The effect of differences in data base on the determination of absorbed dose in high-energy photon beams using the American Association of Physicists in Medicine protocol.
Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.
Mijnheer
B J
BJ
Joint Center for Radiation Therapy, Harvard Medical School, Boston, Massachusetts 02115.
Chin
L M
LM
D003037:Cobalt Radioisotopes; D005720:Gamma Rays; D006077:Government Agencies; D055585:Physical Phenomena; D010825:Physics; D011829:Radiation Dosage / Q000592:standards*; D012955:Societies, Medical; D014481:United States
D003037:Cobalt Radioisotopes