A new approach to measure dwell position inaccuracy in HDR ring applicators – quantification and corrective QA

As part of quality assurance (QA) in high dose rate brachytherapy, it is necessary to verify that the source dwell positions correspond to the radiographic markers used in simulation and treatment planning. The procedure is well established for linear tandem applicators. However, with the advent of ring applicators, this has become more critical and challenging. This work describes a new approach to determine positional inaccuracies for ring applicators in which the dummy markers are imaged just once and their dwell positions characterized with respect to an applicator-defined axis. The radiograph serves as a reference for dummy markers for comparison with all subsequent measurements in which the active sources are autoradiographed at different offsets - thus obviating the back-and-forth transferring of setup between afterloader and simulator. The method has been used specifically to characterize the Varian GammaMed 60° ring applicator, but it may be adapted to any other applicator. The results show that an offset of 1-2 mm minimizes the overall inaccuracy to within ± 2 mm.