Synchronous Differentiated Thyroid Carcinoma in Non-Invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features (NIFTP): Prevalence, Pathological Profile, and Limits of Pre-Operative Risk Prediction in a Multicentre Cohort

Background/Objectives: Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) was reclassified as a non-malignant entity in 2016 to reduce overtreatment. A proportion of NIFTP-containing glands nonetheless harbour a synchronous differentiated thyroid carcinoma (DTC). We quantified its prevalence and pathological profile and assessed whether routinely collected pre-operative variables identify affected patients. Methods: A multicentre retrospective cohort of 1185 adults with NIFTP confirmed at second histological review, operated on between 2017 and 2023 in 22 centres. The outcome was a synchronous DTC in the surgical specimen. A weighted random forest and an elastic-net logistic regression based on 13 pre-operative predictors were evaluated with 20-times repeated five-fold cross-validation, with all data-driven steps nested within training folds, and with leave-one-centre-out and temporal validation. Results: A synchronous DTC was present in 61 patients (5.1%; 95% CI 4.0–6.6; 6.2% after review of the stored reports), with marked between-centre variation (none recorded in eight centres). Fifty-eight carcinomas (95%) were pT1a; none had gross extra-thyroidal extension or nodal metastasis (nodes examined in 27 patients), and none met the 2025 American Thyroid Association criteria mandating a more extensive operation. No pre-operative variable differed between groups after multiplicity adjustment. The cross-validated AUROC was 0.56 (95% CI 0.47–0.65) for the random forest and 0.58 (0.48–0.67) for the elastic net; the within-centre concordance was 0.48 and the temporal-validation AUROC 0.39, and decision-curve analysis showed no net benefit over default strategies. Conclusions: About one in twenty NIFTP-containing specimens harbours a synchronous DTC, usually a low-risk papillary microcarcinoma. Routinely available pre-operative variables do not identify these patients; management should be guided by the pathology of the carcinoma itself.