S71
Brachytherapy - Gynaecology
ESTRO 2026
differences were assessed using the Wilcoxon signed- rank test. Results: Catheter reconstruction using the PETRA sequence and implant model was successful along their entire length in all patients (Fig. 1). The mean 3D displacement between MRTP and CTP dwell positions was 2.43 ± 1.6 mm, primarily reflecting anatomical and applicator shifts and CT–MRI registration uncertainty in the standard workflow. CTV D90 for MRTP was higher compared to CTP, indicating improved target coverage, while other dosimetric parameters were comparable (Fig. 2A): the absolute EQD2 [DVH] difference was 4.13 ± 5.1 Gy [9.76 ± 8.9%, p < 0.05] for CTV D90, and within ±3 Gy [±5%] for all OAR D2cc values. DSC values exceeded 0.70 across the 125% to 65% isodose levels, confirming high spatial agreement between dose distributions (Fig. 2B).
(4/11) in the freehand group. Conclusion:
Salvage HDR-ISBT is a safe and effective option for central pelvic recurrent gynecologic malignancies post- RT; 3D-PT-assisted HDR-ISBT significantly improves HR-CTV dose coverage and enhances inter-fraction dose consistency and stability. Keywords: 3D-printed templates; Brachytherapy
Proffered Paper 3323
Clinical implementation of an MRI-only workflow for interstitial HDR brachytherapy in gynaecologic cancer Josephine L Tan, Evangelia Kaza, Thomas C Harris, Desmond A O'Farrell, Alicia C Smart, Idalid Franco, Aiven Dyer, Robert A Cormack, Ivan Buzurovic Radiation Oncology, Brigham and Women's Hospital, Harvard Medical School and Women's Hospital, Boston, USA Purpose/Objective: This study reports the clinical implementation of a fully MRI-only treatment planning (MRTP) approach for interstitial high-dose-rate (HDR) brachytherapy in gynaecologic cancers. The challenge of accurate catheter reconstruction due to limited visibility of catheters within both tissue and applicator on standard MRI was addressed by using a pointwise encoding time reduction with radial acquisition (PETRA) MRI sequence. Unlike the standard dual- modality workflow combining CT and MRI, MRTP eliminates CT imaging, thereby avoiding additional radiation exposure, shortening procedure time, and removing uncertainties associated with CT–MRI image registration. Material/Methods: Sixteen patients undergoing interstitial HDR brachytherapy were imaged with CT and MRI at 3 Tesla. For MRTP, catheters in the tissue were reconstructed using PETRA MRI, while catheters in the obturator of the Syed-Neblett applicator were reconstructed using PETRA MRI and an implant model in Oncentra Brachy (Elekta Brachytherapy, The Netherlands). Clinical target volume (CTV) and organs at risk (OARs) were delineated on T2-SPACE MRI. MRTP and standard CT/MRI-based (CTP) plans were independently generated. Reconstruction accuracy was evaluated by measuring 3D dwell position displacements between MRTP and CTP. Dosimetric comparison was performed using dose–volume histogram (DVH) metrics and equivalent dose in 2-Gy fractions (EQD2) for CTV D90 and OAR D2cc. Dice similarity coefficients (DSC) were calculated for volumes enclosed by the 150%, 125%, 100%, 95%, 90%, 80%, and 65% isodose levels. Statistical
Figure 1
Figure 2 Conclusion: The MRI-only approach has been successfully
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