Comments on "Age and Radiation Sensitivity of Rat Mammary Clonogenic Cells" (Radiat. Res. 137, 118-123, 1994)

We read with great interest the rapid communication "Age and Radiation Sensitivity of Rat Mammary Clonogenic Cells" (Radiat. Res. 137, 118-123, 1994). In the section Material and Methods it is stated that clonogen survival after a dose of radiation (in this case 5 Gy) was estimated by dividing the AD50 value at zero radiation dose by the AD50 value for the experimental dose. After a comparative analysis, the authors conclude that there was a significant difference in radiosensitivity between 1-4-week-old clonogens and 6-8-week-old clonogens. If Fig. 1 of the text is examined, it will be clear that the total clonogen number grows steadily from 0 to 12 weeks of age, with a plateau at about 45 weeks of age. This is logically reflected in the top panel of the figure, in which the total number of cells needed to be injected in a graft to induce an alveolar unit in 50% of the cases decreases with age, reaching a lower limit at about 4 weeks.