The Response of a Transplantable Tumor to Fractionated Irradiation: II. Fast Neutrons

Abstract

A transplantable mouse mammary was used to compare the relative effect of single doses, 2 and 5 fractions of X-rays, and cyclotron-produced fast neutrons. The hypoxic cell radiosensitizer Ro-07-0582 [1-(2-nitroimidazole)-3-methoxyisopropanol] was also tested in combination with single doses of fast neutrons. The relative biological efficiency of the fast neutrons was high for all fractionation schedules, but the therapeutic gain factor (i.e., tumor RBE[relative biological effectiveness]/skin RBE) decreased for the fractionated treatments. The relative therapeutic efficiencies for the X-ray and neutron treatments were compared by estimating the tumor delay that was caused by the dose necessary to produce a constant level of skin damage. An advantage of fractionated X-rays relative to single doses was previously demonstrated. All neutron schedules caused more tumor delay than the corresponding X-ray schedules. A single dose of neutrons plus the radiosensitizer Ro-07-0582 was the most effective treatment of all. The effectiveness of fast neutrons was compared with recently published work using X-rays plus the radiosensitizer Ro-07-0582. When a high dose of this drug was used with X-rays, the resulting schedules were similar in efficiency to the fast neutron schedules (without drug), for single doses and 2 and 5 fractions at 2 day intervals.