Small spot size versus large spot size: Effect on plan quality for lung cancer in pencil beam scanning proton therapy

Abstract

The purpose of the current study was to evaluate the impact of spot size on the interplay effect, plan robustness, and dose to the organs at risk for lung cancer plans in pencil beam scanning (PBS) proton therapy Methods: The current retrospective study included 13 lung cancer patients. For each patient, small spot (∼3 mm) plans and large spot (∼8 mm) plans were generated. The Monte Carlo algorithm was used for both robust plan optimization and final dose calculations. Each plan was normalized, such that 99% of the clinical target volume (CTV) received 99% of the prescription dose. Interplay effect was evaluated for treatment delivery starting in two different breathing phases (T0 and T50). Plan robustness was investigated for 12 perturbed scenarios, which combined the isocenter shift and range uncertainty. The nominal and worst-case scenario (WCS) results were recorded for each treatment plan. Equivalent uniform dose (EUD) and normal tissue complication probability (NTCP) were evaluated for the total lung, heart, and esophagus. Results: In comparison to large spot plans, the WCS values of small spot plans at CTV D 95% , D 96% , D 97% , D 98% , and D 99% were higher with the average differences of 2.2% (range, 0.3%-3.7%), 2.3% (range, 0.5%-4.0%), 2.6% (range, 0.6%-4.4%), 2.7% (range, 0.9%-5.2%), and 2.7% (range, 0.3%-6.0%), respectively. The nominal and WCS mean dose and EUD for the esophagus, heart, and total lung were higher in large spot plans. The difference in NTCP between large spot and small spot plans was up to 1.9% for the total lung, up to 0.3% for the heart, and up to 32.8% for the esophagus. For robustness acceptance criteria of CTV D 95% ≥ 98% of the prescription dose, seven small spot plans had all 12 perturbed scenarios meeting the criteria, whereas, for 13 large spot plans, there were ≥2 scenarios failing to meet the criteria. Interplay results showed that, on average, the target coverage in large spot plans was higher by 1.5% and 0.4% in non-volumetric and volumetric repainting plans, respectively. Conclusion: For robustly optimized PBS lung cancer plans in our study, a small spot machine resulted in a more robust CTV against the setup and range errors when compared to a large spot machine.In the absence of volumetric repainting, large spot PBS lung plans were more robust against the interplay effect. The use of a volumetric repainting technique in both small and large spot PBS lung plans led to comparable interplay target coverage.

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

“…First, planning strategies (e.g., robust optimization with setup and range uncertainties, optimization on average intensity projection CTs (AIP-CTs) vs. multiple 4DCTs) utilized in these studies were different. A robust optimization technique was applied in the studies by Liu et al 19 and Rana et al, 20 while it was not applied by Grassberger et al 17 and this present study. The use of robust optimization, which includes setup and range uncertainties, inevitably generates plans that are more robust against the interplay effect.…”

“…In the other rescanning study, 18 Grassberger et al showed that mitigation of interplay effect for tumor motion amplitudes up to 20 mm (seen in 4/5 patients in their study) required two repaintings for large,and six for small,spot machines.Liu et al 19 reported comparable interplay effect between large and small spot machines, where similar target dose coverage (D 95% ) was observed. In the study by Rana et al, 20 they showed that large spot plans were more robust against the interplay effect in the absence of repainting, with target coverage higher by 1.5% compared to small spot plans. Following VR, the interplay target coverage in small spot plans was found to be comparable to that of the large spot plans.…”

“…The impact of spot sizes on the interplay effect for treating NSCLC with PBS have been reported in previous studies. [17][18][19][20] In one of the studies reported by Grassberger et al, 17 for the largest motion studied, the interplay effect resulted in a decrease of the minimum dose in the clinical target volume (CTV) to 49% of the prescribed dose for the small spot plans, compared to 78% for the same patient using the large spot plans, in the absence of rescanning. In the other rescanning study, 18 Grassberger et al showed that mitigation of interplay effect for tumor motion amplitudes up to 20 mm (seen in 4/5 patients in their study) required two repaintings for large,and six for small,spot machines.Liu et al 19 reported comparable interplay effect between large and small spot machines, where similar target dose coverage (D 95% ) was observed.…”

“…Investigations of interplay effects on spot sizes for lung cancer have shown the plan robustness and quality in PBS IMPT for lung cancer are affected. [17][18][19][20] Small spot machines have been shown to produce more conformal plans and better OAR sparing compared to large spot machines. 17,19,20 To reduce the interplay effect, however, small spot machines might require more repaintings, and consequently longer times for delivery if multiple repainting is applied.…”

“…[17][18][19][20] Small spot machines have been shown to produce more conformal plans and better OAR sparing compared to large spot machines. 17,19,20 To reduce the interplay effect, however, small spot machines might require more repaintings, and consequently longer times for delivery if multiple repainting is applied. 18 In contrast, comparable interplay effects between small and large spot machines were reported.…”