Cribriform Morular Thyroid Carcinoma: Unreported Cytological Features With Histologic Correlation and Diagnostic Lessons From a Case Lacking Morules

Cribriform morular thyroid carcinoma (CMTC) is a rare thyroid neoplasm. Due to its rarity, cytomorphological diagnosis is challenging, especially in cases lacking morules. Cytomorphological overlap with papillary thyroid carcinoma (PTC) contributes to misdiagnosis. A 29-year-old woman presented with a solitary nodule in the left thyroid lobe, which on ultrasound was predominantly solid with foci of microcalcification (TIRADS 4). Fine-needle aspiration cytology demonstrated moderately cellular smears. Cells were arranged as microfollicles, sheets, and occasional papillary fragments. Focal nesting arrangement with peripheral palisading was noted, mimicking the jigsaw puzzle pattern observed in cylindroma. Hyaline material was identified within some follicular lumina. There was focal cellular swirling. The tumor cells were cuboidal to elongated, exhibiting nuclear elongation, crowding, mild enlargement, focal pseudostratification, and minimal nuclear membrane irregularity. Colloid was absent. The cell block showed similar features. The cells were TTF-1 positive and CDX2-negative. Overall, the cytological features were suspicious for the columnar cell subtype of PTC (Bethesda category V). Total thyroidectomy revealed histomorphological and immunohistochemical findings consistent with CMTC, with the notable absence of morules. On retrospective evaluation, the cytological patterns correlated well with the histological architectures, with the microfollicles corresponding to cribriform areas and the novel "jigsaw pattern" representing thick tumor trabeculae. Awareness of the entity and its key cytological features, like complex architectural patterns, including the "jigsaw pattern," absent/subtle nuclear features of PTC, and absent colloid, is crucial for a correct cytological diagnosis.