Cure, cell killing, growth delay and fragmentation of X-irradiated human melanoma HMV-I multicellular spheroids

Human melanoma, HMV-I, multicellular spheroids were irradiated and cure was determined by the absence of cellular outgrowth. Their cellular radiosensitivity was measured by the colony-forming ability of cells dispersed from the spheroid. Analysis of radiocurability of spheroids in terms of their cellular radiosensitivity predicted three necessary conditions: a linearity of dose versus the double-minus logarithm of curability; constancy of a critical cell number; and constancy of cellular radiosensitivity. These conditions were found to exist in the observed data for each of three size classes of spheroids. Analysis suggests that cellular radiosensitivity in multicellular spheroids with diameters of 250 and 400 microns was different from that of monolayers, and that the increase of spheroid-control doses was found to be a function of cellular radiosensitivity, total cell number per spheroid and a critical cell number. The critical cell number increased from 0.8 in a 150 microns spheroid to 4 in a 250 microns spheroid and to 57 in 400 microns spheroid. This number is a unique characteristic of multicellular systems and is one important factor in determining their radiocurability. X-ray-induced growth delay of spheroid size was increased with increasing dose. At high doses a sharp increase in delay time was seen, sometimes accompanying fragmentation of spheroids at late postirradiation times. The clonogenic activity of these fragments may serve as a model of exfoliation, the first step of radiation-induced metastasis.