Cellular Recovery Kinetics for Postirradiation Treatments

Relationship between DNA double-strand breaks, cell killing, and fibrosis studied in confluent skin fibroblasts derived from breast cancer patients

PURPOSE

To investigate the relationship between DNA double-strand breaks (dsbs), cell killing, and fibrosis using skin fibroblasts derived from breast cancer patients who received postmastectomy radiotherapy.

METHODS AND MATERIALS

Experiments were performed with 12 lines of normal skin fibroblasts derived from recurrence-free breast cancer patients. Cells were irradiated in confluence and cell survival was determined either after immediate or delayed (14 h) plating using a colony-forming assay. Dsbs were measured by constant-field gel electrophoresis. The "excess risk of fibrosis" was previously scored by Johansen et al. (IJRB 1994;66:407-412).

RESULTS

The 12 cell lines showed a typical spectrum of radiosensitivity. The mean value of surviving fraction after 3.5 Gy (SF3.5) was 0.063 for immediate and 0.174 for delayed plating with a coefficient of variation (CV) of 44 and 39%, respectively. There was also a broad variation in the extent of recovery from potentially lethal damage (RPLD), which was not correlated with the immediate sensitivity. The number of initial dsbs as well as the half-times of dsb repair showed little variation, whereas there were considerable differences in the number of residual dsbs (CV = 29%). The number of residual dsbs after 100 Gy was correlated significantly only with SF3.5 after delayed (r2 = O.59; p = 0.006) but not after immediate plating (r2 = 0.21, p = 0.16). There was also no significant relationship between residual dsbs and the "excess risk of fibrosis" determined for the respective patients.

CONCLUSION

It is shown that the number of residual dsbs measured in confluent human fibroblast lines can be used to predict the cellular radiosensitivity after delayed but not after immediate plating and also not to predict the excess risk of fibrosis of the respective breast cancer patients.

Cell recovery kinetics for split-dose, multifractionated and continuous irradiation in the DSB model

The recovery kinetics for split-dose, multifractionated and continuous irradiations were considered in the framework of the DSB model (Ostashevsky 1989) with the assumption of a cooperative type of DSB repair. Two types of in vitro split-dose experiments can be distinguished on the basis of the time of cell plating: (i) IP, where cells are plated immediately after the second dose; and (ii) DP, where cells are incubated for a long period after the second dose before plating. The model predicts that the IP split-dose recovery kinetics depend mainly on the first dose and are not described by a single exponential curve. The rate of these recovery kinetics is faster than that of DSB repair. In contrast, the DP split-dose recovery kinetics are dose-independent and described by a single exponential curve, the time constant of which coincides with that for DSB repair. The equation for cell survival after multifractionated and continuous irradiations, derived in this work, are different from those in other models (e.g. incomplete repair (IR)) and represent an alternative which is worth testing. The values of the repair time constant estimated from these equations are at least 1.5-2-fold longer than those estimated from the IR equations applied to the same experimental data.