Answer: Papillary carcinoma, follicular variant

Histology: solitary, circumscribed and encapsulated. Unlike case 5, the tumor does not invade its surrounding capsule or extend into blood vessels within the capsule. The tumor demonstrates a follicular growth pattern. Many of the follicles are elongated and irregularly shaped, and the colloid filling these follicles is dark-staining. Well-developed papillae are not present, but small abortive papillae protrude into some of the elongated follicles. The cells lining the follicles have atypical nuclei that are characterized by crowding, lack of polarity, clear chromatin, elongation and prominent grooving.

Discussion: The follicular variant of papillary thyroid carcinoma (FVPTC) is the most common variant of PTC, exceeded in numbers only by the conventional form of PTC. It shares with other follicular neoplasms a prominent follicular growth pattern, but the tumor cells exhibit the same nuclear irregularities that define PTC. Unlike follicular carcinoma, demonstration of invasion is not required to establish the diagnosis of malignancy (although it can be very helpful when found!). In the absence of tumor invasion, the distinction between FVPTC and follicular adenoma rests solely on the pathologist’s perception of the nuclear qualities of the follicular cells. Unfortunately, there are no strict guidelines that help define the lower threshold for diagnosing FVPTC. As a result, opinions (even among experts in thyroid neoplasms) diverge when it comes to diagnosing encapsulated follicular neoplasms where the nuclear atypia of PTC is only partially developed. When this type of tumor is encountered, some prefer the designation of “well-differentiated tumor of uncertain malignant potential”. In the present case, the nuclear atypia is quite striking rendering the diagnosis of FVPTC straightforward.