A prospective clinical and dosimetric study on the feasibility and adaptability of MR-Linac in the treatment of cervical cancer: Initial experience from a tertiary institution.

Abstract

e17527 Background: Cervical cancer is a leading cause of cancer burden globally, with a disproportionate burden in LMICs like India. The pelvis is a highly dynamic anatomical region; tumour regression, variable bladder and rectal filling, and small bowel displacement produce substantial inter- and intra-fraction changes that challenge a single pre-treatment plan. We treated cervical cancer patients with standard protocol, with reduced anisotropic margins and daily online adaptive radiotherapy, evaluated the feasibility, dosimetric impact, OAR sparing, tumour regression and bladder dynamics & target coverage with reduced margins. Methods: 7 patients with FIGO IB–IIIC2 cervical cancer received EBRT to 45 Gy in 25 fractions with concurrent weekly cisplatin, followed by MRI-guided brachytherapy. CT - MR based delineation was done for GTV, CTV and OAR's with an anisotropic PTV margins of 5–13 mm. Daily HR 3D MRI was acquired on Unity prior to each fraction. Fractions with minor changes were treated with Adapt-to-Position (ATP), and those with significant anatomical variation (>5 mm shift, >10% volume change, bowel/bladder deformation) with Adapt-to-Shape (ATS). After adaptation, Iso-centre MR scan was acquired immediately pre-beam in 123/175 fractions to assess intra-fraction changes and re-optimise plans when required. Dosimetry of scheduled and adapted plans was compared for PTV, bladder, rectum and small bowel. Also, the MR plans were compared with the plans generated with Standard PTV margis as per the EMBRACE 2 protocol. Serial GTV volumes were observed for tumour regression. Results: All patients completed chemoradiation with 175/175 fractions. Mean total treatment time was 35.2 ± 5.8 minutes per fraction. Baseline median GTV, CTV and PTV were 29.03, 121.52 and 291.13 cm³ respectively. Mean PTV D95% was 98.53 ± 1.26%, with 100% of fractions achieving ≥95% coverage despite reduced margins. All patients demonstrated marked tumour regression: baseline GTV 12.51–52.40 cm³ decreased to 0–2.35 cm³, with mean GTV regression of 96.8 ± 2.4%. Inter-fraction variability was dominated by bladder and bowel motion: mean inter-fraction bladder volume difference between scheduled and adapted scans was 128.92 cm³. Intra-fraction analysis universal significant bladder filling: 100% fractions had increased intra fraction bladder volume [mean increase 128.92 ± 70 cc], 91.1% showing >50 cc change. Conclusions: Daily online adaptive MRgRT for cervical cancer is feasible with a ~35-minute workflow, achieves excellent target coverage with reduced PTV margins. The combination of near-complete tumour regression, substantial small bowel sparing, and profound inter- and intra-fraction bladder and bowel motion establishes the same. This supports MR-LINAC–based adaptive radiotherapy as a promising precision platform in cervical cancer.