Volumetric and Geometric Assessment of Offline Adaptation in Cervical Cancer External Beam Radiotherapy

INTRODUCTION: Offline adaptive replanning is an appealing strategy in cervical cancer external beam radiotherapy (EBRT) to account for tumour shrinkage. It remains unclear whether volume reduction with adaptation improves geometric coverage in the presence of daily pelvic variation. This study evaluated the volumetric and geometric impact of offline EBRT adaptation in this setting. METHODS: Seventeen patients who completed definitive radio(chemo)therapy and brachytherapy were retrospectively assessed. An MRI in the last 2 weeks of EBRT was used to generate adaptive targets and organs at risk, which were compared with original volumes for changes in gross tumour volume (GTV), internal target volume (ITV) and planning target volume (PTV). Geometric coverage by the low-risk ITV (ITV-T-LR) and PTV was assessed on 128 cone-beam CTs. RESULTS: The mean GTV regressed by 47.9% (p = 0.0101), corresponding to a 24.2% reduction in ITV-T-LR (p = 0.0038) and a 10% reduction in PTV (p = 0.0011). Despite this, offline adaptation reduced ITV-T-LR coverage from 75% with the original volumes to 59% (p = 0.024). A 5 mm PTV margin increased coverage to 96% for the original plan and 77% for the adapted plan. On multivariable analysis, each 1% increase in on-treatment bladder volume relative to simulation was associated with a 1.3% increase in odds of anterior ITV-T-LR coverage (p = 0.0279) but a 0.7% decrease in odds of posterior coverage (p = 0.0017). CONCLUSIONS: The offline adaptation approach used in this study reduced tumour and target volumes for this patient cohort, but worsened geometric target coverage, failing to compensate for daily pelvic anatomical variation. These findings raise concerns about the suitability of offline adaptation for cervical cancer EBRT.