Merkel cell carcinoma- particularities and morphological aspect of a unique and rare entity

Introduction: Merkel cell carcinoma is a very rare malignant neoplasm which presents high aggressivity, high recurrence rate and has metastatic potential. Our purpose is to present the histological and immunohistochemical particularities of Merkel Cell Carcinoma while reviewing potential differential diagnoses and challenges that we can encounter in daily practice.
Case presentation: We present the case of an 86-year-old female patient who presented with a nodular tumour located in the left forearm, raising suspicion of a soft tissue tumour. The histological appearance of this unique type of cancer is highlighted on the Haematoxylin-eosin stain as a solid tumour composed of nests and chords of monomorphic cells. The nuclei of these tumoral cells appear characteristically as enlarged with dispersed chromatin. The immunohistochemical reactions have been performed and it was observed that the tumoral cells exhibited positivity for synaptophysin, CD56, NSE, EMA, as well as a “dot-like” expression for CK20. These histopathological and immunohistochemical features were consistent with a diagnosis of MCC, stage pT3, based on the assessment of tumour size.
Conclusions: Sometimes, differentiating this tumour from other primary malignant neoplasms of the skin or even cutaneous metastases can be difficult. Immunohistochemistry remains the most important tool of diagnosis, especially for differentiating this neoplasm from metastatic neuroendocrine tumours that can affect the skin.