Differential effect of hyperthermia and x-irradiation on regrowth rate and tumour-bed effect for a rat sarcoma

The regrowth rate of the rat sarcoma SSB1a was assessed, following treatment with either X-rays or hyperthermia. The growth rate of untreated tumours implanted in pre-irradiated or pre-heated normal tissue was also measured. For treatments giving similar levels of tumour regrowth delay, the regrowth of irradiated tumours was markedly slower than that of untreated tumours, whilst the regrowth rate of tumours subjected to hyperthermia did not differ from that of untreated tumours. The growth rate of untreated tumours implanted in pre-irradiated skin was also slower than that of control tumours, though the growth-rate reduction was less dramatic than for regrowth following tumour irradiation in situ. Tumours implanted in pre-heated skin grew at the same rate as control tumours. Slower regrowth in situ after X-rays than after hyperthermia would produce a larger growth delay for X-rays than for hyperthermia, for equal levels of tumour-cell kill or probability of tumour sterilization. This effect could lead to a systematic underestimation of the efficacy of hyperthermia relative to X-rays, when efficacy is assessed by growth delay.