Simon Downes
Nelune Comprehensive Cancer Centre, Prince of Wales Hospital, Randwick NSW
Chief Physicist

Maximilian Hanlon
Student
School of Science, RMIT University

Ryan Smith
Senior Medical Physicist
Alfred Health Radiation Oncology, Alfred Hospital, Melbourne

Rick Franich
Professor of Medical Physics
School of Science, RMIT University

Background and Purpose: The Venezia applicator is a new type of gynaecological applicator released by Elekta in 2017. The applicator design has ‘Vienna ring’ geometry but has additional divergent needle positions to offer greater lateral coverage of disease involving the parametrium. Coverage of vaginal disease is also easier with additional catheter fixation. The aim of this work was to commission a Venezia applicator and to assess its reproducibility, and explore possible causes of, observed inconsistencies using advanced source tracking and imaging.

Methods: Initial commissioning consisted of all generic tests relevant for brachytherapy applicators including checks of specified size and construction, imaging and autoradiographs with a Flexitron HDR afterloader. Autoradiographs were performed in multiple ways to investigate why initial tests with gafchromic film at the Nelune Comprehensive Cancer Centre (NCCC) were not passing. Mobile x-ray unit imaging confirmed these results. Further testing was performed at the Alfred Health Radiation Oncology (AHRO) with a microSelectron afterloader, and at Peter MacCallum Cancer Centre (PMCC) with another Flexitron. At each of these sites, the brachytherapy source tracking system developed by Smith et. al. [Med Phys 43(5) 2435 (2016)] was used to acquire the source position in the Venezia applicator.

Results: All generic tests passed acceptably however the autoradiographs performed at the NCCC revealed the source position frequently does not reach the expected source position in one or both lunar ovoid channels. Using multiple Ir-192 sources over many months, results show that when the source was driven to a position of 300 mm, the difference between actual and expected positions was between 0 and 4 mm. This also occurred when the source was ‘stepped’ through the ovoids such as what may occur in a clinical treatment. Checks of the source position and using different transfer tubes supplied by the vendor did not help identify a cause for this discrepancy. However, results of the tests performed at AHRO and PMCI did not show the same discrepancy.

Conclusions: The discrepancies found highlight the importance of thorough commissioning, and add weight to advanced commissioning techniques that may detect errors that generic tests do not. Discussion with the vendor has resulted in two replacement lunar ovoid parts being sent but at the time of submitting this paper, no definite cause of the discrepancy has been identified. As a result, the Venezia applicator has not yet been placed in clinical use at the NCCC.


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