Changes in TCD 50 as a measure of clonogen doubling time in irradiated and unirradiated tumors

Abstract

Dose-cure experiments have been carried out on a moderately well differentiated murine mammary carcinoma, designated MCA-4, at different stages of growth after tumor-cell inoculation or after 8 mm established tumors had been exposed to 60 Gy. TCD 50 assays were performed at 1, 3, 7, 14, or 21 days after tumor cell inoculation, or when tumors reached a size of 6 or 8 mm. Likewise, TCD 50 assays were performed at 0.25, 1, 3, 5, 8, 12, 16, or 21 days after 8 mm tumors had been exposed to a 60 Gy priming dose, or when the recurrent tumors reached 6 or 8 mm. All irradiations were performed under hypoxic conditions. The TCD 50 (95% confidence limits) was 64.0 (61.7–68.3) Gy for the 6-mm and 71.9 (70.1–73.9) Gy for the 8 mm tumors, and these values were unaffected by preirradiation. Direct analysis was used for the simultaneous estimation of D 0, clonogen number, and clonogen doubling time from the pooled data. There was no significant difference between D 0 estimates for the preirradiated and control tumors, and the pooled estimate was 10.6 (9.6–11.8) Gy for tumors assayed at specified time points where the size was unknown. This is clearly higher than in tumors of known size [estimate for 6- and 8-mm tumors: D 0= 5.4 (4.5–6.6) Gy] owing to size and other heterogeneity. The clonogen doubling times (T clon) were 3.4 (3.0–4.0) days in the preirradiated tumors and 5.8 (4.9–7.1) days in the unirradiated tumors. It is not unreasonable to assume that the systematic error due to heterogeneity was approximately the same for D 0 and T clon (since variable clonogen number is likely the predominant source of heterogeneity), and thus the ratio of Do for tumors of unknown sizes (10.6 Gy) and D 0 for tumors of known sizes (5.4 Gy) can be used to “correct” the T clon estimates, with the result that T clon (preirradiated) = 1.7 days and T clon (unirradiated) = 3.0 days. We conclude that the clonogen doubling time was shorter in tumors exposed to a single high-dose irradiation than in unirradiated controls, which implies the existence of faster cell repopulation in irradiated tumors.