Fine needle aspiration cytology of metastatic Merkel cell carcinoma

Merkel cell carcinoma (MCC) is uncommon cutaneous malignant neuroendocrine tumour of the elderly people with rapidly growing skin nodules found frequently on sun-exposed areas of the body. MCC is often an aggressive tumour with high tendency for local recurrence, lymph node involvement and distant metastases. This paper reports a case of metastatic MCC diagnosed by fine needle aspiration cytology (FNAC), flow cytometric deoxiribonucleated acid (DNA) analysis, pathohistology and electron microscopy. The cytological features in aspirate (stained with Papenheim and Papanicolaou staining) included increased cellularity, discohesive groups of small-to-medium size malignant cells with uniform, round-to-oval nuclei with moulding effect, fine chromatin, multiple micronucleoli and scanty cytoplasm. DNA flow cytometric analysis of the aspirate showed unexpected results for clinically aggressive behaviour of this tumour (the patient died after 21 months), and revealed that tumour contained diploid peak with DNA index of 1.1. The proliferation was high with elevated S-phase fraction (21%). The cytological diagnosis of possible metastatic MCC was confirmed by histological one as well as by electron microscopy presented the pathognomonic features for this tumour: dense-core neurosecretory granules with diameter of 100-250 nm surrounded by whorls of intermediate filaments. MCC provides an enormous challenge for the morphologist because of a wide range of differential diagnosis and for the clinician because this tumour has a highly malignant potential for local recurrence, nodal and distant spread and very often is combined with other tumours. Therefore it is important to perform FNAC of different lesions in the same patient because it can distinguish MCC from the other tumours.