Rachael Wilks
RBWH
Medical Physicist

Samuel Peet
Medical Physicist
RBWH

Background: The Varian ring and tandem applicators were purchased in early 2018, with commissioning tests commencing in August.

Purpose: The ring and tandem applicators were to be commissioned for use with interstitial, intrauterine gynaecological treatments. While the tandems agreed with the expected geometry and source positions, the source positions in the rings did not match those planned by the treatment planning system.

Method: An investigation into the dwell positions within the rings was implemented. To mark the end of the applicator, a radiograph of a marker wire within the plastic rings was obtained using a superficial kV unit and radiochromic film. Without altering the position of the applicators on the film, the assembly was transferred to the HDR unit. An autoradiograph was then taken by driving the source to several source positions. Delivered source positions were then compared to those planned.

Results: It was observed that the source positions, relative to the end of the applicator, did not coincide with those predicted by the planning system. Additionally, the error in the position varied with distance along the applicator, preventing a constant offset to be applied during planning as recommended by the manufacturer.

Conclusions: The ring applicator has not been released for clinical use, and further testing will be undertaken.


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