Radiotherapy and Oncology, 16 (1989) 41-53 Elsevier 41 RADION 00611 Analysis of late effects data using dose-response models: application to human skin telangiectasia data Dimos Baltas ~, Dieter Fehrentz ~ and Ingela Turesson 2 1Department of ClinicalRadiology, Universityof Heidelberg, Heidelberg,F.R.G., and 2Department of Oncology, The Universityof Gothenborg, Sahlgrenska Hospital, S-413 45 Gothenborg, Sweden (Received 3 August 1988, revision received 9 February 1989, accepted 23 February 1989) Key words: Radiotherapy optimization, dose-response models, late effects, dose accuracy Summary The clinical data for skin telangiectasia from previous prospective studies at the Radiotherapy Department in Gothenborg are reanalyzed using two dose-response models - the general formulations of the well known linear-quadratic (LQ) and NSD isoeffect models. Assuming that essentially no repopulation appears in the vessel endothelium for overall treatment times up to 68 days, the 0t/fl-value of 2.75 Gy is obtained for the LQ-model. The time factor is found not to be significant by the NSD-model for the treatment times used ( _< 68 days) at the 95 Y/o level of confidence. The estimated value of the exponent of the number of fractions, A, is 0.321. The obtained values of the at//~-ratio and of A show high sensitivity of the vessel endothelium to changes in the dose per fraction. Our results show that within the interval of the number of fractions used, 10-35 fractions, the NSD-model gives predictions comparable to those of the LQ-model. For number of fractions smaller than 5, a high discrepancy occurs between the two models, the NSD-model predicting higher values of the isoeffective total dose. The maximal deviation between the models appears for N = 1; 25 ~ and 27 ~ at the 5 and 50 ~o level of effect, respectively. For large N and especially at low effect probabilities the NSD-model again predicts higher isoeffective doses: the dose predicted by the NSD-model for a regimen with 40 fractions and for 5~o probability of telangiectasia is 7.5 ~o, higher than that predicted by the LQ-model. Based on the estimated dose-response curves, considering the telangiectasia as the decisive late tissue effect, the requirement for the combined uncertainty in the dose delivery is estimated between 3 and 4.5~. Address for correspondence: Dimos Baltas, Dipl. Phys, Department of Clinical Radiology, University of Heidelberg, D-6900 Heidelberg, F.R.G. 0167-8140/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)