Failure to produce greater than additive sister chromatid exchange induction with X rays and BCNU

In 1984, Tofilon and Deen reported that X irradiation of 9L cells immediately after treatment with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) produced greater than additive induction of sister chromatid exchanges (SCEs) compared to the sum of SCEs induced by each agent alone [Radiat. Res. 97, 171-177 (1984)]. We (C.P.S., D.F.D.) repeated experiments conducted by Tofilon and Deen, scored SCEs in a blind manner, and were unable to reproduce the original findings. Instead, we found that X rays and BCNU induced SCEs in an additive manner. The slides prepared in the original study are extant; they were coded and recounted blind. Data obtained in this reevaluation do not substantiate the previous report. We conclude that the original findings were apparently the result of bias introduced in the SCE scoring process. Our experience with the SCE assay emphasizes the need to recode SCE slide preparations and count SCEs blind.