Comparison of calibration procedures for (IR)-I-192 high-dose-rate brachytherapy sources

Purpose: To compare the efficacy of different calibration procedures for Ir -192 high-dose-rate (KDR) brachytherapy sources and to determine their suit ability in clinical practice. In addition the manufacturer's calibration is compared with our experimental measurements so that the accuracy of the so urce strength on the manufacturer certificate which is supplied with each n ew Ir-192 source can be accessed. Methods and Materials: We compared three types of calibration system: well- type chambers (HDR-1000 and SDS), cylindrical phantom, and plate phantom, T he total number of measurements we obtained was 365, The number of sources used for the calibration procedure comparison was 20 and the number used fo r comparison with the manufacturer's calibration was 46. This study was mad e during the period 1989-1997. Also, Physikalisch-Technische Bundesanstalt (PTB) calibrated one of our sources using their PTB protocol so that the re sults could be compared with our own. Results: The sensitivity of each system on scattering from the room walls w as studied. It was found that different minimum lateral distances from the walls were required for the different systems tested: 15 cm and 25 cm for t he well-type chambers, 75 cm for the cylindrical phantom, and 13 cm for the plate phantom, The minimum thickness required to reach phantom scattering saturation for the plate phantom setup is 24 cm, The influence of the appli cator material used in the calibration setup was found to be 1.7% for the s tainless steel dosimetry applicator compared to the plastic 5F applicator. The accuracy of source positioning within the applicator can lead to dosime tric errors of +/-1.2% for the radial distance of 8.0 cm used with both sol id phantoms, The change in the response for both well-type chambers was onl y 0.1% for changes in the source position within +/-7.5 mm around the respo nse peak. Good agreement was found between all dosimetry systems included i n our study. Taking the HDR-1000 well-type chamber results as a reference, we observed percentage root mean square (RMS) values of 0.11% for the SDS w ell-type chamber, 0.44% for the cylindrical, and 0.60% for the plate phanto m setup. A comparison of our results using the cylindrical phantom with tho se of the manufacturer showed a percentage RMS value of 3.3% with a percent age fractional error range of -13.0% to +/-6.0%. The comparison of our cali bration results with those of PTB gave deviations less than 0.4% for all sy stems. Conclusions: Our results have shown that with careful use of all calibratio n system protocols an accurate determination of source strength can be obta ined. However, the manufacturer's calibration is not accurate enough on its own, and it should be mandatory for clinics to always measure the source s trength of newly delivered Ir-192 brachytherapy sources. The influence of t he applicator material, metal or plastic, should always be taken into accou nt. (C) 1999 Elsevier Science Inc.