Investigation of the Dosimetric Impact from Positional Deviations of a Radioactive Source within a Ring Applicator during Gynaecological Brachytherapy Treatments

High-Dose-Rate (HDR) brachytherapy with an Iridium (Ir)-192 source remains a cornerstone in the management of locally advanced cervical cancer. However, the accuracy of dose delivery depends critically on precise source positioning within the applicator geometry. This study aims to investigate the dosimetric impact of small positional deviations of a radioactive source within a 26-mm diameter ring applicator and assess how such deviations influence target coverage and doses to surrounding Organs at Risk (OAR) during gynaecological brachytherapy. A retrospective analysis was performed on twenty previously treated patients using the Elekta Oncentra Treatment Planning System (TPS) at Steve Biko Academic Hospital. Reference treatment plans were generated with ten dwell positions distributed along the ring. Two deviation scenarios, 3 mm clockwise and 3 mm counter-clockwise shifts, were simulated relative to the nominal source path. Dose values were extracted at four reference points: A1 and A2 (prescription points), A3 (bladder), and A4 (rectum), in accordance with the International Commission on Radiation Units and Measurement (ICRU) Report 38 recommendations. The simulated deviations produced observable dosimetric changes. Mean percentage dose differences at A1 and A2 ranged from ±1.7% to ±3.2%, indicating that even minor geometric shifts can alter the target coverage. In terms of OARs, an average dose increase of 2.5% - 3.0% was observed at the bladder and rectum points, with the rectum displaying the highest sensitivity to posterior (counter-clockwise) displacement. These findings confirm that positional deviations of only a few millimetres can lead to clinically significant alterations in dose distribution, particularly in regions adjacent to steep dose gradients. The study underscores the importance of meticulous applicator reconstruction, regular mechanical quality assurance, and an accurate treatment planning system applicator modelling library, which should be established through commissioning of the virtual applicator geometry against physical measurements of the clinical device. Furthermore, incorporating correction factors or applicator-specific calibration during commissioning may further mitigate such uncertainties by enhancing target dose coverage during treatments while ensuring patient safety in HDR brachytherapy.