Effects of materials and beam currents on ozone generation in proton beam irradiation: a phantom study

Some patients undergoing radiation therapy have been reported to experience olfactory sensation. Radiation-induced ozone generation has been proposed as a potential cause, yet the underlying mechanism remains elusive. Previous studies have focused on ozone generation in photon- and electron-beam environments; however, data specific to proton beams remain limited. Using proton irradiation, we measured ozone concentrations at four clinically used beam currents (50, 100, 150 and 200 nA) and under material conditions. A custom-designed phantom was developed to investigate ozone generation during proton beam irradiation using three materials: air, water and bolus. Before conducting the main measurements, we used a high-sensitivity ozone monitor to assess the background ozone concentration in the treatment room over two treatment days, averaging 1.26 ± 0.97 ppb, which is substantially lower than clinically accepted safety thresholds. In air, ozone concentration increased linearly with beam current (R2 = 0.992, P = 0.004), reaching a maximum of 33.60 ppb and a mean of 13.47 ± 2.08 ppb at 200 nA. Water conditions also followed a linear increase (R2 = 0.988, P = 0.006), albeit at a more gradual rate compared to air, with a mean of 8.69 ± 1.12 ppb at 200 nA. However, bolus conditions showed no linear relationship (R2 = 0.081, P = 0.715), with a mean of 8.57 ± 0.87 ppb at 200 nA. Our findings indicate that moisture and airflow can affect ozone concentrations during proton therapy and provide foundational data for elucidating the relationship between ozone levels and patient-reported olfactory sensation in further clinical investigations.