Abstract
During the last decades, the histopathological classification of thyroid gland tumors has changed, with stricter criteria for the diagnosis of follicular thyroid carcinoma (FTC), the establishment of poorly differentiated carcinoma, and superposition of nuclear atypia of papillary thyroid carcinoma (PTC) above architecture. Furthermore, the increase in imaging studies of the neck area results in the detection of a high number of thyroid nodules. Fortunately, however, thyroid nodules harbor cancer in only 5% of cases (1).
In recent issues of Thyroid, both aspects of thyroid cancer epidemiology and the reclassification of FTC were covered, with interesting observations (2,3). Vaccarella et al. estimated that diagnostic changes may account for ≥60% of thyroid cancer cases diagnosed during recent years in certain developed countries (2). In a study by Cipriani et al. (3), FTC cases diagnosed during 1965–2007 were re-evaluated by three pathologists. A remarkable 71% of FTC cases were reclassified as follicular adenomas or PTCs. Elimination of follicular adenomas and PTCs led to the finding of a decreased survival in the patients with FTC diagnosed according to current criteria. Moreover, inter- and intra-observer variability in histologic interpretation leads to diagnostic challenges in follicular lesions (4).
We analyzed thyroid cancers in the Pirkanmaa region of Finland. During 1981–2002, a total of 495 patients with differentiated thyroid cancer were treated at the Tampere University Hospital (5). We performed a registry-based follow-up until December 2011. In our cohort, 71 (13%) tumors were initially classified as FTCs. In the patients with FTC, disease-specific mortality was 19 (27%), and mortality from other causes affected 19 (27%) cases during the observation period.
We performed reclassification of FTCs according to the latest World Health Organization guidelines, with the following results: 5/71 tumors (7%) were reclassified as follicular adenomas, and 3/71 (4%) tumors were reclassified as PTCs. The mean age at diagnosis was 49 ± 16 years in PTC patients and 59 ± 18 years in FTC patients. None of the reclassified cases with follicular adenomas or PTC died of thyroid malignancy. Among the reclassified FTC patients, 19 (30%) patients had FTC-related death, and 17 (27%) patients had non-FTC-related death. The median follow-up time was 6.9 years (range 0.1–25 years) for all FTC patients. Patients with FTC-related death had a median follow-up time of 2.5 years (range 0.1–11 years), and patients dying of other causes had a median follow-up time of 5.1 years (range 0.2–18 years).
Our results corroborate that the diagnosis of FTC may be inaccurate in previous historical pathology reports. Inter- and intra-observer variation in the evaluation of follicular thyroid lesions is considerable (4). Nevertheless, in our series, the number of reclassified cases included only 8/71 (11%) tumors. These figures are much lower than the ones reported in the study by Cipriani et al. (3), in which a considerable 71% of FTC cases were reclassified as follicular adenomas or PTCs. Differences in patient material and institutional practices may explain these discrepancies. The material collection time was shorter and more recent in our study than in the study by Cipriani et al. (1981–2002 vs. 1965–2007, respectively), which may have an impact on the decreased quantity of reclassified FTCs. Our cohort consists of patients of a single center with dedicated endocrine pathologists with relatively high volumes of thyroid samples per pathologist, which may also explain the observed difference.