Adenoid Cystic Carcinoma of Thyroid - Unusual Primary Site and Diagnostic Considerations in Cytology

Abstract Adenoid cystic carcinoma (ACC) is a rare head and neck tumor, typically arising from both major and minor salivary glands and sweat gland epithelium. This case report presents a 38-year-old female with primary thyroid ACC, an exceedingly rare entity with only a very few cases reported in the literature. The patient presented with a solitary thyroid nodule, Fine needle aspiration cytology revealed basaloid cells in three-dimensional clusters with many luminal hyaline globules, consistent with ACC. Immunohistochemistry supported the diagnosis by demonstrating CD117 positivity and subsequent histopathology confirmed ACC limited to the thyroid without extrathyroidal extension. ACC, characterized by perineural invasion and late distant metastasis, necessitates thorough clinicoradiological assessments to rule out direct extension or metastasis from adjacent structures into thyroid gland before considering primary from thyroid. Differential diagnoses, such as adenoid cystic variant of papillary thyroid carcinoma, require careful cytological observation. ACC treatment involves complete surgical excision, and postoperative radiotherapy may be considered in cases with perineural invasion or high-grade tumors. This case highlights the importance of fine needle aspiration cytology and immunohistochemistry in diagnosing primary thyroid ACC , emphasizing the need for differentiation from other thyroid tumors with similar morphology and direct thyroid invasion from neighbouring structures. Given its rarity, the establishment of standard treatment protocols for primary thyroid ACC remains challenging, underscoring the importance of long-term follow-up and consideration of adjuvant radiotherapy in certain cases. Keywords: Cytology, adenoid cystic carcinoma, hyaline globules, thyroid nodule, fine needle aspiration cytology.