Depopulation and repopulation of the R1H rhabdomyosarcoma of the rat after X-irradiation

Experiments were carried out to study the kinetics of depopulation and repopulation of the solid transplantable rhabdomyosarcoma R1H of the rat following local irradiation with a single X-ray dose of 15 Gy. Several parameters were sequentially measured over a time interval of 25 days after irradiation: The ratio of tumour to host cells was determined by flow cytometry; the numerical density of tumour cells was obtained by stereological analysis of histological slides; the clonogenic fraction of tumour cells was assayed by plating an appropriate number of tumour cells and scoring the colonies; tumour volume was assessed by measuring two tumour diameters at right angles to each other. All parameters investigated, except tumour volume, undergo drastic changes during the first 2 weeks after irradiation. From the directly measured parameters the following values and their variation with time could be derived. The number of host cells per tumour increased by a factor of 10 within the first 10 days after irradiation, probably due to infiltration by blood-borne host cells. During the same time interval, the number of tumour cells decreased by a factor of 5, whereas the total number of cells per tumour showed an increase by a factor of 4. Since the host cells are considerably smaller than the tumour cells, the cellular numerical density increased by a factor of 3, but approached the control level by Day 18 after irradiation. From the number of clonogenic and non-clonogenic tumour cells the kinetics of repopulation and depopulation was obtained. Repopulation of irradiated tumours by surviving tumour cells as well as removal of inactivated tumour cells began immediately after irradiation and proceeded with exponential kinetics. Repopulation occurred with a doubling time of 4.4 ± 0.3 days whereas inactivated tumour cells disintegrated with a halving time of 3.5 ± 0.7 days. There were no indications that proliferation of doomed cells contributed significantly to tumour growth after X-irradiation.