Purpose: Despite new technology offered through BRAVOS afterloader from Varian, the issue of discrepancy between planned source dwell position and delivered in the ring applicator still exists. Purpose of our study was to propose a methodology and quantify the offset due to snaking effect in ring applicators before clinical use.
Methods: 6 Ring applicators (2 sets of 30/45/60 degrees) from Eckert & Ziegler were used. The applicators are taped to Gafchromic™ EBT3 film and irradiated to 400MUs with a 16MeV electron beam on a linac with the x-ray marker BV 320mm inserted. A treatment plan was created and delivered with 10 equally spaced 1cm source dwell positions at a nominal dwell time of 2 seconds. Combination of applicator image and dwell positions in same radiograph helps in confirming the structural integrity and identifying the tip of the lumen better to compare the expected vs delivered source dwell positions. Films were scanned using Epson Expression 11000XL scanner and the analysis was done in Eclipse Brachytherapy 2D Entry.
Results: Arc length was measured from the center of each delivered source/dwell position to the center of the ring. Subtracting with 0.25 (source center from distance tip) gives the path length. Subtracting the path length with applicator length (150 cm) will give us the shift in delivered source position. Results show an average of 5mm (σ +/-0.6mm) difference between planned and delivered source dwell position. The results also revealed that the offset increases with the angle of arc length. Applying a consistent correction offset of 4mm to all planned dwell position improved the positional accuracy to less than 1mm (σ= +/-0.07mm).
Conclusion: The data from this study warrants the need for robust applicator commissioning. Quantifying one consistent offset for ring applicators instead of multiple offset values will improve efficiency/workflow without compromising the safety/delivery accuracy.