Increased 252Cf therapeutic advantage for an advanced murine lymphoma (LSA) growing in vivo.

The relative biological effectiveness (RBE) was determined for 252Cf irradiation of the murine LSA lymphoma growing in vivo. Irradiation was performed on Days 1-3, i.e. on the young and rapidly growing tumour, on Day 5, i.e. the early stationary phase, or on Day 7, i.e. the advanced tumour. Mice inoculated with 10(5) cells would die on about Day 9 with disseminated tumour if untreated. Irradiation with acute 60Co or low-dose-rate 252Cf total-body irradiation increased the mean survival time (MST). For 252Cf the MST increased linearly with dose, the effectiveness being independent of the day of irradiation. For 60Co, the MST increased linearly with dose for the tumours irradiated on Days 1-3 but two-component curves were noted after irradiation on Days 5-7. Tumour response showed an increasing photon-resistant component which gave no increase in MST with dose. The RBE was approximately 4 for the early tumours but was greater than 5 for the advanced and more photon-resistant tumours. 252Cf was effective for the treatment of early or advanced stages but was especially effective against the advanced, more radioresistant tumours.