E. Sadiki et al.: Radioprotection 2026, 61( 2), 140 – 145 141
calculations of dwell times and source positions within the applicators.
Despite these advancements, a crucial but often overlooked factor is the potential impact of rotational displacements of the applicator during treatment. Rotations whether lateral, vertical, or axial can distort the planned dose distribution, even with small shifts, compromising both treatment efficacy and safety. Recent studies suggest that such rotational deviations, caused by factors such as patient movement, anatomical changes, or setup errors, can lead to underdosage of portions of the HR- CTV and overdosing of critical OARs, which may increase the risk of tumor recurrence and toxicity. This study aims to investigate the dosimetric effects of applicator rotations in HDR brachytherapy for cervical cancer. By modeling rotational shifts along the X, Y, and Z axes, we will quantify their impact on the dose distribution to both the HR-CTV and OARs, offering critical insights for refining treatment protocols and enhancing patient safety.
2 Materials and methods
2.1 Patient cohort
This study was conducted at the Radiotherapy Department of a University Hospital on a cohort of 20 patients, aged between 30 and 70 yr, diagnosed with locally advanced cervical cancer. All patients underwent intracavitary highdose-rate( HDR) brachytherapy as part of their treatment plan. The CT treatment planning images were acquired between January 2021 and June 2023. Intracavitary applicators were inserted under general anesthesia to ensure accurate placement and patient stability, with all procedures supervised by an experienced radiation oncologist.
2.2 Treatment protocol
All patients received 46 Gy in 23 External Beam Radiation Therapy( EBRT) fractions, followed by four fractions of 7 Gy HDR brachytherapy, as per GEC-ESTRO recommendations.
2.3 Imaging and volume delineation for treatment planning
Treatment planning began with the acquisition of CT images using a Siemens Somatom Sensation Open scanner( slice thickness: 3 mm), selected for its balance between resolution and precision. These images were used to delineate the high-risk clinical target volume( HR-CTV) and organs at risk( OARs), including the bladder and rectum. The data were then transferred to Varian SOMAVISION Focal workstations for accurate contouring of the target volumes and OARs, ensuring precise treatment planning.
2.4 Treatment plan optimization and dose calculations
Treatment plans were optimized using Eclipse 10( Varian Medical Systems) with the goal of delivering 7 Gy to 90 % of the HR-CTV( D90 %) while minimizing dose to the bladder and rectum. Dosimetric constraints followed GEC-ESTRO and AAPM TG-43 guidelines, including a minimum HR-CTV D90 % of 90 %, and the dose to the highest irradiated 2 cubic centimeters( 2cc) D2cc limits of 90 Gy Equivalent Dose in 2Gy Fractions( EQD2) for the bladder and 75 Gy for the rectum( Rivard et al., 2004).
2.5 Simulation of rotational displacements
To simulate real-world conditions, rotational displacements of the intracavitary applicators were modeled within the Varian TPS. Prior to the simulations, a visual and spatial comparison of CT images from successive treatment fractions was performed using rigid registration Figure. 1. This process revealed that, despite consistent applicator type, insertion parameters, and patient preparation, applicator rotations between 3 ° and 10 ° were frequently observed, particularly along the vertical axis. These angular deviations, noted during routine post-insertion control CTs in our institution, reflect clinically relevant positioning uncertainties. Based on these observations, we simulated displacements in the X, Y, and Z axes( representing lateral, vertical, and longitudinal directions) with rotations of ± 3 °, ± 5 °, ± 8 °, and ± 10 °, consistent with realworld variations encountered inr clinical practice.
The rotational displacements were then manually applied to the applicator geometry in the treatment planning system to simulate their dosimetric effect Figure. 2. After each displacement simulation, the applicator’ s position was adjusted, and dose recalculations were performed. Dose-volume histograms( DVHs) were generated to evaluate the dosimetric impact of these displacements on the HR-CTV and OARs. Key dosimetric metrics such as D90 % for the HR-CTV and D2cc for the OARs were calculated for each rotation scenario
3 Results and discussion
3.1 Dose variations along the X-axis( lateral)
Rotations along the X-axis resulted in moderate dose variations. The dose to the CTV ranged from �2.49 % to þ0.92 %, the bladder dose ranged from þ0.174 % to þ2.13 %, and the rectum dose varied from �0.63 % to þ1.61 %. While these fluctuations are relatively small, they can have clinically meaningful consequences, especially when the organs at risk( OARs) are located close to the applicator. Previous studies, such as Jayarathna et al.( 2023), Schindel et al.( 2013) and Yong et al.( 2016), have demonstrated that lateral rotations, although generally less problematic, can lead to significant dose discrepancies, particularly in cases of complex anatomy. These findings highlight the need for 3D planning and realtime imaging correction strategies, such as MRI or CBCT, to minimize these placement errors. Figure 3 presents a histogram illustrating the lateral dose variation as a function of rotation angles.
3.2 Dose variations along the Y-axis( vertical)
Displacements along the vertical axis showed the most significant dose variations in this study. The bladder dose increased by up to þ12.97 %, the rectum dose increased by þ16.6 %, while the CTV dose decreased by up to �10.11 %.