S131
Brachytherapy - Physics
ESTRO 2026
intrauterine tandem (IU), and 3 plastic catheters. The phantom contains two slots for a microDiamond (PTW) detector 25 and 15 mm away from the IU of the applicator, called “side point” and “front point” respectively. The detector was calibrated by measuring its response at a 5.0 cm distance from an Ir-192 source and calculating the dose rate of the source at the same distance using the TG-43 algorithm [3] with the measured air-kerma rate from the calibrated well chamber. Four separate end-to-end dose verification measurements were performed with the following workflow: MRI-imaging of the phantom, creation of 3 or 5 treatment plans with the Oncentra Brachy treatment planning system (OB) containing various combinations of dwell positions in the IU, ring and catheters, and simulated treatment delivery using an afterloader and dose measurement in water filled phantom. The measured doses in the detector locations were compared with the doses calculated by OB. Results: On average, the difference between measured and calculated dose was +2.6% (Table 1), and the average distance to agreement (DTA) was 0.6 mm. Generally, the dose differences were larger at the front point located closer to the applicator, and when measuring plans containing dwell positions in the ring. Conclusion: End-to-end dose verification measurements of an Ir- 192 cervical HDR brachytherapy procedure were performed using a custom phantom. The results show that the tested procedure is quite accurate with an average DTA of less than 1 mm, and the measurements were easy to set up and perform. We are planning to modify this phantom to use the more widely used Venezia-applicator, to introduce it to clinical QA use, and to create an end-to-end phantom to be used in ultrasound guided HDR prostate brachytherapy. Keywords: Quality Assurance (QA), End-to-End testing References: [1] Tachibana, H., Watanabe, Y., et al. (2022). Multi- Institutional Study of End-to-End Dose Delivery Quality Assurance Testing for Image-Guided Brachytherapy Using a Gel Dosimeter. Brachytherapy, 21(6), 956– 967. [2] Krause, F., Risske, F., et al. (2018). End-to-end test for computed tomography-based high-dose-rate brachytherapy. Journal of contemporary
variations, and an example of how QA intercepted a manufacturing aberration." Journal of Applied Clinical Medical Physics 22.8 (2021): 72-82.
Digital Poster Highlight 2852
Verifying the accuracy of MRI-guided iridium-192 cervical HDR brachytherapy using an end-to-end phantom Patrik Ikäheimonen, Mikko Tenhunen, Satu Strengell, Tomi Miettinen, Vappu Reijonen, Anna Rintala Comprehensive cancer center, HUS, Helsinki, Finland Purpose/Objective: End-to-end testing is an approach to quality assurance (QA), in which a complex process is performed in its entirety to quantify its cumulative uncertainties. While it has been used to measure the uncertainty of external beam radiotherapy for some time, its application to brachytherapy is still relatively new [1, 2]. To verify the accuracy of a brachytherapy procedure, we present an end-to-end phantom design for MRI-guided iridium-192 cervical HDR brachytherapy, as well as the results of several dose verification measurements performed with the phantom. Material/Methods:
The end-to-end measurements were performed using an in-house built phantom shown in Figure 1, consisting of a holder for a Vienna-applicator with a 30 mm diameter ring and a 40 mm long, 4 mm thick
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