Biological dosimetric impact of dose-delivery time for hypoxic tumour with modified microdosimetric kinetic model

BACKGROUND: An improved microdosimetric kinetic model (MKM) can address radiobiological effects with prolonged delivery times. However, these do not consider the effects of oxygen. The current study aimed to evaluate the biological dosimetric effects associated with the dose delivery time in hypoxic tumours with improved MKM for photon radiation therapy. MATERIALS AND METHODS: Cell survival was measured under anoxic, hypoxic, and oxic conditions using the Monte Carlo code PHITS. The effect of the dose rate of 0.5–24 Gy/min for the biological dose (D(bio)) was estimated using the microdosimetric kinetic model. The dose per fraction and pressure of O(2) (pO(2)) in the tumour varied from 2 to 20 Gy and from 0.01 to 5.0% pO(2), respectively. RESULTS: The ratio of the D(bio) at 1.0–24 Gy/min to that at 0.5 Gy/min (R(DR)) was higher at higher doses. The maximum R(DR) was 1.09 at 1.0 Gy/min, 1.12 at 12 Gy/min, and 1.13 at 24 Gy/min. The ratio of the D(bio) at 0.01–2.0% of pO(2) to that at 5.0% of pO(2) (R(oxy)) was within 0.1 for 2–20 Gy of physical dose. The maximum R(oxy) was 0.42 at 0.01% pO(2), 0.76 at 0.4% pO(2), 0.89 at 1% pO(2), and 0.96 at 2% pO(2). CONCLUSION: Our proposed model can estimate the cell killing and biological dose under hypoxia in a clinical and realistic patient. A shorter dose-delivery time with a higher oxygen distribution increased the radiobiological effect. It was more effective at higher doses per fraction than at lower doses.