Biology of high single doses of IORT: RBE, 5 R’s, and other biological aspects
PURPOSE Determine the radiobiological effectiveness (RBE) for low-energy X-rays (average energy of 23 KeV) produced by the Photon Radiosurgery System (PRS). METHODS AND MATERIALS RBE values were assessed by comparison with survival data obtained for cells irradiated with either low-energy X-rays from a GE Maxitron 100 machine or high-energy photons from a clinically used Varian 6 MV LINAC. The output of the GE and PRS sources was determined using Baldwin-Farmer and Markus thin window ionization chambers calibrated with 50 kVp X-rays and cross-checked against figures supplied by Photoelectron Corporation. The dose-rate for the PRS was 1.2 Gy/min at a distance of 35 mm with a field flatness of +/-2%. RESULTS The RBE for the PRS low-energy X-ray source (at 1-mm depth) was greater than either the GE or Varian machines and varied with cell survival. For Chinese hamster ovary (CHO) cells, the PRS was 1.25 and 3.3 times more effective than 90 kVp X-rays and 6 MeV photons at 0.5% cell survival, respectively; by comparison, the PRS was 1.2 and 1.9 times more effective at 0.05% cell survival, respectively. Similar RBE values of 1.4 and 1.2 were obtained for human U373 and T98 glioblastoma cells grown in vitro irradiated with the PRS or GE sources, respectively. Other studies showed that the RBE for the PRS low-energy X-ray source increased with depth. The RBEs for the PRS source at 1-mm and 4-mm depth were 1.2 and 2.5 (0.5% survival) and 1. 2 and 1.9 (0.05% survival). CONCLUSIONS The biological and physical properties of the PRS low-energy X-rays offer, under the right conditions, a significant advantage for patient treatment over conventional external beam, stereotactic, or brachytherapy treatment.