized with larger target margins, different beam angles, density override with a higher HU number, increased number of repainting, reduced spot spacing, and/or 4D robust optimization within our in-house treatment planning system. 21,36 Linear energy transfer (LET)-based plan evaluation was performed for all patients treated by proton therapy.…”

Cyclotron and linear accelerator generated scanning proton beams for lung cancer SBRT: Interplay effects and mitigations

“…31 Precisely simulating the time of beam delivery could prove valuable in modeling the interplay effect. 32 Lastly, the results observed here may be different if another patient cohort was investigated.…”

“…The beam time structure can vary among different synchrotron facilities and consequently impact the interplay effect, depending on factors such as accelerator design and intended applications (e.g., continuous or pulsed beams) 31 . Precisely simulating the time of beam delivery could prove valuable in modeling the interplay effect 32 . Lastly, the results observed here may be different if another patient cohort was investigated.…”

Quantifying dose uncertainties resulting from cardiorespiratory motion in intensity-modulated proton therapy for cardiac stereotactic body radiotherapy

“…The interplay effects of dynamic delivery and tumor motion degrade the plan quality, particularly in IMPT ( 14 ). The degree of motion-induced dose uncertainty depends on the delivery system and varies according to spot size ( 15 ), fractionation ( 13 ), delivery time ( 16 ), scanning technique ( 17 ), and patient motion amplitude ( 18 ).…”