Dosimetric comparison of CT and CBCT-guided HDR brachytherapy planning in cervical cancer sequential treatment

Background: Anatomical changes and/or applicator displacement between planning computed tomography (CT) and high-dose-rate brachytherapy (HDR-BT) delivery may affect target coverage and organs-at-risk (OARs) dose. However, dosimetric agreement between CT-optimized plans and pre-treatment cone-beam CT (CBCT)-based recalculations remains insufficiently characterized. This study evaluated the consistency of target coverage and OAR sparing between the two approaches. Methods: This retrospective single-center observational cohort study included 10 consecutive patients with cervical cancer treated at A.O.U. "G. Martino", Messina, Italy, between June 2022 and December 2024 with sequential external beam radiotherapy followed by intracavitary HDR-BT, for a total of 33 fractions. Eligible fractions required an available CT-based plan and an evaluable pre-treatment CBCT dataset; fractions with incomplete data, inadequate image quality, or unsuccessful CT-CBCT registration were excluded. Patients were treated using a CT-compatible Fletcher-type applicator. For each fraction, treatment planning was optimized on CT and CBCT was acquired immediately before irradiation. CT and CBCT datasets were deformably registered, contours were propagated and reviewed, and CT-optimized plans were recalculated on CBCT using unchanged dwell positions and dwell times. High-risk clinical target volume (HR-CTV) coverage was assessed using V90% and D90, while OAR exposure was evaluated using D2cc for bowel, rectal wall, sigmoid colon, and bladder. Dosimetric parameters were compared using descriptive statistics and paired tests; cumulative doses were expressed as equivalent dose in 2-Gy fractions (EQD2). Results: Across 33 fractions, HR-CTV V90% ranged from 82.16% to 99.16% on CT and from 84.03% to 100.03% on CBCT, with mean values of 91.48% and 92.09%, respectively. Mean CT versus CBCT D2cc values were 3.14 versus 2.95 Gy for bowel, 3.93 versus 3.94 Gy for rectal wall, 2.81 versus 3.17 Gy for sigmoid colon, and 5.39 versus 5.31 Gy for bladder. No significant paired differences were observed for bowel (P=0.08), rectal wall (P=0.87), or bladder (P=0.85), whereas an isolated significant difference was observed for sigmoid colon (P=0.02). Cumulative HR-CTV D90 EQD2 ranged from 63.1 to 78.1 Gy on CT and from 60.1 to 81.4 Gy on CBCT. Bowel D2cc showed the strongest association with prescription-dose modulation. Conclusions: Pre-treatment CBCT-based recalculation showed overall dosimetric consistency with CT-optimized HDR-BT plans while identifying patient- and fraction-specific variations before treatment delivery. These findings support CBCT as a complementary verification tool within a standardized CT-based workflow, particularly when a non-negligible interval exists between planning imaging and irradiation. Larger studies are warranted to define the patients and clinical scenarios in which CBCT provides the greatest added value.