A comparison of three and five times weekly fractionation on the response of normal and malignant tissues of the C3H mouse

A comparison was made of the response of the KHT sarcoma, transplanted into the thighs of syngeneic C3H mice, and the response of the normal tissues of the irradiated leg to two fractionation régimes; six fractions in two weeks and ten fractions in two weeks. The response of the tumour, which was measured by the delay in growth to a fixed size, was found to be consistent with a fractional difference between the two régimes of approximately 11 per cent for dose fractions of between 200 and 600 rads. For the normal tissues, early skin reactions and deformity 48 weeks after irradiation were measured, and the mean fractional difference between the two régimes was 12 per cent for the early and 13 per cent for the late reaction. The values of the “fraction number” exponent in the nominal standard dose equation of Ellis were calculated for both the tumour and normal tissues and found to be consistent with the value of 0·24.