Cell Population Kinetics of the Rhabdomyosarcoma R1H of the Rat after Single Doses of X-rays

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Abstract
The kinetics of depopulation and repopulation of the solid transplantable rhabdomyosarcoma R1H of the rat following local irradiation with single subcurative X-ray doses of 7·5, 15 and 30 Gy was studied. Several parameters were sequentially measured over a time interval of 4 weeks after irradiation: the ratio of the number of tumour to host cells, and the cellular DNA content of tumour and host cells, were determined by flow cytometry; the amount of DNA per gram of tumour tissue was determined biochemically; the clonogenic fraction of tumour cells was obtained from in vitro colony assay; and the tumour volume was assessed by in situ caliper measurements. From the amount of DNA per gram and the average DNA content per cell, the total number of cells per gram of tumour tissue was obtained. From this and the other parameters measured, the number of clonogenic tumour cells, non-clonogenic tumour cells and nucleated host cells per tumour, as well as their variation with time and dose, could be derived. The results showed that there was a lag period prior to depopulation amounting to 3·8 ± 1·4, 1·4 ± 0·8 or 0 ± 0·7 days for 7·5, 15 or 30 Gy, respectively. The rate of depopulation of non-clonogenic tumour cells increased with dose; the halving times of non-clonogens were 4·7 ± 1·8, 2·6 ± 0·7 or 2·1 ± 0·4 days for the three doses applied. There were no indications that proliferation of doomed cells contributed significantly to tumour growth after irradiation. After lag periods that were similar in length to those prior to depopulation, a massive immigration of host cells was observed. Under certain conditions more than 97 per cent of the cells present in irradiated tumours were found to be of host origin. There was a lag period before the onset of repopulation by clonogenic tumour cells, the length of which increased from 2·7 ± 0·7 to 5·0 ± 0·8 or 6·3 ± 1·0 days for 7·5, 15 or 30 Gy, respectively. The initial rate of repopulation increased with radiation dose; after the end of the lag period the doubling time of clonogenic tumour cells (in controls amounting to 3·7 ± 0·2 days) was 3·1 ± 0·1, 2·1 ± 0·1 and 1·1 ± 0·1 days for the three doses applied. Nevertheless, all repopulation curves could be described by one particular Gompertz function (whose parameters also give a good fit to the volume growth of the undisturbed tumour), indicating that the rate of repopulation corresponds to the growth rate of untreated tumours that contain a comparable number of clonogens as are left in the irradiated tumours.