The influence of pre-irradiation on the survival of two head and neck squamous cell carcinoma lines.

Induction of radioresistant tumor cells by previous irradiations may be a problem because the chance to treat a patient presenting with a recurrence with radiotherapy may be lost if a resistant subpopulation was induced by the initial treatment. We here report on experiments with two squamous carcinoma cell lines where cultures were initially irradiated with graded single doses (2-6 Gy) and were re-irradiated after 4 days with single and split doses (2-6 Gy). Both cell lines showed no alteration in radiosensitivity. Also, the split-dose recovery was not changed by the previous irradiations. Flow cytometric analysis showed no significant differences in cell cycle distribution, so that a possible influence of cell cycle redistribution could be excluded. Although preliminary, the results from this study give no indication that previously irradiated cells become more resistant to radiation. In view of the clinical importance of radioresistant clones in tumours, it seems important to perform similar experiments to those reported upon in this study with several radiation fractions as pre-irradiation treatment, to impose a stronger selection pressure on the surviving cells.