PHSOR04 Presentation Time: 9:15 AM

Purpose

Afterloader integrated electromagnetic tracking (EMT) in pelvic high dose rate brachytherapy (HDR-BT) has shown sub-millimeter accuracy in detecting dwell positions in interstitial implants. However, larger residual errors were found when including dwell positions from gynaecological applicators [van Heerden et. al (2021)]. The purpose of this study is to analyze EMT measurements in the Venezia™ intracavitary gynaecological applicator (Elekta AB, Stockholm, Sweden), and assess their agreement with manufacturer-specified dwell positions in the treatment planning software (TPS).

Materials and Methods

Measurements with the afterloader integrated EMT system (Elekta AB, Stockholm, Sweden) were performed on 8 applicators. We included two different applicator configurations: a small configuration consisting of 22 mm/60° ovoids (OVs) with a 40 mm/30° intrauterine tandem (IU), and a large configuration consisting of 26 mm/60° OVs with a 70 mm/30° IU. For each of the two configurations, 4 applicators were included: 2 old applicators being phased out of clinical use, and 2 new applicators before clinical introduction. Dwell positions with an interval of 5 mm were measured along the full length of each of the three applicator channels. Measurements in each applicator were repeated 5 times to determine maximum measurement deviation, including repositioning of the applicator and reconnecting to the transfer tubes. The measurements from the four distinct applicators within each configuration were compared to evaluate maximum inter-applicator deviation. Applicator configuration measurements were compared with manufacturer-specified dwell positions, as defined in the Oncentra-Brachy TPS (Elekta AB, Stockholm, Sweden). Manufacturer-specified dwell positions consist of centerline data for the IU and measured source position data for the OVs. In all comparisons, Euclidean distances (ED) between corresponding dwell positions after registration (Procrustes analysis) were used to determine deviation.

Results

Maximum intra-applicator measurement deviation was below 0.8 mm for all applicators. A maximum inter-applicator deviation of 1.3 mm was observed, invariant across configurations and irrespective of applicator age. Comparing the measured dwell positions to the manufacturer-specified dwell positions, EDs of up to 3.8 mm and 4.2 mm were found for the small and large configuration, respectively. The largest deviation between EMT measurements and manufacturer-specified dwell positions were found at the IU tip for both applicator configurations, see Fig. 1. For the straight part and curved part of the IU, maximum deviations were considerably lower at 2.1 mm and 1.9 mm, for the small and large applicator configuration respectively. In the OVs the maximum deviations were 2.5 mm and 2.4 mm, for the small and large applicator configuration respectively.

Conclusion

EMT measurements had an intra-applicator measurement deviation similar to that found in other studies [Durrbeck et. al (2024)]. A slight increase was noted when looking at inter-applicator deviations, suggesting low geometric variations between different applicators, irrespective of applicator age or configuration. When comparing EMT measured dwell positions to the manufacturer-specified ones, a larger deviation than the measurement uncertainty was found. This was especially the case in the IU tip, where EMT measurements did not follow the manufacturer-defined dwell positions (Fig 1.b). Therefore, we recommend the introduction of experimentally defined dwell positions in the IU.