Purpose: Commissioning of HDR applicators requires dwell position verification. This is particularly challenging for ring applicators, which are known to have unreliable offsets of the distal dwell due to the sharp curvature of the ring channel. Historically, this verification was performed via autoradiograph: first by imaging the applicator with an inserted dummy source of nominal dwell positions and then by delivering an HDR plan at the same expected dwell positions to radiographic film. This required the use of film processors, which have since become largely extinct in radiation oncology departments. Therefore, this work aims to investigate a novel and accessible method to generate an autoradiograph with a linac in order to commission a ring applicator.
Methods: Varian 45o plastic and titanium ring applicators were affixed to Gafchromic EBT3 radiochromic film and secured using a clamping device. In order to determine the optimal energy/MU settings, it was irradiated with 6, 12, and 18MeV electrons at MU=100, 500, 1000 and 1500 using a 10x10 cone and SSD=106cm. The applicator/film geometry was then carefully transferred for HDR plan delivery with 10s per dwell for plastic and 15s for titanium, respectively. Films were qualitatively assessed for resolution and quantitatively analyzed for contrast.
Results: Plastic and titanium applicator image resolution improved with increasing energy and contrast increased with MU. For the titanium applicator contrast/MU plateaued above 1000MU and showed no further improvement (1000MU: 0.07, 1500MU: 0.07).
Conclusion: The method presented demonstrates a novel and efficient approach to generate an autoradiograph, thereby enabling accurate dwell position verification during applicator commissioning. It was evident that appropriate energy/MU selection will vary with applicator material/thickness and is the subject of future investigation.