Cystic papillary thyroid carcinoma in fine needle aspiration may represent a subset of the encapsulated variant in WHO classification.

This study audits the reliability of ultrasound-guided fine needle aspiration (FNA) in excluding papillary thyroid carcinoma (PTC) in thyroid cysts containing mural nodules, and investigates the histological counterpart of cystic PTC diagnosed on FNA. Using a 10-5 MHz ultrasound probe and a 27-gauge needle, solid portions of thyroid nodules were sampled and assessed immediately using both Diff-Quik and Ultrafast Papanicolaou stains. Unlike usual PTCs that demonstrate hypercellularity, the aspirates of cystic PTC showed low cellularity and consisted of papillae with atypical nuclei scattered in abundant thin colloid. Over a period of 13 years, histological follow-up was obtained from 11 women and 6 men in whom cystic PTC was reported on FNA. This represented 4.4% of 383 cases of PTC reported and 0.25% of all thyroid FNAs performed. In all 17 cases, histopathology showed encapsulated PTC in various stages of cystic degeneration. Sonography correlated well with histopathology, where findings ranged from cysts with small mural nodules to solid nodules with pockets of thin colloid. In 87 patients with thyroid cysts containing mural nodules, FNA findings were benign, as was clinical follow-up that ranged from 1 to 12 years. In conclusion, ultrasound-guided FNA, if performed in the manner described, can reliably distinguish cystic PTC from a benign cyst with a mural nodule. Cystic PTC on FNA in this series correlates to a subset of the encapsulated variant of PTC, an entity described in the 1988 WHO Histological Typing of Thyroid Tumours in the good prognostic category.