HDR source calibration methods and discrepancies.

PURPOSE: To report the calibration discrepancies observed in HDR sources and our methods of calibration of a new source. METHODS AND MATERIALS: 21 sources have been calibrated since March 1991 to February 1996 and discrepancies are reported in comparison to the certificate value. The calibration methodology used for two different systems: the NEL Farmer system and reentrant well chamber (HDR-1000) are described. A lead shield insert developed for use with the well chamber for timer linearity measurement is described. RESULTS: Eleven sources had discrepancy from 0 to 1%, 7 sources had discrepancy from 1 to 5%, and 3 sources had discrepancy from 5 to 10%. The maximum discrepancy observed was -8.04 % for one source. Our procedure of dealing with situations when discrepancy exceeds the range of +/-5% value is discussed. CONCLUSION: (a) The discrepancies that we observed in our study are compared with other users and found to be within the range of values observed. (b) Due to the easy set up procedure, less prone to error in the use of well chamber and with the use of a new lead insert, we conclude in our case that, the well system can be used for monthly calibration of HDR source and timer linearity measurements. (c) Because the Farmer system has a first generation calibration factor, obtained by interpolation of two calibration energies from ADCL, we prefer to use the Farmer system for determining the activity of the new HDR source.