RESUMEN
Merkel cell carcinoma (MCC), a rare primary cutaneous small cell neuroendocrine carcinoma, is a tumor with distinct cytological features. In many cases, immunohistochemical staining (IHC) is required for the differentiation from other small round cell malignancies. Here we describe the cytological findings of Merkel cell carcinoma; these findings contributed to the diagnosis prior to performing IHC. A lower eyelid mass was excised and submitted for frozen section diagnosis. The frozen section diagnosis was consistent with a malignancy, but the more specific diagnosis was limited by the lack of specific histological features. Touch imprint cytology revealed a high cellularity with loosely cohesive small to large sized cells. The tumor cells showed hyperchromatic nuclei with fine chromatin and inconspicuous nucleoli, and thin-rimmed-cytoplasm including the characteristic eosinophilic button-like paranuclear inclusion, previously described as a pathognomonic cytological finding of MCC; this was not found in the H&E frozen section. In conclusion, we suggest that the touch imprint cytology may help in the differential diagnosis of small round cell neoplasms prior to performing IHC especially in frozen section diagnosis.