Thyroid carcinoma: Pathologic classification with data on prognosis

Abstract

In spite of a complex variety of microscopic patterns, thyroid carcinoma can be classified pathologically into four basic subtypes, each with a more or less predictable rate of growth, mode of spread, and expected outcome after treatment. These four subtypes are designated as papillary, follicular, medullary, and anaplastic. The name selected for each subtype is not intended to describe the total microscopic structure, but merely to call attention to one diagnostic facet of each type. Thus, papillary cancer, the common tumor among children and young adults, usually is composed of a mixture of papillations and neoplastic follicles. However, either of these structural patterns may dominate the microscopic picture, or the tumor may contain a large, solid component. The exact makeup of microscopic structure in this subtype, while of possible interest in terms of uptake of 131I, does not influence mode of spread, surgical treatment, or prognosis. Tiny papillary tumors arc extremely common, and they come to the attention of clinician or pathologist only when associated with palpable, metastatically involved lymph nodes or when discovered incidentally in a surgical or autopsy specimen. Follicular carcinoma is that subtype of well-differentiated thyroid carcinoma characterized by encapsulation, slow growth, and a tendency toward vascular invasion. Spread is via the bloodstream to lungs or bones, but cervical lymph nodes arc seldom inolved. Microscopically, papillary structure is not seen, but rather, various mixtures of solid and follicular pattern are identifiable; Hürthle cell variants are also included in this subtype. Uptake of 131I, important in the treatment of distant metastasis, depends on the extent of follucular differentiation in a given tumor. Tumors of medullary type, derived from parafollicular cells, are of undifferentiated microscopic structure and contain varying amounts of stromal amyloid. The tumors grow slowly and spread locally to cervical nodes and distantly to lungs or liver. “Anaplastic carcinoma” is the name applied to a subtype of undifferentiated tumors with a variety of microscopic patterns. All are characterized by microscopic evidence of rapid growth and a uniformly fatal prognosis.