Physics of rotating gamma systems for stereotactic radiosurgery.

PURPOSE The purpose of this study was to evaluate the characteristics of a new rotating gamma system for stereotactic radiosurgery by comparison with a well accepted system. METHODS AND MATERIALS A novel gamma unit for stereotactic radiosurgery has been developed and distributed to 15 hospitals in China. The unit contains 30 cobalt-60 gamma radiation sources with initial activity of 200 Ci (7.4 x 10(12) Bq) each. The sources are positioned along 30 arcs, and rotate continuously as a group in an axis orthogonal to the patient's body. Measurements have been made on a representative unit installed in the Auhai Radiosurgery Center at the Beijing Navy General Hospital in the People's Republic of China. Ionization chambers calibrated by an American accredited dosimetry calibration laboratory were used for these measurements, as well as radiochromic film and thermoluminescent dosimeters. The unit tested utilizes collimators of nominal diameters of 4, 8, 14, and 18 mm. Radiochromic film samples from a Leksell Model U Gamma Knife were evaluated by the same laboratory and are presented for comparison. The treatment planning system was not evaluated. RESULTS Radiation-absorbed dose rates and profiles measured for this unit are comparable to those previously measured with the same techniques for the Leksell Model U Gamma Knife units in San Diego and Atlanta. CONCLUSION This unit is capable of producing well collimated beams of high energy photons, suitable for stereotactic radiosurgery. It has similar physical characteristics to those previously reported for the Leksell Model U Gamma Knife unit.