COOPERATIVE EFFECTS OF PHOTODYNAMIC TREATMENT OF CELLS IN MICROCOLONIES

Microcolonies of 2-8 Madison-Darby canine kidney cells (MDCK II) and C hinese hamster lung fibroblasts (V79) cells were incubated with the ph otosensitizer Photofrin and exposed to light, and the resulting number of dead cells per colony was determined, The distribution of this num ber was found to be incompatible with the assumption that cells are in activated independently, The experimental distributions were significa ntly different from the binomial distribution expected from this assum ption, but in accordance with a model in which an inactivated cell can inactivate adjacent cells with a certain probability, These findings are contrary to the common view that damage caused by radiation is lim ited to the cell in which the primary damage takes place, Our findings clearly indicate some kind of cooperativity between cells treated wit h Photofrin and light.