Oxygen distributions partly explain the radiation response of human squamous cell carcinomas

The oxygen tension of human tumours has often been thought to alter tumour response to radiation therapy. The purpose of this analysis is to determine to what extent the observed results of radiotherapy fit predictions based on in situ human tumour pO2 distributions. The radiation dose-response curve for patients treated with radiation alone for squamous cell cancers of the cervix and oropharynx were calculated based on published data. pO2 histograms were obtained from 30 women with cervical cancer and 11 patients with neck nodes from head and neck cancers. An average of 76 +/- 35 (range 28-174) measurements were made from each patient. Hypoxia was assumed to be a purely dose-modifying factor with a maximum OER of 2.5. Assuming patients are treated with daily radiation doses of 2 Gy, the squamous cell carcinoma alpha/beta ratio is 10 Gy, and that tumours have a mean of 10(8) clonogens, it was possible to estimate tumour control probability. Tumour oxygenation was an extremely important modifier of the slope of the dose-response curve and alone was sufficient to account for the slope of the clinically observed dose-response curve for neck nodes. The response curve for uterine cervical cancers is very shallow, and the oxygen distribution did not completely account for heterogeneity of response of these tumours. The results support the conclusion that oxygen tension distribution is an important modifier of human radiation treatment response.