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

PURPOSE

To provide beam quality correction factors ( ) for detectors used in Ir 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 cm) and small (< 0.1 cm) cavity chambers, and a synthetic microdiamond detector. The k correction factors were calculated at distances from 1 to 10 cm away from an Ir source, using the EGSnrc Monte Carlo (MC) code. All detectors were calibrated in a Co 10 × 10 cm reference field provided by standard calibration laboratories. The impact of the central electrode, stem and wall on the detectors' responses in Ir photon energies was investigated. An experimental procedure was additionally applied for dose measurements around a microSelectron-v2 Ir 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 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 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 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)cGyhU was found in agreement with the literature (within 0.5%).

CONCLUSIONS

k values were calculated for dose measurements around Ir 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.