Calibration in water versus calibration in air for cobalt-60 gamma rays.
In the United States it is common practice to calibrate Cobalt-60 teletherapy machines "in air," despite recommendations by the International Commission on Radiation Units and Measurements (ICRU) and other organizations that calibration be accomplished by measurement at 5-cm depth in a water phantom. A comparison has been made between the results of ionization measurements in air at 80.5-cm distance from the source and in water at 80-cm source-skin distance (SSD) for the determination of absorbed dose at three depth (5, 10, and 15 cm) for each of three fields sizes (6 X 6, 10 X 10, and 20 X 20 cm2), for a total of 42 Cobalt-60 machines. The mean of the ratio, absorbed dose from in-water measurements to absorbed dose at the same depth calculated from in-air measurements, ranged frt 5-cm depth for a 20 X 20-cm2 field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.
D000388:Air; D003037:Cobalt Radioisotopes; D005720:Gamma Rays / Q000592:standards*; D011839:Radiation, Ionizing / Q000592:standards*; D011867:Radioisotope Teletherapy / Q000295:instrumentation / Q000592:standards*; D011879:Radiotherapy Dosage / Q000592:standards*; D014867:Water
D003037:Cobalt Radioisotopes; D014867:Water