Patient specific quality assurance ensures the patient’s treatment plan can be accurately delivered. For complex, external beam radiotherapy treatments, this is usually verified by comparing the calculated treatment plan to phantom measurements. However, due to the high dose gradients and associated detector setup uncertainties inherent in brachytherapy, reliable phantom measurements are problematic. This study assesses the use of a water phantom in terms of accuracy, precision, and practicality for the purpose of treatment plan verification in brachytherapy.
A 3D printed jig was designed and fabricated in-house to suspend a brachytherapy applicator vertically within a PTW BeamScan water phantom. A PinPoint 3D ionisation chamber was positioned parallel to the applicator, at a depth in line with the source. Two isodose planes, with 2.5 mm resolution, were then measured both parallel and radial to the source axis. Each 2D scan took approximately 6 minutes.
The 3D printed jig allowed the applicator to be suspended securely and reproducibly within the water phantom. The time required to attach the jig and applicator was negligible compared to that required to fill the water phantom. This indicates this QA procedure may not be feasible for every treatment, but could be practicable for commissioning or verification of a particularly complex treatment. It was observed that the dose distributions parallel and radial to the source axis precisely demonstrate the Monte Carlo derived distributions from the literature.
The dose distribution around a single dwell position was measured using a water phantom primarily used for external beam reference dosimetry. The mechanical precision and accuracy of the detector positioning proved suitable for the measurements of high dose gradients inherent in brachytherapy. For more complex treatments with multiple dwell positions or channels, a single dose point could easily be compared to a calculated dose point in the expected applicator geometry.
Miss Rachael Wilks – Senior Medical Physicist RBWHRachael Wilks is the Senior Brachytherapy Medical Physicist at the Royal Brisbane and Women’s Hospital, currently undertaking a PhD on the subject of 3D printing in brachytherapy. Her focus is on refining complex intracavitary brachytherapy treatments through improvements in the treatment design and associated quality assurance.
AProf Scott Crowe – Senior Research Physicist RBWHScott Crowe is a medical physics researcher at the Royal Brisbane & Women’s Hospital, with appointments at QUT and UQ. His research interests include applications of 3D printing in radiation oncology, the quantitative assessment of radiotherapy treatment quality and complexity, and radiation dosimetry.