The hazard of accelerated tumor clonogen repopulation during radiotherapy.

It is suggested that, on average, clonogen repopulation in squamous cell carcinomas of the head and neck accelerates only after a lag period of the order of 4 +/- 1 weeks after initiation of radiotherapy and that a dose increment of about 0.6 Gy per day is required to compensate for this repopulating.