Repopulation of FaDu human squamous cell carcinoma during fractionated radiotherapy correlates with reoxygenation

Purpose: FaDu human squamous cell carcinoma (FaDu-hSCC) showed a clear-cut time factor during fractionated radiotherapy (RT) under ambient blood flow. It remained unclear whether this is caused solely by proliferation or if r adioresistance resulting from increasing hypoxia contributed to this phenom enon. To address this question, repopulation of clonogenic FaDu cells durin g fractionated RT under clamp hypoxia was determined by local tumor control assays, and compared to the results after irradiation with the same regime n under ambient blood flow. Methods and Materials: FaDu-hSCC was transplanted into the right hind leg o f NMRI nu/nu mice. In the first set of experiments, irradiation was perform ed under clamp hypoxia. After increasing numbers of 3 Gy fractions (time in tervals 24 h or 48 h), graded top-up doses were given to determine the TCD5 0 (dose required to control 50% of the tumors). In the second set of experi ments, all 3 Gy fractions were applied under ambient conditions, but as in the previous experiments the graded top-up doses were given under clamp hyp oxia. A total of 26 TCD50 assays were performed and analyzed using maximum likelihood techniques. Results: With increasing numbers of daily fractions, the top-up TCD., under clamp hypoxia decreased from 39.4 Gy [95% CI 36, 42] after single dose to 19.8 Gy [15, 24] after 18 fractions in 18 days and to 37.8 Gy [31, 44] afte r 18 fractions in 36 days. The results were consistent with biphasic repopu lation, with a switch to rapid repopulation after about 22 days [13, 30]. T he clonogen doubling time (T-clon) decreased from 9.8 days [0, 21] in the b eginning of RT to 3.4 days after 22 days. Under ambient blood flow the top- up TCD50 decreased from 37.6 Gy [34, 40] after single dose irradiation to 0 Gy [0, 1] after 18 fractions in 18 days and 22.4 Gy [18, 27] after 18 frac tions in 36 days. Similar to results from irradiations under clamp hypoxia, the ambient data were consistent with a biphasic course of clonogen inacti vation. Comparison of both data sets revealed significant reoxygenation aft er 12 fractions. Conclusions: Our data are most consistent with a biphasic course or clonoge n repopulation during fractionated RT of FaDu-hSCC under clamp hypoxia with a switch in T-clon after about 22 days of treatment ("dog-leg"). A similar biphasic course of cell repopulation was observed under ambient conditions . The temporal coincidence between repopulation and reoxygenation suggests that the latter might be the stimulus for proliferation in FaDu tumors. (C) 2001 Elsevier Science Inc.