Purpose: To evaluate how the sub-lethal damage repair influences on the biological effectiveness of proton in prolonged scanning irradiation. Previous studies revealed the reduction of biological effectiveness caused by the repair effect in passively-scattered irradiation where the dose rate is independent of position over target volume. However, the effect in scanning irradiation was not clarified because of the different dose delivery time structure.
Methods: Spread-out Bragg-peak (SOBP) irradiation was simulated in water phantom with one- and two-field plans. Three scenarios (instantaneous, passively-scattered and layered-scan) were considered with corresponding models of dose delivery time structures for each irradiation technique. The repair effect was evaluated with the biological dose (dose × relative biological effectiveness (RBE)) obtained from linear quadratic model combined with theory of dual radiation action. McNamara model was used to include the LETd dependence of RBE. In addition, the simulation using clinical plans with one and two fields (post-operative seminoma and prostate tumor, respectively) were carried out.
Results: Dose delivery time structure which differs for each scenario was found to be an important factor for evaluating the repair effect. In the passively-scattered scenario, relative dose decrease against the instantaneous scenario was about 3.3% over SOBP with 19 min of total irradiation time. On the other hand, layered-scan scenario resulted in the variable dose decrease with 3.5% and 0.5% at the proximal and distal side of SOBP, respectively. The variability of dose decrease was mainly caused by the spatial dependence of dose delivery time structure in scanning irradiation. The similar results were obtained from the clinical plans indicating the potential distortion of biological dose distribution in layered-scan scenario.
Conclusion: Consideration of the dose delivery time structure in scanning proton irradiation is important for the evaluation of the repair effect.