© E. Sadiki et al., Published by EDP Sciences 2026

1 Introduction

High-dose-rate (HDR) brachytherapy remains a cornerstone in the treatment of cervical cancer, offering precise radiation delivery that significantly improves local control and overall survival rates while minimizing toxicity to surrounding healthy tissue. By placing the radioactive source either inside or close to the high-risk clinical target volume (HR-CTV), HDR brachytherapy ensures a highly conformal dose distribution. The integration of advanced imaging techniques, such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), has further enhanced the precision of HDR planning, enabling 3D treatment approaches that optimize dose delivery while sparing organs at risk (OARs), such as the bladder and rectum.

As Viswanathan et al. (2012) and Kim et al. (1995) have emphasised in their respective studies, precise applicator positioning is paramount to avoid hot or cold spots in the CTV area. These studies underscore the critical importance of precise positioning to ensure optimal treatment outcomes. The optimization goal in cervical cancer brachytherapy is to achieve an ideal, pear-shaped dose distribution, effectively covering the HR-CTV while minimizing radiation exposure to nearby OARs. This delicate balance between maximizing therapeutic dose to the target and protecting critical structures is essential for achieving high local control rates with minimal side effects. Optimization methods rely on accurate 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 high-dose-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 real-world 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

Fig. 1 Example of rigid registration between two CT acquisitions showing angular deviation of the intracavitary applicator between fractions.

Fig. 2 Illustration of applicator rotations along the three principal axes: (a) Lateral (X-axis), (b) Vertical (Y-axis), and (c) Axial rotation (Z-axis).

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 +0.92%, the bladder dose ranged from +0.174% to +2.13%, and the rectum dose varied from −0.63% to +1.61%. 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 real-time 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.

Fig. 3 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the lateral (X-axis).

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 +12.97%, the rectum dose increased by +16.6%, while the CTV dose decreased by up to −10.11%. These variations are particularly concerning because they suggest that vertical displacements (e.g., patient movements or transportation between imaging and treatment rooms) can significantly affect the radiation distribution. Previous studies, including Pötter et al. (2009) and Hoskin et al. (1996), have shown that such displacements can lead to underdosing of the CTV and overdosing of OARs, thereby increasing the likelihood of local recurrence and toxicity. Real-time imaging, such as MRI, can help detect and correct these displacements before dose delivery. Additionally, adaptive planning strategies could be implemented to adjust the treatment plan according to the variations in applicator positioning. Figure 4 presents a histogram illustrating the Vertical dose variation as a function of rotation angles.

Fig. 4 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the lateral (Y-axis).

3.3 Dose variations along the Z-axis (axial rotation)

Rotations around the Z-axis (applicator axis) caused minor dose variations. The bladder dose varied by a maximum of +1.02%, the rectum by +0.24%, and the CTV by −0.94% to +0.55%. While these variations are relatively small, they can affect dose distribution, particularly when structures like the bladder or rectum are close to the applicator. The impact of axial rotations is generally less critical than that of vertical displacements but is still important. Figure 5 presents a histogram illustrating the Axial Rotation dose variation as a function of rotation angles.

Fig. 5 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the axial rotation (Z-axis).

3.4 Clinical implications

Applicator displacements, especially along the vertical axis, frequently occur during patient transportation between imaging and treatment rooms or during applicator insertion. These shifts may not be immediately visible but can lead to significant dosimetric deviations, jeopardizing treatment accuracy. Studies have documented notable geometric variations in applicator positioning and demonstrated their significant impact on doses delivered to organs at risk (Kim et al., 1995; Hoskin et al., 1996; Garipagaoglu et al., 2006).

From a radiation therapy optimization perspective, these positional uncertainties present two critical risks: the potential underdosing of the clinical target volume (CTV), reducing tumor control probability, and the overdosing of organs at risk (OARs), increasing the likelihood of treatment-related toxicity. Maintaining the delicate balance between effective tumor irradiation and sparing of healthy tissues relies heavily on precise applicator placement and stable geometry.

Repeated imaging has been shown to reduce these uncertainties by enabling verification and adjustment of OAR volumes and dose distributions throughout the treatment course (Hellebust et al., 2001). Real-time imaging techniques such as MRI and cone-beam CT (CBCT) offer promising tools to detect and correct applicator displacements immediately before dose delivery (Damato et al., 2015), thereby improving treatment accuracy. The EMBRACE II study further emphasizes the crucial role of image-guided adaptive brachytherapy in managing anatomical variations and applicator positioning to optimize clinical outcomes (Pötter et al., 2018).

However, these imaging modalities have limitations. CBCT contributes additional radiation dose to the patient cumulative imaging dose from repeated CBCT scans can be clinically relevant (Islam et al., 2006) and both CBCT and MRI may increase treatment time and require specialized equipment, which may not always be available. Therefore, while real-time imaging is advantageous, its use must be judiciously balanced against these practical considerations.

4 Conclusion

Managing applicator displacement continues to be a significant concern in HDR brachytherapy, as it directly affects treatment precision and patient outcomes. Ensuring accurate dose delivery requires a comprehensive strategy that combines real-time tracking, adaptive treatment planning, and effective immobilization techniques. Tailored solutions such as patient-specific fixation devices, optimized vaginal packing, and robust optimization methods can help counteract the uncertainties linked to patient positioning. By integrating these approaches, clinicians can better protect organs at risk while maintaining adequate coverage of the clinical target volume, ultimately enhancing both the safety and effectiveness of HDR brachytherapy.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

Data will be made available on request.

Author contribution statement

Elmehdi Sadiki: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Conceptualization. Omar Berradi: Writing – review & editing, Writing – original draft, Investigation, Validation, Conceptualization. Mohammed Ait Erraisse , Kaoutar Soussy , Samia khalfi and Nourredine Slassi: Writing – review & editing, Validation, acquisition, Resources. Mohammed Najeh: Writing – review & editing, Validation. Rodouan Touti: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Supervision.

Ethics approval

This retrospective study was conducted in accordance with the ethical principles of the 1964 Declaration of Helsinki and its later amendments. The protocol was reviewed and approved by the Ethics Committee of CHU Hassan II of Fez.

Informed consent

The Committee waived the requirement for individual informed consent due to the fully anonymized nature of the clinical data used in this research.

References

All Figures

	Fig. 1 Example of rigid registration between two CT acquisitions showing angular deviation of the intracavitary applicator between fractions.
In the text

	Fig. 2 Illustration of applicator rotations along the three principal axes: (a) Lateral (X-axis), (b) Vertical (Y-axis), and (c) Axial rotation (Z-axis).
In the text

	Fig. 3 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the lateral (X-axis).
In the text

	Fig. 4 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the lateral (Y-axis).
In the text

	Fig. 5 Relative dose variation histogram for the bladder, rectum and CTV high-risk during simulated rotational displacements along the axial rotation (Z-axis).
In the text