Papillary thyroid carcinoma with hobnail features: histopathologic criteria to predict aggressive behavior.

Recent reports indicate that papillary thyroid carcinoma with hobnail features, also designated as micropapillary variant of papillary thyroid carcinoma, is a rare but very aggressive variant of papillary thyroid carcinoma. We examined the histopathologic and immunohistochemical features of 24 cases of papillary thyroid carcinoma with hobnail/micropapillary component to determine the prognostic significance of the amount of hobnail/micropapillary features in these tumors. The patients included 18 women and 6 men. Ages ranged from 28 to 78 years (mean, 57 years). Tumor size ranged from 1 to 5.8 cm (mean, 3 cm). The average follow-up time was 106 months (range, 4-274 months). Twelve cases (50%) of papillary thyroid carcinoma showed more than 30% hobnail/micropapillary features, and all but 3 cases were associated with an aggressive behavior. During the follow-up, 6 of these patients died of disease after a mean of 44.8 months, and 3 patients remained alive with extensive disease after a mean follow-up of 32.3 months. Metastases to lymph nodes or distant organs showed a hobnail pattern of growth similar to the primary tumor. The remaining 3 patients with prominent hobnail/micropapillary features were alive with no evidence of disease after a mean follow-up of 125.3 months. The other 12 papillary thyroid carcinoma cases (50%) showed less than 30% hobnail/micropapillary features. Nine of these patients were alive without disease after a mean of 162 months, and 1 patient died of sepsis, which was not related to thyroid tumor after 155 months. Two patients in this group died of disease after 21 and 163 months, respectively. These findings confirm earlier observations that papillary thyroid carcinoma with hobnail/micropapillary features is an aggressive variant of papillary thyroid carcinoma. Tumors with more than 30% hobnail/micropapillary features were often very aggressive, although 2 patients with tumors with 10% hobnail/micropapillary features also had poor outcomes.