Characteristics of dose-response relationships for late radiation effects: an analysis of skin telangiectasia and of head and neck morbidity.

The dose-response characteristics were analysed for late skin telangiectasia in patients for 1, 2 and 5 fractions per week and 3 to 4 dose levels per schedule. Altogether 286 fields were used. Skin telangiectasia was scored on an arbitrary scale and dose-response analysis was performed at 10 year's follow-up for various degrees of telangiectasia, score greater than or equal to 1 to greater than or equal to 4. The following parameters were determined for each schedule and the equivalent single dose-response curves for each endpoint using probit analysis: the ED50, the absolute steepness, measured as the probit width, K, the relative steepness, K/ED50, and the normalised effect gradient, gamma 50. The inverse radiosensitivity, Doeff or Do, was estimated using the Poisson and LQ-models for tissue response. The alpha/beta value was found to be independent of the degree of telangiectasia used as endpoint. The absolute steepness of the dose-incidence curve increased with increasing dose per fraction and was correlated to the degree of damage. The relative steepness was independent of the dose per fraction when the dose-response curve was generated by a fixed dose per fraction, and was less than if generated by a fixed number of fractions. The relative steepness increased with higher degree of damage. Doeff decreased with increasing dose per fraction, and also with higher degree of telangiectasia. The highest steepness determined for telangiectasia score greater than or equal to 4 (partially confluent or more) at 10 years corresponded to K = 0.8 Gy, K/ED50 = 5%, gamma 50 = 7 and Do = 0.7 Gy. The dose-response characteristics found for late skin telangiectasia score greater than or equal to 2 to greater than or equal to 4 were consistent with those determined for necrosis and fatal complications 5 years after radiotherapy to head and neck tumours in our department.