OC20.05: External validation of International Ovarian Tumour Analysis (IOTA) for classification of adnexal masses suspected to be endometriomas

External validation of the IOTA classification of adnexal masses as benign or malignant in women with suspected endometriomas. Retrospective study including women referred to an endometriosis tertiary referral centre between 2011-2016. All women were evaluated by ultrasound (TVUS) using the IOTA classification simple rules and expert opinion. The reference standard was definitive histology after mass removal at laparoscopy. The diagnostic performance was estimated by calculating the sensitivity, specificity, positive and negative predictive values, positive and negative likelihood ratios and overall accuracy. 621 women evaluated for endometriosis, all were premenopausal (mean age 33.5 ± 6.5 years). 208 women had an adnexal mass with a maximal median size 49, IQR 40.5-65.5 mm. We divided the women into 4 groups: endometrioma on TVUS and confirmed on surgery (Group 1 = 181), endometrioma on TVUS but other benign cysts were found on surgery (Group 2 = 9), functional clear cysts on TVUS but endometriomas on surgery (Group 3 = 2), masses classified as other findings or suspicious for malignancy on TVUS and confirmed on surgery (Group 4 = 5 malignant, 11 benign). This gave a sensitivity 98.9%, specificity 64%, positive 95.3% and negative 88.9% predictive values, positive 2.74 and negative 0.017 likelihood ratios and 94.7% overall accuracy. The surgical diagnosis for the 5 masses suspected to be malignant was: borderline serous tumour (2), borderline mucinous tumour (2), and endometrioid lesion with complex hyperplasia without atypia (1). 1 mass was diagnosed as an atypical endometrioma (Group 2) and the pathological diagnosis was inconclusive (epidermal inclusion cyst/benign cystic Brenner tumour and/or benign mucinous cyst with extensive squamous metaplasia). The IOTA classification simple rules and expert opinion performs well for classifying adnexal masses suspected to be endometriomas as benign or malignant on external validation. The most common malignant masses were borderline tumours.