REPOPULATION CAPACITY DURING FRACTIONATED-IRRADIATION OF SQUAMOUS-CELL CARCINOMAS AND GLIOBLASTOMAS IN-VITRO

Purpose: Determination of clonogenic cell proliferation of three highl y malignant squamous cell carcinomas (SCC) and two glioblastoma cell l ines during a 20-day course of fractionated irradiation under in vitro conditions, Methods and Materials: Tumor cells in exponential growth phase were plated in 24-well plastic flasks and irradiated 24 h after plating with 250 kV x-rays at room temperature, Six fractions with sin gle doses between 0.6 and 9 Gy were administered in 1.67, 5, 10, 15, a nd 20 days, Colony growth was monitored for at least 60 days after com pletion of irradiation, Wells with confluent colonies were considered as ''recurrences'' and wells without colonies as ''controlled.'' The d ose required to control 50% of irradiated wells (WCD50) was estimated by a logistic regression for the different overall treatment times, Th e effective doubling time of clonogenic cells (T-eff), was determined by a direct fit using the maximum likelihood method, Results: The incr ease of WCD50 within 18.3 days was highly significant for all tumor ce ll lines accounting for 7.9 and 12.0 Gy in the two glioblastoma cell l ines and for 12.7, 14.0, and 21.7 Gy in the three SCC cell lines, The corresponding T(eff)s were 4.4 and 2.0 days for glioblastoma cell line s and 2.4, 4.2, and 1.8 days for SCC cell lines, Population doubling t imes (PDT) of untreated tumor cells ranged from 1.0 to 1.9 days, showi ng no correlation with T(eff)s. T-eff was significantly longer than PD T in three of five tumor cell lines, No significant differences were o bserved comparing glioblastomas and SCC, Increase of WCD50 with time d id not correlate with T-eff but with T-eff InSF2 (surviving fraction at 2 Gy), Conclusion: The intrinsic ability of SCC and glioblastoma ce lls to repopulate during fractionated irradiation could be demonstrate d, Repopulation induced dose loss per day depends on T-eff and intrins ic radiation sensitivity, proliferation during treatment was decelerat ed compared to pretreatment PDT in the majority of cell lines, Pretrea tment cell kinetics did not predict for tumor cell proliferation durin g treatment. (C) 1997 Elsevier Science Inc.