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

Abstract

Purpose 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. Methods A purpose-built pressure vessel was constructed previously which could achieve pressures ranging from 74.661 kPa to 106.66 kPa (560 mmHg to 800 mmHg). Three Cesium Blu sources (¹³¹Cs) from Isoray Inc. and three CivaDots (¹⁰³Pd) from CivaTech Oncology Inc. were tested over this pressure range in increments of 2.7 kPa (20 mmHg) 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 101.325 kPa, and averaged with runs of the same combination. The chamber response was also simulated using MCNP6 to validate the experimental results. 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, P_A = k₁(P)^k2, 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. Conclusions 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 ¹⁰³Pd sources leads to unique correction coefficients. The higher energy of the Cesium Blu source with respect to ¹⁰³Pd and ¹²⁵I 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. This article is protected by copyright. All rights reserved