Integrating spot-scanning proton arc therapy with functional avoidance strategies to reduce pulmonary toxicity

Functional avoidance radiotherapy has emerged as a promising technique using functional imaging to minimize pulmonary toxicity by reducing doses to functional lung. This study aims to investigate the potential dose-volume advantages of a novel spot-scanning proton arc (SPArc) therapy for functional avoidance radiotherapy with four-dimensional computed tomography (4DCT)-based ventilation imaging.

Twenty-five patients from a prospective functional avoidance clinical trial treated with intensity-modulated photon radiotherapy were included. Robustly optimized intensity modulated proton therapy (IMPT) and SPArc plans were generated in RayStation. Functional lung contour was derived from the 4DCT-based ventilation imaging and utilized as an optimization structure in photon as well as IMPT and SPArc functional planning. The dose distributions were compared, and normal tissue complication probability (NTCP) models were applied to estimate the probability of pulmonary toxicity.

Using clinical photon plans as the baseline for comparison, both proton plans achieved equivalent target coverage and reduced dose to organs at risk. Compared with photon plans, the median absolute reduction of fV_20Gy (the volume of functional lung receiving ≥ 20 Gy) was 3.7 percentage points (pp) with IMPT and 13.0 pp with SPArc. Using fV_20Gy for NTCP estimation, the median reduction of probability of grade ≥ 2 pneumonitis was 5.0 pp with IMPT and was 14.9 pp with SPArc.

Our study highlighted the potential of SPArc to spare dose to functional lung. NTCP results further indicated that the risk of pulmonary complications can be reduced with SPArc compared to photon or IMPT for functional avoidance radiotherapy.