Object: This study aimed to investigate the impact of tumor hypoxia on treatment outcome for metastases commonly treated with radiosurgery using 1 fraction of radiation and the potential gain from reoxygenation if the treatment is delivered in a few radiation fractions.
Methods: In silico metastasis-like radiosurgery targets were modeled with respect to size, density of clonogenic cells, and oxygenation. Treatment plans were produced for the targets using Leksell GammaPlan, delivering clinically relevant doses and evaluating the tumor control probability (TCP) that could be expected in each case. Fractionated schedules with 3, 4, and 5 fractions resulting in similar biological effective doses were also considered for the larger target, and TCP was determined under the assumption that local reoxygenation takes place between fractions.
Results: The results showed that well-oxygenated small- and medium-size metastases are well controlled by radiosurgery treatments delivering 20 or 22 Gy at the periphery, with TCPs ranging from 90% to 100%. If they are moderately hypoxic, the TCP could decrease to 60%. For large metastases, the TCPs from single-fraction treatments ranged from 0% to 19%, depending on tumor oxygenation. However, for fractionated treatments, the TCP for hypoxic tumors could significantly increase up to 51%, if reoxygenation occurs between fractions.
Conclusions: This study shows that hypoxia worsens the response to single-fraction radiosurgery, especially for large tumors. However, fractionated therapy for large hypoxic tumors might considerably improve the TCP and might constitute a simple way to improve the outcome of radiosurgery for patients with hypoxic tumors.
Keywords: BED = biological effective dose; HF = hypoxic fraction; HI = heterogeneity index; TCP = tumor control probability; fractionation; metastases; radiosurgery; stereotactic radiosurgery; tumor hypoxia.