Effect of well chamber altitude pressure corrections for cesium Blu 131 Cs and CivaDot 103 Pd brachytherapy sources.

Abstract

Previous publications have described how the standard temperature and pressure correction will overcorrect measurements with a low-energy photon low-dose rate brachytherapy source at low ambient air pressures. To account for this effect, an additional correction factor is applied after the standard temperature and pressure correction. This additional correction is dependent on the source being measured and the chamber it is measured in. Well chamber corrections for two sources and findings regarding aspects that may affect the altitude response of the sources are presented. A purpose-built pressure vessel was constructed previously, which could achieve pressures ranging from 74.661 to 106.66kPa (560-800mmHg). Three Cesium Blu sources (131 Cs) from Isoray Inc. and three CivaDots (103 Pd) from CivaTech Oncology Inc. were tested over this pressure range in increments of 2.7kPa (20mmHg) in three HDR 1000 Plus chambers, and the Cesium Blu sources were also tested in two IVB 1000 chambers. Both chamber models are air communicating well-type ionization chambers produced by Standard Imaging Inc. Multiple runs of each source/chamber combination were completed, corrected with the standard temperature and pressure correction, normalized to the result at 101.325kPa, and averaged with runs of the same combination. The chamber response was also simulated using MCNP6 to validate the experimental results. Measurements of both sources in all chambers followed the expected power dependence on ambient pressure as seen in previous studies. The Cesium Blu source, however, demonstrated a significant difference in response in the HDR 1000 Plus chamber versus the IVB 1000 chamber. For an altitude correction factor of the form, PA =k1 (P)k 2 , new coefficients are proposed for both sources for pressure units of kPa and mmHg. The Monte Carlo calculated chamber response agreed with the experimental results within 2% for all sources and chambers at all pressures. Altitude correction coefficients for two new low-energy photon low-dose rate brachytherapy sources are provided. The directional dependence of the CivaDot has no bearing on its dependence on pressure; however, the difference in construction materials from other 103 Pd sources leads to unique correction coefficients. The higher energy of the Cesium Blu source with respect to 103 Pd and 125 I sources yields a difference in correction factors depending on which model chamber is used for air-kerma strength calculations. Clinics must be careful to select the correct pair of coefficients for the chamber model they used.