The Effect of Hypoxia on Relative Biological Effectiveness and Oxygen Enhancement Ratio for Cells Irradiated with Grenz Rays

SIMPLE SUMMARY: Grenz rays are low energy X-rays and have been used for dermatological radiotherapy for decades. This study calculates the relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) for DNA double-strand break (DSB) induction and cell survival under hypoxic conditions for Grenz-ray therapy (GT). The RBE values for cell survival for cells irradiated by 15 kV, 10 kV and 10 kVp and titanium K-shell X-rays (4.55 kV) relative to (60)Co γ-rays are 1.0–1.6 at an aerobic condition (21% O(2)) and further increase to 1.2–2.1 in conditions of severe hypoxia (0.1% O(2)). The OER values for DSB induction relative to (60)Co γ-rays are ~2.4 for GT, but the OER for cell survival is 2.8–2.0 as photon energy decreases from 15 kV to 4.55 kV. ABSTRACT: Grenz-ray therapy (GT) is commonly used for dermatological radiotherapy and has a higher linear energy transfer, relative biological effectiveness (RBE) and oxygen enhancement ratio (OER). GT is a treatment option for lentigo maligna and lentigo maligna melanoma. This study aims to calculate the RBE for DNA double-strand break (DSB) induction and cell survival under hypoxic conditions for GT. The yield of DSBs induced by GT is calculated at the aerobic and hypoxic conditions, using a Monte Carlo damage simulation (MCDS) software. The RBE value for cell survival is calculated using the repair–misrepair–fixation (RMF) model. The RBE values for cell survival for cells irradiated by 15 kV, 10 kV and 10 kVp and titanium K-shell X-rays (4.55 kV) relative to (60)Co γ-rays are 1.0–1.6 at the aerobic conditions and moderate hypoxia (2% O(2)), respectively, but increase to 1.2, 1.4 and 1.9 and 2.1 in conditions of severe hypoxia (0.1% O(2)). The OER values for DSB induction relative to (60)Co γ-rays are about constant and ~2.4 for GT, but the OER for cell survival is 2.8–2.0 as photon energy decreases from 15 kV to 4.55 kV. The results indicate that GT results in more DSB induction and allows effective tumor control for superficial and hypoxic tumors.