EXPERIMENTAL RADIOTHERAPY OF A RAT RHABDOMYOSARCOMA WITH 15 MEV NEUTRONS AND 300 KV X-RAYS .I. EFFECTS OF SINGLE EXPOSURES

Experiments have been carried out to evaluate various responses of a rhabdomyosarcoma transplantable in an inbred strain of rats to different single doses of 300 kV X-rays and 15 MeV neutrons. Survival curves were derived by a direct in vitro cloning technique for cells irradiated in vitro or in vivo. In addition, growth delay and cure rates were determined and the rate of repopulation of the tumour by surviving cells was measured. The results demonstrate that for cells irradiated in vitro the survival curves for 300 kV X-rays can be described approximately by a mean lethal dose D0 equal to 120 rad and an extrapolation number N=10 for cells in equilibrium with air, while a D0 of 295 rad and N=10 were derived for cells in hypoxic conditions. For 15 MeV neutrons the corresponding values are D0=85 rad, N=3 and D0=120 rad, N=3 for oxygenated and hypoxic conditions, respectively. For tumours irradiated in hypoxic conditions, the survival curves of the constituent cells can be represented by D0=305 rad, N=10 and D0=120 rad, N=3 for 300 kV X-rays and 15 MeV neutrons, respectively. The survival curves for cells from tumours irradiated in living animals indicate that approximately 15% of the cells are severely hypoxic. The measurements of proportion of tumours cured indicate that a TCD 90 is attained at 6700 rad of 300 kV X-rays, in good agreement with an expected value of 6500 rad obtained by extrapolation of the survival curve. For 15 MeV neutrons proportion of tumours cured could not be measured due to the low output of the present generator, but extrapolation of the survival curve yields a TCD 90 of 2400 rad. The RBE of 15 MeV neutrons for a TCD 90 is equal to 2·8. Measurements of growth delay show that the RBE of fast neutrons is equal to about 3·3 over a wide range of doses. Measurements of changes in the number of clonogenic cells in irradiated tumours show that after an initial lag period, surviving cells repopulate the tumour at a high rate, corresponding to a doubling time of 30 hr, which is a factor of 3 larger than the rate of increase of cells in an unirradiated tumour, corresponding to a doubling time of about 96 hr. © 1969.