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

Abstract

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 ⩾ 1 to ⩾ 4. The following parameters were determined for each schedule and the equivalent single dose-response curves for each endpoint using probit analysis: the ED 50, the absolute steepness, measured as the probit width, K, the relative steepness, K ED 50 , and the normalised effect gradient, γ 50. The inverse radiosensitivity, D 0eff or D 0, was estimated using the Poisson and LQ-models for tissue response. The α β 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. D 0eff decreased with increasing dose per fraction, and also with higher degree of telangiectasia. The highest steepness determined for telangiectasia score ⩾ 4 (partially confluent or more) at 10 years corresponded to K = 0.8 Gy, K ED 50 = 5%, γ 50 = 7 and D 0 = 0.7 Gy. The dose-response characteristics found for late skin telangiectasia score ⩾ 2 to ⩾ 4 were consistent with those determined for necrosis and fatal complications 5 years after radiotherapy to head and neck tumours in our department.