MRI Accuracy for Detecting Deep Myometrial Invasion in Low-Grade Endometrial Cancer: Insights From a Seven-Year UK Experience

BackgroundEndometrial cancer is the most common gynaecological malignancy in high-income countries, with approximately 10,200 new cases each year in the United Kingdom, around 34% of which are attributable to overweight and obesity.Depth of myometrial invasion determines International Federation of Gynaecology and Obstetrics (FIGO) stage IA versus IB and therefore both the extent of surgical staging and whether surgery is undertaken locally or at a cancer centre.Pelvic magnetic resonance imaging (MRI) is widely used for this assessment, although diagnostic performance in routine practice varies and the influence of body mass index (BMI) remains uncertain. MethodsThis retrospective single-centre diagnostic accuracy study included consecutive women with biopsy-proven grade 1-2 endometrioid endometrial carcinoma who underwent preoperative pelvic MRI followed by hysterectomy within a UK NHS pathway between January 2017 and July 2025.Women whose MRI demonstrated radiological stage greater than IB were excluded, so these estimates apply to women with radiologically presumed stage I disease.Final histopathology was the reference standard, with deep myometrial invasion (DMI) defined as invasion of 50% or more of the myometrial thickness.Diagnostic performance measures were calculated with 95% confidence intervals.A post hoc subgroup analysis compared performance by BMI category. ResultsA total of 204 patients were included.Median age was 65 years (range: 26 to 88), and median BMI was 34 kg/m² (range: 21 to 62; mean: 34.4 kg/m²); of 189 women with recorded BMI, 46 (24.3%) had a BMI below 30 kg/m² and 143 (75.7%) had a BMI of 30 kg/m² or above.MRI demonstrated sensitivity of 63.33% (95% CI: 49.90% to 75.41%), specificity of 82.64% (75.45% to 88.44%), positive predictive value of 60.32% (50.34% to 69.50%), negative predictive value of 84.40% (79.37% to 88.38%), and accuracy of 76.96% (70.57% to 82.55%) for detecting DMI.MRI stage and final histopathology were concordant in 157 of 204 cases (77.0%);MRI over-staged 25 women and under-staged 22. Direct between-group comparison showed higher diagnostic accuracy among women with BMI ≥30 kg/m² than among those below, but this comparison was unadjusted; the subgroups were unequal in size, and the prevalence of deep invasion also differed between them (26.6% versus 34.8%). ConclusionIn this single-centre NHS cohort, preoperative MRI demonstrated moderate diagnostic accuracy for detecting DMI in women with grade 1-2 endometrioid endometrial carcinoma, with higher specificity and negative predictive value than sensitivity.MRI supports preoperative risk assessment but does not reliably exclude deep invasion, and false-negative cases remain clinically important.Performance appeared higher among women with BMI ≥30 kg/m², but this post hoc finding is hypothesis-generating and requires prospective validation.