Impact of Interplay Effects In Lung SBRT for Intensity-Modulated Proton Therapy and Spot-Scanning Proton Arc Therapy
Abstract
Purpose
This study assessed 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 (SPArc®).
Methods
We used two published proton therapy system (PTS) delivery sequence models in this study: IBA Proteus®ONE and IBA Proteus®PLUS. Interplay effects were retrospectively evaluated in ten proton lung SBRT patients (4-5 fractions) using 4D dynamic dose accumulation. Two-field IMPT and SPArc® treatment delivery were simulated on each PTS. We used a single-fraction approach where every fraction of a patient’s treatment course is assumed to start on the same breathing phase. We evaluated data across ten possible starting phases. Reported dose volume histogram metrics for the GTV, as contoured on the 50% phase image, include D99%, D1%, 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 possible single-fraction dose distributions and all patients, D99% decreased 2.4%/4.0%/2.7%/2.7% for IMPTProteus®ONE/IMPTProteus®PLUS/SPArcProteus®ONE/SPArcProteus®PLUS, respectively. D1% increased 2.4%/4.2%/2.1%/2.1%. Mean dose decreased by 0.1%/0.1%/0.3%/0.3%. HI increased 2.6%/4.7%/2.9%/3.0%. QC decreased 3.5%/5.2%/3.8%/4.0%. 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 Proteus®ONE treatment delivery was more robust to interplay effects than cyclotron-based Proteus®PLUS, 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 SPArc® plans, where delivery is spread over the arc trajectory. The magnitude of the decrease in D99% and QC of the target and the increase in HI is larger for both SPArc® plans than for IMPTProteus®ONE plans. Thus, interplay effect evaluation is critical for the implementation of the proton arc modality.