[Experimental studies of multiple fractions per day (MFD) at smaller doses than 2 Gy]

Effects of twice daily fractionations (B.I.D.) at smaller doses per fraction than 2 Gy on mammary carcinoma of C3H/He mouse (alpha/beta ratio = 15-20 Gy) were studied. Sixty percent doses of conventional fractionations (Q.D.) were irradiated twice daily with 4 hours interval. Though B.I.D. at 120 R, 150 R were less effective than Q.D. at 200 R, 250 R, B.I.D. at 180 R were more effective than Q.D. at 300 R, equivalent to the increments of the total doses. B.I.D. at smaller doses than "flexure dose" (not equal to 0.1 x alpha/beta) seemed to be less effective for tumors. Human tumors transplanted to nude mice might be useful to study therapeutic gain ratio.