Proton arc therapy reduces the dose delivered to circulating blood compared to intensity‐modulated proton therapy for head‐and‐neck cancer

Background Compared to intensity‐modulated proton therapy (IMPT), proton arc therapy (PAT) has various dosimetric advantages. However, questions have been raised over whether the larger low‐dose bath during PAT may increase radiation‐induced lymphocyte killing, which is correlated with adverse clinical outcomes. Purpose The purpose of this study is to quantitatively compare radiation dose delivery to circulating blood during IMPT and PAT. Methods For each of 10 head‐and‐neck cancer patients, one IMPT and one PAT treatment plan were created, and the hematological dose (HEDOS) blood dose model was used to simulate radiation dose delivery to circulating blood. HEDOS simulations utilized gantry angle‐wise dose distributions (including for PAT) and considered gantry rotation and energy layer switching as well as irradiation times. Blood dose histograms as well as the mean dose to all simulation blood particles (D mean ) and the dose delivered to the 90% and 10% of blood particles receiving the highest dose (D 90% and D 10% ) were then compared between IMPT and PAT. Results For IMPT, blood dose metrics (in the form of mean over all patients ± one standard deviation) were (0.522 ± 0.204)Gy(RBE) for D 90% , (0.563 ± 0.212)Gy(RBE) for D mean , and (0.605 ± 0.219)Gy(RBE) for D 10% . For PAT, D 90% was (0.430 ± 0.184)Gy(RBE), D mean was (0.483 ± 0.195)Gy(RBE), and D 10% was (0.538 ± 0.207)Gy(RBE). The Wilcoxon signed‐rank test indicated statistically significant ( p < 0.05) differences between blood dose metrics of IMPT and PAT for all patients, and IMPT did not increase the percentage of blood particles which received no radiation dose during the delivery of a single treatment fraction. Conclusions Compared to IMPT, proton arc therapy was associated with significantly lower radiation dose delivery to circulating blood across a variety of different metrics. Proton arc therapy for head‐and‐neck cancer is therefore not expected to result in higher radiation‐induced killing of circulating lymphocytes compared to IMPT.