Beam quality correction factors for dose measurements around 192Ir brachytherapy sources.

Abstract

Purpose: To provide beam quality correction factors ( $k_{Q,Qo}$ ) for detectors used in ¹⁹²Ir brachytherapy dosimetry measurements.

Materials and methods: Ten detectors were studied, including the PTW 30013 and Exrading12 Farmer large cavity chambers, seven medium (0.1-0.3 cm³) and small (< 0.1 cm³) cavity chambers, and a synthetic microdiamond detector. The k_Q,Qo correction factors were calculated at distances from 1 to 10 cm away from an ¹⁹²Ir source, using the EGSnrc Monte Carlo (MC) code. All detectors were calibrated in a ⁶⁰Co 10 × 10 cm² reference field provided by standard calibration laboratories. The impact of the central electrode, stem and wall on the detectors' responses in ¹⁹²Ir photon energies was investigated. An experimental procedure was additionally applied for dose measurements around a microSelectron-v2 ¹⁹²Ir high dose rate (HDR) brachytherapy source using a motorized water phantom.

Results: Farmer chambers underestimated the dose near the source due to signal volume averaging effects, resulting in k_Q,Qo values ranging from 1.177 and 1.317 at 1 cm, decreasing with distance to between 0.980 and 1.005 at 10 cm. Small cavity volume detectors should be used close to the source. The k_Q,Qo for the studied small and medium cavity volume detectors were found to be close to unity (within 1.3%), showing also a small dependence on source-to-detector distance, except for ion chambers containing high-Z materials in their construction. The presence of high-Z materials caused an overresponse in these detectors, resulting in k_Q,Qo values ranging from 0.950 at 1 cm to 0.729 at 10 cm away from the source. A dose rate constant of (1.114 ± 0.023)cGyh^-1U^-1 was found in agreement with the literature (within 0.5%).

Conclusions: k_Q,Qo values were calculated for dose measurements around ¹⁹²Ir brachytherapy sources. Farmer chambers should be preferred for measurements at increased distances, whereas small or medium cavity volume detectors, not containing high-Z materials, should be used close to the source.