Angular measurement of the Cobalt-60 emitted radiation spectrum from a radiosurgery irradiator

The photon energy spectrum emanating from a Leksell Gamma Knife, Model 2300 4B, was measured between 0.250 and 3.5 MeV with the sources exposed. Measur ements were made using a 2X2 inch NaI detector enclosed in a lead-shielded apparatus having a 1/4 inch diameter measurement aperture, which reduced th e amount of radiation received by the crystal. All measurements were made o ne meter above the floor within a quadrant toward one side of the Gamma Kni fe couch. The measured spectra displayed the expected Co-60 doublet of phot on peaks at energies of 1.17 and 1.33 MeV. These peaks appeared in spectra beginning at similar to 50 degrees, as one proceeds from a paint directly l ateral to the source enclosure (0 degrees) toward the foot of the couch (90 degrees). The average photon energy of the spectrum shifts to lower values as the doublet decreases in magnitude with increasing angle until almost v anishing at an angle equal to 90 degrees. Inserting a 16 cm diameter plasti c sphere phantom, provided with the Gamma Knife, into the radiation beams i ncreases the low energy photon emissions appearing in the spectrum, especia lly for measurements at the foot of the couch. Implications for the design of shielding a treatment room containing the Gamma Knife, Model B, and esti mation of the radiation exposure to personnel during an emergency procedure in the treatment room with the sources exposed are discussed. (C) 2001 Ame rican Association of Physicists in Medicine.