BIOLOGICAL EFFECTIVENESS OF FRACTIONATED DOSE OF PIONS IN MICROSCOPICSCCVII TUMORS - COMPARISON BETWEEN TUMOR-CONTROL DOSE AND TUMOR-GROWTH TIME ASSAYS

The relative biological effectiveness (RBE) of fractionated pions for tumor growth time (TGT) assay changes with the endpoints, so it is ess ential to determine the RBE for tumor control dose (TCD) assay. For th is purpose, the TCD50 of fractionated pions was compared with that of photons, and the RBEs for TGT and TCD assays were concurrently compare d as a function of the effect level, A ''convenient'' RBE (cRBE) was s ubstituted for the RBE when the comparison was made between similar fr actionation schedules with different dose per fraction, SCCVII tumors (2 X 10(4) or 2 X 10(5) cells) were implanted into the feet of C3H mic e and irradiated starting from 2 days after implantation at a total do se range of either 9.6-38.4 Gy pions (2.4-6.4 Gy per fraction) or 14.4 -50.4 Gy photons (3.6-7.2 Gy per fraction) in 2-10 fractions over 56 d ays. The cRBE and the RBE at the iso-effective level of 30 days TGT we re 1.53-1.60 for 2.4-4.8 Gy pions and 1.50 for 4-fractionated pions, r espectively: there were only small differences within these schedules used, However, the cRBE values decreased from 1.60 to 1.15 with increa sing TGT from 30 to 75 days. In contrast, the cRBE values for TCD50 in creased from 1.08 to 1.40 (95% confidence Limits [CL]; 1.18-1.63) with increasing evaluation time from 60 to 100 days: pions significantly i nhibited late tumor appearance. The TCD50 at 100 days was 28.7 Gy (CL; 25.0-32.5 Gy) for pions and 40.3 Gy (CL; 36.3-44.2 Gy) for photons. I n conclusion, the RBE for TCD50 was not predictable from the RBE for T GT assay. The cRBE value of 1.4 for microscopic tumor control was in c lose agreement with the reported values for skin reaction.