Solid Subtype of Papillary Thyroid Carcinoma: A Case Series Highlighting Aggressive Features.

Papillary thyroid carcinoma (PTC) is the most common malignant thyroid neoplasm. Its solid/trabecular subtype (STPTC) is rare, and its aggressiveness remains controversial. We describe the clinical, histopathological, and immunohistochemical characteristics of three STPTC cases, highlighting their biological behavior and impact on current classification. Two patients exhibited aggressive clinical courses with extensive lymphatic metastases and possible distant spread, while the third case showed a more indolent progression. Immunohistochemistry revealed positivity for cytokeratin 7, cytokeratin 19, thyroglobulin, TTF-1, and PAX8, with negativity for CD56, synaptophysin, and p63, among others. Although the 2022 WHO classification does not consider STPTC an aggressive subtype, the literature and our findings suggest reconsideration. Immunohistochemistry and the Ki-67 proliferation index may be key tools for differential diagnosis and prognosis. STPTC can exhibit aggressive behavior, particularly in young patients. Early diagnosis and individualized therapeutic approaches are essential to improve clinical outcomes.