Studies of the Cellular Radiosensitivity of Transplants of Murine Ependymomas Irradiated in Vivo

Summary

Cell-survival curves for in vivo tumour irradiation have been obtained for two subcutaneously transplantable murine ependymomas, using end-point dilution assays in isologous hosts.

No significant transplantation immunity could be detected in either tumour system, but both exhibited a ‘Révész effect’.

The TD₅₀ for No. 15 tumour was about 300 unirradiated cells in the initial assays, but in a later series this was reduced to 20. The TD₅₀ of tumour No. 48 was about 100 cells. The survival curves gave D₀ = 260 rads for tumour No. 15, and D₀ = 288 rads for tumour No. 48, suggesting that a large fraction of the viable tumour cells were hypoxic in vivo.

For unirradiated tumours, doubling-times of 21 hr for No. 48 were estimated both from the latency data and from caliper measurements of small tumours. After high radiation doses (3500 rads), these values were increased to as long as 5 days, although such long doubling-times were not always maintained as the tumours increased in size.