Tall cell variant of papillary thyroid carcinoma without extrathyroid extension: biologic behavior and clinical implications.

Background: The tall cell variant (TCV) is a histologic subtype of papillary thyroid carcinoma (PTC) that is more aggressive than "classical" PTC. Most authors believe that TCV's worse prognosis is related to older age at presentation, larger tumor size, and high frequency of extrathyroid tumor extension (ETE). To assess the biologic and clinical behavior of TCV without ETE, we performed a detailed comparative clinicopathologic analysis of classical PTC and TCV without ETE.

Methods: TCV was defined as a PTC harboring >50% tall cells, while classical PTC was restricted to those tumors containing >1% papillae and <30% tall cells. Microscopic analysis and chart review identified 62 cases of TCV and 83 classical PTC without ETE. These patients were analyzed for various pathologic, imaging, and clinical parameters including outcome.

Results: There was no statistical difference between TCV and classical PTC in relation to age, gender, tumor size, risk stratification, type of therapy, and length of follow-up. TCV displayed more invasion of the tumor capsule and more often infiltrated into the thyroid capsule (p = 0.047 and 0.0004, respectively). Among patients with microscopically assessable regional lymph node (LN), 33 of 49 (67.3%) patients with TCV had LN metastasis at presentation, while only 24 of 60 (40%) classical PTC had positive nodes (p = 0.004). In multivariate analysis, histologic subtype (TCV vs. classical PTC) was the only independent factor associated with LN metastases (p = 0.007). In patients with adequate follow-up, 4 of 62 (6.5%) classical PTC and 7 of the 47 (14.9%) TCV had thyroid cancer recurrence (p = 0.202). TCV recurred at a distant site (3 of 47, 6.4%) while none of the 62 classical PTC developed distant metastases (p = 0.077).

Conclusion: TCV without ETE is biologically a more aggressive tumor than classical PTC without ETE independent of age, gender, and tumor size.