A nomogram integrating a novel uterine isthmus ratio (APTUIR), sonographic signs and clinical symptoms for predicting posterior deep endometriosis

OBJECTIVE: This study aims to propose a novel, reproducible transvaginal ultrasound (TVUS) measurement-anterior-posterior wall thickness of the uterine isthmus ratio (APTUIR)-and to construct a nomogram model integrating this parameter with other ultrasound assessments and clinical symptoms for predicting the risk of posterior pelvic deep endometriosis (DE). METHODS: This retrospective study included 362 patients with endometriosis (153 with posterior pelvic DE; 209 without) who underwent TVUS and surgery between 2021 and 2024. APTUIR, uterine white sliding lines (WSL), and clinical symptoms were assessed. LASSO regression was used to identify independent predictors and construct a nomogram. Model performance was evaluated by area under the curve (AUC), calibration curves and decision curve analysis (DCA) with internal validation (500 bootstrap samples). RESULTS: Results showed statistically significant differences between groups in dyspareunia, menstrual defecation abnormalities, CA125, uterine anteroposterior diameter, concomitant endometrial polyps (EPs), uterine posterior isthmus echo, uterine anterior WSL, uterine posterior WSL, ovarian kiss sign and APTUIR (p < 0.05). Four key predictors were identified: APTUIR, posterior uterine wall WSL, EPs, and menstrual bowel dysfunction. The nomogram demonstrated excellent predictive performance: AUC 0.933 (95% CI: 0.901-0.957), sensitivity of 85.0%, specificity of 88.0% and accuracy of 86.7%. The calibration curve of the nomogram demonstrated good consistency, whilst the decision curve indicated favourable clinical utility. CONCLUSION: The APTUIR-based nomogram provides a non-invasive, highly accurate tool for identifying high-risk posterior pelvic DE. It facilitates objective risk stratification and timely specialist referral, particularly in non-specialised settings.