Repopulation capacity during fractionated irradiation of squamous cell carcinomas and glioblastomas in vitro

Determination of clonogenic cell proliferation of three highly malignant squamous cell carcinomas (SCC) and two glioblastoma cell lines during a 20-day course of fractionated irradiation under in vitro conditions.

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 single doses between 0.6 and 9 Gy were administered in 1.67, 5, 10, 15, and 20 days. Colony growth was monitored for at least 60 days after completion of irradiation. Wells with confluent colonies were considered as "recurrences" and wells without colonies as "controlled." The dose required to control 50% of irradiated wells (WCD50) was estimated by a logistic regression for the different overall treatment times. The effective doubling time of clonogenic cells (T[eff]) was determined by a direct fit using the maximum likelihood method.

The increase of WCD50 within 18.3 days was highly significant for all tumor cell lines accounting for 7.9 and 12.0 Gy in the two glioblastoma cell lines 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 lines and 2.4, 4.2, and 1.8 days for SCC cell lines. Population doubling times (PDT) of untreated tumor cells ranged from 1.0 to 1.9 days, showing no correlation with T(eff)s. T(eff) was significantly longer than PDT in three of five tumor cell lines. No significant differences were observed comparing glioblastomas and SCC. Increase of WCD50 with time did not correlate with T(eff) but with T(eff) InSF2 (surviving fraction at 2 Gy).

The intrinsic ability of SCC and glioblastoma cells to repopulate during fractionated irradiation could be demonstrated. Repopulation induced dose loss per day depends on T(eff) and intrinsic radiation sensitivity. Proliferation during treatment was decelerated compared to pretreatment PDT in the majority of cell lines. Pretreatment cell kinetics did not predict for tumor cell proliferation during treatment.