Variants of Papillary Thyroid Carcinoma: Multiple Faces of a Familiar Tumor

Classically characterized by its papillary appearance, papillary thyroid carcinoma is the most common malignant neoplasm of the thyroid. Several variants of papillary carcinoma have been described considering size (microcarcinoma); nature of the tumor boundaries (encapsulated); architecture (follicular, macrofollicular, cribriform-morular, solid, micropapillary); cellular characteristics (tall cell, columnar, oncocytic, clear cell, hobnail); additional tumor components (papillary carcinoma with focal insular component, papillary carcinoma with spindle and giant cell carcinoma, papillary carcinoma with squamous cell carcinoma, papillary carcinoma with mucoepidermoid carcinoma); stromal features (papillary carcinoma with fasciitis like stroma); or a combination of the abovementioned characteristics (diffuse sclerosing, Warthin-like). Although most variants do not pose any diagnostic problems; some histological variants can be challenging to the pathologist and some are of clinical significance, because of prognostic implications or accompanying clinical conditions. It is not rare to find a papillary carcinoma showing characteristics of more than one variant. For example: an individual tumor can show hobnail features and present as a microcarcinoma; another case can show solid architecture and clear cytoplasm. In such cases, it is more important to comment on clinically relevant morphological characteristics of the tumor, rather than religiously trying to classify an individual tumor into a particular variant. In this review, pathologic and clinical features of papillary carcinoma variants are summarized.