Effect of preirradiation of transplantation site on growth characteristics and hypoxic fractions in human colon tumor xenografts

The volumetric growth curves and hypoxic fractions of seven different human colon tumor lines (clone A, clone D, WiDR, SW480, SW620, DLD-2, and HCT-8) xenografted into the flank regions of either unirradiated nude mice or mice that had received 17.5 Gy of 250-kVp X-rays 1 day prior to implantation were biomathematically analyzed using the Verhulstian equation. Significant variation was found among tumors with respect to both initial growth rates (r, days-1) and theoretical final volumes (carrying capacities, K, mm3). In radiation-damaged normal tissue, tumors grew relatively well for about the first 2 wk postimplantation, attaining volumes of about 70 to 155 mm3. Then, tumor growth rates altered. This effect varied from relatively minor effects on growth rate (tumors of clones A and D) to inhibition of growth, with actual decreases in tumor volume (e.g., WiDr, SW480, SW620, HCT-8, and DLD-2). After this short-term transience in growth kinetics, neoplasms began to steadily regrow at about 3 wk postimplantation, albeit at a slower rate than that seen in controls. Tumor bed effect values were calculated using the ratio of times at which control tumors and tumors growing in the radiation-injured tissue reached a volume of 7.5% of the K values derived from the respective control growth curves. Values for clone D, clone A, and WiDR, SW480, SW620, DLD-2, and HCT-8 tumors were, respectively, 1.89, 2.41, 3.48, 3.62, 2.82, 3.66, and 3.65, indicating that tumor bed effect responses varied by almost 100%, even for cancers of the same neoplastic class. Also, the hypoxic fractions of all tumors growing in radiation-damaged sites were increased as compared with levels in controls.