Purpose: To quantify the uncertainties associated with the first dwell position given by the CT and MR markers used for HDR brachytherapy treatments and to compare the intended source position with the actual position of the source, using an autoradiographic-free approach.
Methods: In-house QA devices were created using decommissioned HDR GYN tandem and ring applicators. Ring applicators of diameters 26mm, 30mm, and 34mm and 60mm tandem applicator were used for the QA devices. The ring applicators were modified by removing their top cap and replacing it with a 1mm thick transparent plastic plate and 2cm segment was removed from the tip of the tandem. This allowed for proper visualization of both the CT, MR marker tips, and the real source position. A cellphone was used to acquire photographs of each applicator/marker combination to be analyze. For the real source, the Elekta Flexitron system is equipped with a high-resolution camera system that was used to acquire photos of the source.
Results: Two observers independently measured the arc length of the marker or source path traveled. We found that each observer found similar results (SD < 0.14mm). The observed variations of the first dwell positions from multiple runs were 0.04mm, 0.08mm, and 0.05mm for ring applicators of diameters 26mm, 30mm, and 34mm, respectively. The SD of the observed first dwell positions for tandem was 0.08mm. The offsets of the first dwell positions indicated by the CT and MR markers were accurately determined for all ring and tandem applicators. We also observed the nontrivial (up to 5mm) but consistent and predictable offsets between the observed and the expected dwell positions in all ring applicators.
Conclusion: This work provides the user with an autoradiographic-free approach for the commissioning and QA of Elekta MR compatible HDR tandem and ring applicators.
Not Applicable / None Entered.