Encapsulated neoplasms of the thyroid gland

Encapsulated thyroid gland lesions, defined by complete or partial confinement within a fibrous capsule, are common findings in endocrine pathology but frequently pose diagnostic challenges. The primary difficulty lies in distinguishing benign, low-risk, and malignant neoplasms, particularly within the spectrum of follicular-patterned tumors. Accurate classification can be hindered by pitfalls such as differentiating true tumor capsule from peritumoral fibrosis, identifying capsular or vascular invasion versus reactive changes from preoperative fine-needle aspiration, and accounting for histologic and cytologic heterogeneity. In this review, we discuss the definition of true capsule and vascular invasion and how to contrast from mimics. We describe the wide spectrum of both follicular and non-follicular lesions encountered in the thyroid, and we propose a systematic diagnostic approach to encapsulated thyroid neoplasms, integrating ultrasonographic, cytologic, histologic, immunohistochemical, and molecular data, in an effort to optimize diagnostic accuracy and guide appropriate clinical management.