Calibration in water versus calibration in air for Cobalt‐60 γ rays

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Abstract
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 field sizes (6×6, 10×10, and 20×20 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 from 1.031±0.013 at 15‐cm depth for a 6×6‐cm2 field size to 1.009±0.007 at 5‐cm depth for a 20×20‐cm2 field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.

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