S138
Brachytherapy - Physics
ESTRO 2026
Purpose/Objective: Prior to using gynaecological brachytherapy applicators clinically, verification of dimensions, material strength, connectivity to transfer tubes, as well as source position placement within the applicator is important. The latter is generally proposed to be performed by autoradiography [1], [2]. At our institution, we have since 2018 commissioned all new gynaecological applicators using the mobile c-arm to image the active source wire dwelling at preset positions within the applicator. This has given us a unique opportunity to compare actual dwell positions with planned, with a much higher level of accuracy than when using film. Material/Methods: The Ziehm Vision RFD 3D (Ziehm Imaging GmbH) c- arm is used to image the active source wire in the applicator. The mobile exposure control station has a mirrored screen and is brought outside of the brachytherapy theatre in order to time the exposure with the source dwelling. Careful operation of the settings is required to avoid image saturation.A height- adjustable carbon fibre patient couch is positioned between the x-ray tube and the flat-panel detector, both carefully levelled. The applicator is meticulously placed on the table top (Figure 1), with its source path perpendicular to the detector. For accurate scaling, a radiopaque cm marker is placed close to the applicator.
is parallel to the x-ray images.By matching the magnification factor of both images and superimposing the printout on the x-ray image (Figure 2), the deviation between planned and actual dwell position is marked on the transparent paper, and the distance is measured and noted. After applicator commissioning is done, the transparent paper is digitised and the original is wiped down for re- using.
Results: We have found that in applicators with a curved lumen, such as a ring or a half-ring (ovoid), there is a deviation of approximately 1-2 mm that is increasing the more distal the dwell position. In applicators with a straighter lumen, such as intrauterine applicators, the deviation is less pronounced, < 1 mm. We relate this to the snaking effect of the source wire, which increases the further it is pushed within a curved lumen applicator. Conclusion: The well-elaborated technique ensures quick assessment of the source path within the applicator and comparisons with planned dwell positions are digitised for future reference. Keywords: gynaecological applicator, commissioning References: [1] Venselaar J, Pérez-Calatayud J, editors. A practical guide to quality control of brachytherapy equipment: European guidelines for quality assurance in radiotherapy. ESTRO Booklet No. 8. Brussel: European Society for Therapeutic Radiology and Oncology, ESTRO; 2004.[2] Paulsen Hellebust, T, et al. Recommendations from Gynaecological (GYN) GEC- ESTRO Working Group: Considerations and pitfalls in commissioning and applicator reconstruction in 3D image-based treatment planning of cervix cancer brachytherapy. Radiotherapy and Oncology. 2010; 96(2):153-60. doi:10.1016/j.radonc.2010.06.004
From the treatment planning system (TPS), a cross- section view of the applicator with the source centre marked, and scale applied, is printed and copied to transparent paper. It is important that the view angle
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