Follicular Thyroid Carcinoma: How Have Histologic Diagnoses Changed in the Last Half-Century and What Are the Prognostic Implications?

Background: Follicular thyroid carcinoma (FTC) comprises 10% of differentiated thyroid cancers. Diagnostic controversy and interobserver variability render the practical diagnosis of FTC difficult. Overall survival rates vary (46-97%). The aims of this study were to review FTC histologically at the authors' tertiary care institution and to evaluate long-term survival and recurrence.

Methods: Diagnostic slides from 66 FTC cases (1965-2007) were reviewed by three pathologists from two institutions (blinded to clinical outcomes), and consensus was obtained. Patient demographics, tumor characteristics, and treatment, survival, and recurrence data were collected. Thyroid cancer-specific and recurrence-free survival were calculated by original and reclassified diagnoses.

Results: Forty-seven cases (71%) were reclassified: 24 (36%) to papillary thyroid carcinoma (PTC), 18 (27%) to follicular adenoma (FA), and five (8%) to poorly differentiated carcinoma (PDC). Nineteen (29%) maintained a diagnosis of FTC. The extent of surgical resection and rates of radioiodine treatment did not differ by reclassification diagnosis. Pre-review FTC-specific survival was 83.5% and 75.1% at 10 and 20 years, respectively. Following contemporary reclassification, FTC-specific survival was 77% and 33.7% at 10 and 20 years, respectively. There were no cancer-specific deaths in the FA or PTC groups.

Conclusions: Over the past 50 years, changes in our understanding of the pathogenesis, histology, and behavior of thyroid carcinoma may partially account for the changes in histologic diagnosis. Elimination of PTC and FA "contaminants" led to decrease in survival following reclassification. Variability in histologic interpretation contributes to diagnostic challenges in follicular lesions. Histologic review of thyroid tumors for research studies is crucial, especially given the ever-changing diagnostic criteria.