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.

High dose rate (HDR) brachytherapy technique: for carcinoma of uterine cervix using Nucletron applicators.

This study describes our existing treatment method for high dose rate (HDR) brachytherapy of carcinoma of uterine cervix using Nucletron applicators. Based on our clinical experience with low dose rate (LDR) brachytherapy since 1986, we deliver 40-45Gy by external and 30Gy by internal radiation therapy. The change in regimen using HDR brachytherapy is that internal radiation dose is given in 5 fractions at weekly intervals following external radiotherapy. We have analyzed the dosimetry of 20 patients; total of 100 treatments done at our center using ring-tandem (R-T) and ovoid-tandem (O-T) applicator combinations. Since O-T applicator has more flexibility of using desired tandem length and reduced rectum and bladder dose due to internal shielding inside the ovoids, we have made the transition from R-T to O-T combination of applicator. The dose volume histogram of the isodose curve indicates that there is an increase in isodose volume with the O-T applicator by as much as 1.5 times as compared to R-T applicator. In majority of the treatments, rectum and bladder doses are less than 70% of target dose, however in very few treatments, the bladder dose has increased to more than the target dose, in which case, the applicators were reseated and added more packing or reduced the dose per fraction, for better tolerance of late responding normal tissues. All our dose calculations are checked with an independent calculation method and agreement was obtained with in 5-7% discrepancy.(ABSTRACT TRUNCATED AT 250 WORDS)