[Experimental study of tumor response to twice-a-day irradiation at smaller doses than 2 Gy]

Tumor response to twice-a-day irradiation (BID) at smaller doses than 2 Gy on mammary carcinoma of C3H/He mouse (alpha/beta ratio = 18 Gy) was studied with comparison of once-a-day irradiation (QD) in the same overall treatment time. The dose per fraction (dBID) was ranged from 110R to 162R in BID, and the dose per fraction (dQD) was ranged from 200R to 300R in QD fractionation (1R = 0.91 cGy). The number of fraction was 10 or 15 in QD, and 20 or 30 in BID. The interval of fractions was 4 hours in BID. Tumor response was assessed by tumor growth delay. MFD effect ratio, i.e., growth delay time of BID/growth delay time of QD, was significantly smaller than the ratio estimated from FDF formula, (alpha/beta + dBID) x dBID x NBID/(alpha/beta + dQD) x dQD x NQD, with decreasing fraction size. In the case of dBID = 120R and dQD = 200R, estimated ratio was 1.16, but MFD effect ratio was 0.58, regardless of 20% increments of daily dose. To get same or more tumor response, it is necessary to prolong the overall treatment time of twice-a-day irradiation. If the overall treatment time is same, it is estimated that the fraction size of BID equivalent to the fractionation of daily 200R is presumably 135R to this mouse tumor.