Cytomorphology of neuroendocrine tumours of the gallbladder

Gallbladder neuroendocrine tumours (GB-NETs) are rare and account for 0.5% of all NETs. GB-NETs have an aggressive behaviour, which depends on the tumour grade. The cytomorphological spectrum of these tumours has never been described in detail. The present study evaluates the cytological features of GB-NETs and grades them according to the World Health Organization (WHO) classification. Furthermore, the expression of thyroid transcription factor-1 (TTF-1) has not been investigated previously in GB-NETs, although found in a subset of extrapulmonary NETs.

Twenty cases of GB-NET among 875 gallbladder carcinomas diagnosed by ultrasound-guided fine needle aspiration cytology (FNAC) over a period of nearly 4 years were studied. The following parameters were evaluated: architectural pattern, nuclear chromatin, nucleoli, mitoses, necrosis, moulding, apoptosis and smudge cells. Cases were categorized into well-differentiated (grades 1 and 2), small cell carcinoma (SCC) (grade 3) and mixed adenoneuroendocrine carcinoma. Nuclear positivity for TTF-1 was considered as positive.

Morphologically, tumour cells were mainly arranged in rosettes in the well-differentiated category; sudden anisonucleosis and rare nuclear moulding with or without mitotic figures were other features. Eleven cases of SCC showed prominent nuclear moulding with frequent smudge cells, mitoses, apoptosis and necrosis. Three mixed adenoneuroendocrine carcinomas showed papillary fragments and an acinar arrangement of tumour cells. Four of the nine SCCs in which TTF-1 was evaluated on de-stained smears showed nuclear positivity. Histopathology was available in two SCCs and showed morphology similar to FNAC.

Cytology plays an important role in the diagnosis of GB-NETs for appropriate subtype characterization, which is necessary for the prognostication of these tumours. TTF-1 may not be used for the differentiation of gallbladder SCCs from pulmonary SCCs.