Absence of contact effects in irradiated EMT6-Rw tumors.

Some cells have been reported to show greater resistance to drugs or radiation when growing with close intercellular contacts in spheroids or in solid tumors than when growing with few intercellular contacts in sparse cultures. In some cases this increased resistance reflects an increased capacity of cells in close contact to repair cytotoxic damage. However, not all tumors show contact effects, and in some tumors and spheroids the increased resistance appears to be produced by environmental factors, such as hypoxia, rather than by changes in the repair capacity of the cells. To assess whether EMT6-Rw cells showed increased intrinsic radioresistance when grown as solid tumors, we compared survival curves for cells in exponentially growing monolayers and in solid tumors in BALB/c mice. To avoid complications arising from regional heterogeneity in oxygenation within solid tumors, these irradiations were performed under conditions of uniform, maximal hypoxia. The two survival curves were indistinguishable. Moreover, survival curves for cells suspended from solid tumors, plated at low densities and irradiated immediately, after 5 h of incubation or after 24 h of incubation, were indistinguishable from one another and were indistinguishable from survival curves for cells suspended from exponentially growing monolayers and irradiated immediately using an identical protocol. It therefore appears that contact effects are insignificant for irradiated EMT6-Rw tumors and that the intrinsic radiosensitivity of these cells is similar in culture and in solid tumors.