Comparison of radiobiologic modeling for one- and two-isocenter dose distributions applied to ellipsoidal radiosurgery targets

Purpose: RTOG protocol 90-05 determined the maximum acutely tolerated dose of single-fraction radiosurgery in patients receiving prior fractionated la rge volume cranial irradiation. Data from 90-05 have suggested that patient s treated with a gamma unit, compared to linac-based therapy, have a tumor control advantage and lower rates of severe complications. This study was p erformed to investigate the radiobiologic effect of using one vs. two isoce nters in single-fraction radiosurgery of ellipsoidal targets. Methods and Materials: For a series of ellipsoidal targets that varied by v olume and radiosensitivity, single and two-isocenter treatment plans were g enerated to approximate those typically employed for gamma unit and linac r adiosurgery. Tumor control probabilities (TCP) and normal tissue complicati on probabilities (NTCP) were generated automatically by the treatment plann ing system based on established parameter values. Results: The modeling data showed that multiple-isocenter plans resulted in improved TCP with equivalent or lesser NTCP, particularly for larger, radi oresistant targets. Multiple-isocenter plans reduce the amount of normal ti ssue that receives high dose, Also, areas within the tumor receive signific antly higher doses than the prescription dose, which contributes to increas ed tumor cell inactivation. Conclusion: For ellipsoidal targets, radiobiologic modeling data are consis tent with the clinical findings of the RTOG 90-05 trial, as they predict im proved outcome with a multiple-isocenter plan relative to a single-isocente r plan. The benefit is most apparent with increasing target volume and decr easing tumor radiosensitivity. (C) 2001 Elsevier Science Inc.