Effects of feeder cells on the X-ray sensitivity of human colon cancer cells.

The survival responses to 250 kVp X-irradiation of 20 different exponentially growing human colon tumor lines have been described using the linear-quadratic equation of cell survival. Because some of these tumor lines (6/20) showed statistically significant increases in colony forming efficiency (CFE) when feeder cells (FCs) were added (10(5) FCs/60 mm dish), radiation survival parameters were determined for all 20 tumor lines with and without added feeder cells. In neither FC independent nor FC dependent lines did addition of FCs significantly affect any of the derived radiation survival parameters, including the alpha and beta inactivation constants, the mean inactivation dose (D, Gy), or the surviving fraction of cells at 2 Gy (S2). The average alpha, beta, and S2 values for these 20 human colon cancer cell lines with added feeder cells were: 0.281 Gy-1, 0.0711 Gy-2, and 0.443. A cumulative frequency distribution plot of the mean inactivation dose (D) which includes other published radiobiological data on human colon cancer cell lines (N = 26) indicates that the D50 value is 2.17 Gy.