Rare Papillary Thyroid Carcinomas

Abstract

Papillary thyroid carcinoma (PTC) is a malignant epithelial tumour showing evidence of follicular cell differentiation and a set of distinctive nuclear features. Following the criteria of the last WHO classification, either papillae or invasion is required for a diagnosis of PTC [1]. Although the diagnosis of PTC is not usually problematic, numerous variants of PTC have been recognized, sometimes raising diagnostic difficulties. In the follicular-patterned tumours with PTC nuclear features, the diagnosis is based primarily on the identification of invasion signs and will not be addressed in this book [1, 2]. It is particularly important to recognize those variants of PTC that are associated with clinical aggressiveness, as is the case with tall cell variant, columnar cell variant, the recently described hobnail (micropapillary) variant of PTC and PTC displaying diffuse nuclear immunoreactivity for P53. Since the hobnail (micropapillary) variant of PTC is the rarest form of the aforementioned three morphologic variants and its diagnosis is more difficult, it will be specially addressed in this chapter, together with other (very) unusual forms of PTC.