The reproducibility and relationship between single and fractionated dose estimates of the surviving fraction at 2 Gy

Purpose: To evaluate the reproducibility and relationship between the surviving fraction at 2 Gy obtained from single-graded dose and multifraction survival curve assays, Methods and Materials: Single-graded dose and multifraction survival curve assays were concurrently performed in five human cell lines, For the multifraction studies, five to six doses at 2 Gy per fraction were administered with a time interval of 5.5 h between fractions. All surviving fraction data was corrected for multiplicity and cell proliferation during treatment, Replicate experiments were performed for each cell line. Results: The precision of the Sf2 estimates obtained from multifraction studies was approximately three times greater than was obtained in the single-dose studies. For the single-dose studies, the average difference between the Sf2s obtained in the initial vs, repeat experiment was 0.11; for the fractionated-dose studies the difference was significantly reduced to 0.032. A rank-order correlation was not obtained between the 2 Gy Sfs in the initial and repeat single-dose assays; in the multifraction studies, the correlation was significant (p < 0.05). The rank-order correlation between the single- and fractionated-dose Sf2 assays was significant only when the two sets of assays were pooled; however, the coefficient of correlation remained low (R(2) = 0.50). Conclusions: The precision of the estimates of the surviving fraction at 2 Gy, obtained from multifraction assays, was substantially greater than was obtained in single dose-survival curve assays.