Acute skin toxicity associated with proton beam therapy in spine and brain patients

Objective The purpose of this retrospective study is to determine the dependence of acute skin toxicity on physical parameters associated with the proton beam treatment. Methods Treatment records from 389 spine and brain patients were retrospectively reviewed. The subgroup of 90 patients was selected based on the study criteria. The severity of the skin reaction was estimated based on the international criteria from patient chart assessment. Skin dose, energy of proton beam incident on the skin, and irradiated skin area were collected from patient’s records, and these parameters were analyzed for skin toxicity. Results The median age of the patients in this study was 11.2 years. Skin reactions were recorded in 77/90 patients (86 %). The presence of grades 1, 2, 3, and 4 of skin reactions were observed for 66.2, 20.8, 10.4, and 2.6 % patients, respectively. The median skin dose for grade 1, 2, and 3 skin reactions were 47, 52, and 55 Gy, respectively. It is shown that the skin dose and proton energy beam incident on the skin are two independent parameters defining skin toxicity in proton therapy. The increase in skin toxicity from grade 1 to grade 2 is associated with higher initial proton energies defining the proximal edge, center, and distal edge of the spread out Bragg Peak ( p = 0.0479, p = 0.0395, and p = 0.0751, respectively). Conclusion The severity of acute skin reaction is associated with higher beam energy of protons independent of delivered dose. These results indicate the presence of radiobiological mechanisms defined by processes specific to the interaction of protons with the skin.