Multi-center Assessment of DLL3 Expression by Immunohistochemistry in Medullary Thyroid Carcinoma.

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine malignancy accounting for 1-2% of thyroid carcinomas. As a neuroendocrine neoplasm, it shares molecular features with other aggressive neuroendocrine carcinomas, including alterations in the Myc and Notch pathways. Delta-like ligand 3 (DLL3), an inhibitory ligand of the Notch pathway and a validated therapeutic target in small cell lung carcinoma, has attracted interest as a biomarker and potential target in other neuroendocrine tumors; however, its relevance in MTC remains poorly characterized. We performed a multicenter retrospective study of 119 MTC cases resected between 2000 and 2024 across five European institutions. DLL3 immunohistochemistry was assessed on whole sections using the Ventana SP347 antibody, with expression categorized as null (< 1%), low (1-49%), or high (≥ 50%). Interobserver agreement between two endocrine pathologists was substantial (weighted kappa = 0.80). DLL3 positivity (≥ 1%) was observed in 89.1% of cases; 53.8% showed low and 35.3% high expression. DLL3-high expression correlated with adverse histopathological features, including larger tumor size, high-grade histology, desmoplasia, positive surgical margins, and lymph node metastases. In survival analyses, DLL3-high expression was associated with significantly shorter disease-free survival (HR 7.96, p = 0.05) and overall survival (HR 11.6, p = 0.01). Our findings indicate that DLL3 is frequently expressed in MTC and its high expression identifies tumors with aggressive pathological characteristics and poor clinical outcomes. These results support DLL3 as a potential prognostic biomarker and therapeutic target in MTC, highlighting the need for further validation and integration into clinical trials of DLL3-directed therapies.