Clinical delivery optimization and toxicity evaluation of head and neck proton arc therapy treatments

A new proton therapy technique, called Static Proton Arc Therapy (PAT), was introduced into routine clinical practice at the Trento Proton Therapy Center in October 2024, for the first time worldwide. Early clinical results showed that PAT can improve organ-at-risk (OAR) dose sparing compared with conventional Multi-Field Optimization (MFO). However, because PAT uses many more gantry directions (up to 30) than MFO (typically 4--6), treatment delivery times may increase. This work investigated alternative static PAT planning approaches to identify configurations that reduce delivery time while preserving plan quality. A cohort of 10 head-and-neck patients was planned with MFO and four PAT configurations with 20, 15, 10, and 8 directions. Across the evaluated metrics, the 10-direction approach maintained target coverage and OAR sparing while reducing delivery time from 26.8 $\pm$ 2.8 min for MFO to 17.5 $\pm$ 3.6 min for PAT with 10 directions. In addition, a Normal Tissue Complication Probability (NTCP) model for acute oral mucositis was selected from the literature and refitted on 120 patients treated with MFO, using \(p(D)=1/(1+(MD_{50}/D)^k)\), where \(D\) is the mean dose to the oral cavity, \(MD_{50}\) is the dose associated with a 50\% probability of developing the complication, and \(k\) controls the steepness of the dose--response curve. The parameter estimates were \(MD_{50}=26.0 \pm 4.6\) Gy and \(k=1.8 \pm 0.4\). The model was applied to an independent cohort of 10 patients to assess whether the dose sparing associated with PAT relative to MFO translated into a lower predicted probability of toxicity. The median predicted probability decreased from \(\tilde{p}_{\mathrm{MFO}}=0.31\) to \(\tilde{p}_{\mathrm{PAT}}=0.10\) (67.7\% reduction). Overall, the PAT technique provides a favorable balance between plan quality, delivery efficiency, and lower predicted toxicity.