3784 - Stereotactic Body Radiotherapy (SBRT) Boost on MRLINAC in Patients of Carcinoma Cervix after External Beam Radiotherapy (EBRT) who are Ineligible for High Dose Rate Brachytherapy- Early Experience at a Single Institution

Purpose/Objective(s): Brachytherapy is the standard boost technique in carcinoma cervix (CaCx) after external beam radiotherapy (EBRT) treatment. But there are scenarios in clinical practice when brachytherapy treatment is not feasible. Stereotactic Radiation Therapy (SBRT) is a well established alternate boost modality for CaCx boost. We present our early experience and feasibility of Magnetic Resonance Image (MRI) guided SBRT (MgRT) boost in eight CaCx patients treated on MRLINAC at a single institution.

Materials/Methods: Eight cases of CaCx received MgRT boost after EBRT on 1.5T MRLINAC between May 2025 till December 2025. Patients were either ineligible for brachytherapy due to anaesthesia fitness or had refused brachytherapy. All patients had received EBRT to a dose of 45Gy in 25 fractions with concurrent weekly cisplatin and had undergone MRI pelvis at the end of EBRT for response evaluation. SBRT dose ranged from 25-27.5Gy in 4-5 fractions delivered on alternate days. All patients underwent daily online adaptive planning with adapt to shape (ATS) workflow on MRLINAC. Dose received by 90% (D90) of high risk clinical target volume (CTV HR) was noted. Cumulative 2cc organ at risk (OAR) doses for bladder, rectum and sigmoid were kept as 85-90Gy, 75Gy and 75Gy. Changes in CTV HR volumes and OAR volumes between reference CT plan and first fraction of MR plans were evaluated. Adapted versus non-adapted MRLINAC plans were also compared for any difference between the plans. Acute genitourinary (GU), gastrointestinal (GI) and hematological toxicities were noted according to CTCAE version 5.

Results: Median age of the patients was 58 years (41-67 years). Median overall treatment time (OTT) was 57.5 days (51-72 days). The plans were adapted for changing OARs and target volumes. The mean CTV HR volume for all patients was 39.6cc (27.76- 58.5cc). The mean D90 CTV HR was 25.4 Gy(24.24-26.8Gy). The mean cumulative tumor EQD2 was 76.34Gy (75.5-79.75Gy). The mean cumulative EQD2 for 2cc bladder, rectum and sigmoid was 82.9Gy, 72.1Gy and 59Gy respectively. Out of 8 patients, 5 patients had statistically significant difference between CTV HR coverage between adapted and non adapted MRLINAC plans. Bladder and rectum were better spared in adapted plans as compared with non adapted plans in four and three patients respectively. None of the patients experienced grade 3 or 4 genitourinary (GU) or gastrointestinal (GI) toxicity. All patients experienced grade 1 GU toxicity mostly as a sequelae of EBRT.

Conclusion: MRgRT SBRT boost in CaCx after EBRT in brachytherapy ineligible patients is a decent alternative which allows for daily online adaptation to highly mobile pelvic structures with excellent soft tissue visualisation, allowing PTV margin reduction at the same time achieving desired target coverage and OAR sparing. Treatment is well tolerated with acceptable acute toxicities.