Challenges in the differential diagnosis of interdigitating dendritic cell sarcoma of intraparotid lymph node vs. metastatic malignant melanoma with unknown primary site.

Interdigitating dendritic cell sarcoma (IDCS) is an uncommon form of malignant histiocytosis affecting dendritic cells. The parotid gland more frequently than other salivary glands has metastasis from extraparotid tumours, which in 80% of cases are melanomas and squamous cell carcinomas. Herein we report our case, a 64-year-old woman who presented with a short history of fluctuating in size swelling below her right ear. Ultrasound scan showed a loculated cystic lesion extending in the parotid parenchyma. Fine needle aspiration (FNA) revealed appearances that were highly suspicious of malignancy, therefore MRI scan was arranged, and parotidectomy planned. The histology of tumour was a malignant spindle cell neoplasm, with immunohistochemical features highly suggestive of metastatic malignant melanoma with divergent differentiation. The challenges in the differential diagnosis of IDCS of intraparotid lymph node vs. metastatic malignant melanoma with unknown primary tumour are described here. The rarity of this neoplasm figures highlights the importance of describing all new cases putting special emphasis on the steps to be taken in order to shorten the diagnosis, management and treatment process.