Comparison of NIST and manufacturer calibrations of 90Sr+90Y ophthalmic applicators.

Since the resumption of the NIST calibration service for 90Sr+90Y beta-particle ophthalmic applicators, 65 sources have been calibrated using the revised technique [C. G. Soares, Med. Phys. 18, 787-793 (1991)]. For 59 of these sources, the manufacturer's calibration results were available for comparison to the NIST calibration results. The 59 sources represent eight different manufacturers, only one of which is still selling new sources. Manufacturer calibration dates range from the present back to 1954. The results of the comparisons are present, broken down by both manufacturer and calibration date; there are interesting and significant trends in both, with average differences of 20% not uncommon. The obsolete unit, "roentgen-equivalent-beta" (reb), in which some of the manufacturer calibrations are expressed, is discussed, and a factor (0.00982 Gy reb-1) for its conversion to absorbed dose is suggested.