3095 - Realistic Treatment Simulation for Interplay Effect Evaluation in Lung SBRT with Multiple Proton Therapy Delivery Sequences

Purpose/Objective(s): We simulated realistic patient treatments to assess the impact of machine-specific delivery sequence in quantifying interplay effects for lung stereotactic body radiotherapy (SBRT) between two proton therapy modalities, intensity-modulated proton therapy (IMPT) and spot-scanning proton arc therapy (PAT).

Materials/Methods: We used two published proton therapy system (PTS) delivery sequence models in this study including a synchrocyclotron-based and a cyclotron-based system. Interplay effects were retrospectively evaluated in ten proton lung SBRT patients (4-5 fractions) using 4D dynamic dose accumulation. Two-field IMPT and PAT treatment delivery were simulated on each PTS. We used a multi-fraction approach where every fraction of a patient’s treatment course was assumed to start on a random (out of ten total) breathing phase. We simulated 25 treatments for each patient. Reported dose volume histogram metrics for the GTV, as contoured on the 50% phase image, include D_99%, D_1%, mean dose, heterogeneity index (HI), and quality of coverage (QC). These are given as percent change from the planned distribution. We also reported estimated total delivery time and beam-on time.

Results: Averaging across all simulated multi-fraction dose distributions and subsequently across all patients, D_99% decreased 0.6%/1.2%/1.5%/1.5% for IMPT_{synchrocyclotron}/IMPT_cyclotron/PAT_{synchrocyclotron}/PAT_cyclotron, respectively. D_1% increased 0.6%/1.2%/0.8%/0.8%. Mean dose decreased by 0.0%/0.1%/0.3%/0.3%. HI increased 0.4%/1.1%/1.2%/1.3%. QC decreased 1.5%/2.1%/2.8%/2.8%. Average total delivery times were 59.8/37.1/200.3/98.7 seconds with beam-on times of 38.3/11.6/83.5/13.4 seconds.

Conclusion: Synchrocyclotron-based treatment delivery was more robust to interplay effects than cyclotron-based, likely due to the burst delivery mechanism, allowing time-average smoothing of the delivered dose distribution. The disparity between the two PTS is reduced in PAT plans, where delivery is spread over the arc trajectory. The magnitude of the decrease in D_99%, mean dose, and QC of the target and the increase in HI is larger for both PAT plans than for either IMPT plan. Thus, interplay effect evaluation is critical for the implementation of the proton arc modality.