Cobalt-60 dose calibration: a comparison of methods.

Cobalt-60 dose determination can be accomplished by a variety of methods. This study investigates dose determination by actual "at-depth" measurement for the depths and field sizes desired as well as by "in-air" or "in-water" calibration measurements for both constant SSD and isocentric techniques. Comparison of the above dose determination methods reveals the existence of both depth-dependent and field size-dependent dose discrepancies between the individual methods studied. For many teletherapy units, the source of these discrepancies lies in the inapplicability of the tabulated percent depth dose and tissue-air ratio values employed. Appropriate percent depth dose and tissue-air ratios can be obtained by performing a series of at-depth dose determinations for a variety of depths and field sizes. When the appropriate values are employed, excellent agreement is found between all of the dose determination methods investigated.