E.A. Kuus1, V.P. Gorelov2, O.L. Vlasova3

1,2 FSBI “North-West District Scientific and Clinical Center n.a. L.G. Sokolov of the Federal Medical and Biological Agency of Russia” (FSBI NWRDSC n.a. L.G. Sokolov FMBA of Russia) (St. Petersburg, Russia)
1,3 FSAEI HE “Peter the Great St. Petersburg Polytechnic University” (FGAOU VO Peter the Great St. Petersburg Polytechnic University) (St. Petersburg, Russia)
1kuus.e@yandex.ru, 2vpgorelov@gmail.com, 3olvlasova@yandex.ru

Migration of iodine-125 microsources after low-dose-rate brachytherapy for prostate cancer is a significant issue, as it can reduce the required dose coverage of the target organ and, consequently, reduce treatment efficacy. Monitoring this migration, assessing and adjusting the dose distribution require postimplantation dosimetry and reimplantation of the sources. However, existing software has limited functionality, preventing the transfer of contours and the overlay of images from different studies. This necessitates the search for new solutions to improve the planning and dosimetry processes for reimplantation of iodine-125 microsources in cases of their migration in the early postimplantation period.

To improve the effectiveness of low-dose-rate (LDR) prostate brachytherapy by developing dosimetric methods for repeat implantation of iodine-125 (I-125) seeds in cases of early post-implant seed migration, with the aim of correcting reduced target dose coverage.

A retrospective comparative analysis of computed tomography (CT) images was performed in 280 patients who underwent LDR brachytherapy in 2023–2024 at the L.G. Sokolov North-Western District Scientific and Clinical Center of the Federal Medical and Biological Agency (FMBA of Russia). CT scans acquired immediately after seed implantation were compared with follow-up scans obtained 4–6 weeks after the procedure to detect seed migration and assess its impact on target coverage. Imaging was performed using a Siemens SOMATOM Sensation 40 CT scanner. Dosimetric evaluation of dose distribution was carried out in PSID 4.1 treatment planning software using the TG-43 formalism. The effectiveness of repeat implantation was assessed by restoration of target D90 and V100 values.

Seed migration after LDR brachytherapy for prostate cancer was observed in 10.4% of cases. In 89.7% of these patients, migration resulted in reduced target coverage, with mean decreases of 32% in D90 and 24% in V100, representing an indication for repeat implantation to restore adequate dose coverage. To support pre-implant planning for repeat brachytherapy, dedicated software was developed to transfer contours of underdosed regions from the post-implant planning dataset to CT images used for pre-planning. Fifteen patients with confirmed seed migration and subsequent undercoverage underwent repeat implantation using the proposed workflow. Post-implant dosimetric analysis demonstrated that repeat implantation guided by the developed software improved D90 by a mean of 35.18%, achieving a final value of 99.5%.

Early post-implant I-125 seed migration can substantially compromise prostate target coverage after LDR brachytherapy. A dosimetry-driven repeat implantation workflow supported by software-based transfer of underdosed-region contours enables effective restoration of dose coverage and provides a standardized approach for repeat pre-implant planning.

The practical significance of this study lies in the development and implementation of new methods for planning reimplantation in prostate cancer brachytherapy. The introduction of these technologies into clinical practice will help ensure timely dose adjustments in patients and improve the quality of treatment.

Kuus E.A., Gorelov V.P., Vlasova O.L. Possibilities of correcting reduced prostate dose coverage when planning reimplantation of
I-125 microsources in cases of their migration // Nanotechnology: development and applications – XXI century. 2026. V. 18. № 2. P. 47–55. DOI: https://doi.org/10.18127/ j22250980-202602-05

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