The tall cell variant of papillary thyroid carcinoma (PTC) was described in 1976 by Hawk and Hazard as a histological variant with a poor prognosis, characterized by cells at least twice as tall as they are wide.1 Since then, multiple changes have been made in the World Health Organization (WHO) diagnostic criteria, changing the required cell height from at least twice as tall as they are wide to cells 3 times as tall as they are wide (Cx3) in 2004, reverting in 2017 to cells between 2 and 3 times as tall as they are wide (C2−3), and returning to Cx3 in 2022. In addition to these changes in cell description, changes have been made in the required percentage of tumor area occupied by tall cells for diagnosis: while the original description required the tall cell component to be clearly predominant, in 2004 it was required that tall cells should make up >50% of the tumor, and in 2017 the cutoff was cut down to 30%.2

Furthermore, numerous studies indicate that the tall cell variant is associated with a worse prognosis. This poor prognosis has been demonstrated regardless of the diagnostic criteria applied: 30% of cells at least twice as tall as they are wide,3 50% of cells at least twice as tall as they are wide,4 30% of C2-3,5 30% of Cx3,6 50% of Cx3,7 or studies with large case numbers obtained from databases where the criteria were likely not homogeneous.8, 9

The greater aggressiveness of the tall cell variant has been determined by larger tumor size,10, 11 higher rates of extrathyroidal extension,10, 11, 12, 13, 14 lymph node metastasis,12, 14, 15, 16, 17 and distant metastasis12, 13, 14, 15, 17, 18 vs classical papillary carcinoma. Additionally, these tumors tend to occur in older patients,10, 11, 18 present more recurrences,10, 13, 15, 16,19 and are associated with lower survival rates.10, 12, 13, 14, 15, 16, 17, 19

Given the disparity in criteria used across different studies over time, which complicates their interpretation, we aimed to elucidate the best criteria that could define the tall cell variant so that its diagnosis would have prognostic significance. To this end, we evaluated cell height and the percentage of tumor area occupied by tall cells, correlating these with tumor characteristics and outcomes across different groupings, establishing the percentage at which the tall cell component becomes relevant and the height these cells should have.