The prognostic significance of tall cells in papillary thyroid carcinoma: A case-control study

The subtype of the papillary thyroid carcinoma tall-cell variant has a worse prognosis than does the conventional papillary type (papillary thyroid carcinoma). The new World Health Organization 2017 classification defines a tall-cell variant as a tumour consisting of over 30% of cells that are two or three times as tall as they are wide. However, thresholds have differed. Our aim was to study how tall cells affect the prognosis of papillary thyroid carcinoma patients and to determine, for such cells, a cut-off percentage. Our cohort included 65 papillary thyroid carcinoma patients who underwent surgery at Helsinki University Hospital between 1973 and 1996: originally, 36 otherwise-matched patient pairs, eventually comprising 34 patients with an adverse outcome plus 31 who had recovered. All samples were digitally scanned and scored by two investigators based on tall cell composition. The cohort was analysed with four tall cell thresholds: 10%, 30%, 50% and 70% with a median follow-up of 22 years. In survival analysis, only the 70% threshold showed a correlation with reduced overall survival, disease-specific survival and relapse-free survival. A correlation also emerged with death from papillary thyroid carcinoma. In multivariate analysis, a 70% cut-off and age at diagnosis significantly affected DSS. Increasing tall cell score correlated with increasing age and extrathyroidal extensions. A tall cell composition of 10%, 30% or 50% showed no correlation with adverse outcome and suggests that the choice of pathologists reporting tall-cell variant should be a 70% threshold.