Encapsulated Medullary Thyroid Carcinoma with Adenoma-Like Features: Diagnostic and Therapeutic Challenges, Case Report and Literature Review

Medullary thyroid carcinoma (MTC) is an uncommon neuroendocrine thyroid malignancy arising from parafollicular C cells. It differs from differentiated thyroid carcinoma in its biochemical behavior, genetic associations, and surgical management. Encapsulated MTC is rare and may appear radiologically and grossly deceptively benign, creating diagnostic uncertainty when morphologic findings conflict with serum biomarkers. 
We report a 47-year-old female with a slowly enlarging, well-circumscribed right thyroid nodule, no cervical lymphadenopathy, and markedly elevated serum calcitonin. Fine-needle aspiration (FNA) showed atypical spindle and polygonal cells with focal calcitonin and synaptophysin positivity, raising suspicion for MTC but precluding confident classification due to adenoma-like follicular architecture. A multidisciplinary review favored definitive surgery because the biochemical and profile outweighed the reassuring imaging findings. Total thyroidectomy with central neck dissection revealed a 3.1 cm well-encapsulated MTC with adenoma-like features, no capsular or vascular invasion, negative central nodes, and a somatic RET mutation. Postoperative calcitonin and CEA declined substantially. 
This case emphasizes that encapsulation and follicular-patterned architecture should not exclude MTC when calcitonin is elevated. It also supports the need for integrated biochemical, cytologic, IHC, molecular, and multidisciplinary assessment in diagnostically discordant thyroid nodules.