Cell Proliferation In Emt6 Tumors Treated With Single Doses of X‐Rays Or Hydroxyurea. I. Experimental Results

EMT6 mouse mammary tumors were treated in vivo with 5 mg/mouse of hydroxyurea (HU) or 300 rads of X-rays. The proliferation of the tumor cells was followed for 28 hr after treatment. Changes in the 3H-TdR labeling index, the mitotic index, the specific activity of the 3H-TdR-labeled DNA, and the proportion of suspended, clonogenic cells in the S phase of the cell cycle were examined and compared. Evidence was found for reassortment of the surviving cells in treated tumors into partially synchronous cohorts. The partial synchrony in the proliferation of the surviving cells was not accurately predicted by the changes in the labeling index and the mitotic index. The changes in DNA specific activity proved unacceptable as an indicator of cell proliferation in solid EMT6 tumors treated with low doses of radiation or HU.