Response of multicellular tumor spheroids to multiple fractions of negative pi mesons and x rays

Two cell lines in the form of multicellular tumor spheroids (MTS) were used as test systems to compare fractionated negative pi meson (pion) radio-therapy (10 daily fractions in 11 to 14 days) with single-fraction pions, as well as single and 10-fraction doses of x rays. The pion irradiations were carried out in the proximal and distal regions of a modulated 8 cm pion peak. The results showed: (1) the relative biological effectiveness (RBE) for pion increased significantly with fractionation, and this increase in RBE with fractionation was greater in the cell line with the smaller D/sub q/ in single cell systems; (2) the RBE was the same in the proximal and distal portions of the pion peak; and (3) the amount of reduction in the threshold region with 10 fractions of pions was less than a simple multiple of the amount of reduction of the threshold region with a single fraction of pions