Axillary lesion in a young adult ends up to a peculiar diagnosis: primitive neuro-ectodermal tumor (PNET) of ulnar nerve: a rare case report.

Introduction and importance: Primitive neuro-ectodermal tumor (PNET) is a highly aggressive tumor composed of small round blue cells, mostly developing in children and young adults. Being a member of Ewing's Sarcoma Family of Tumors (ESFT); it has been discussed in two subcategories of central and peripheral PNET. PNETs of peripheral nerves are very uncommon pathologic findings, as to the best of our knowledge only 12 well-documented cases have been yet reported. Case presentation: A 30-year-old male presented with progressive paresthesia of his right hand's little finger and painless swelling of the right axilla. Magnetic resonance (MR) neurography demonstrated a heterogeneous, high-signal, round mass within the right axilla fossa in proximity to the medial aspect of brachial plexus branches. The clinical and radiological study failed to an accurate diagnosis, thus surgical resection of the tumor was done for tissue evaluation. Histopathologic study of the lesion revealed a neoplasm comprising sheets of small, round, blue cells (Hematoxylin and Eosin stain), which immunohistochemically consisted with the diagnosis of PNET. Clinical discussion: The differential diagnosis of axillary fossa masses, focusses on peripheral nerve tumors like Schwannoma and PNET. MR neurography aids in evaluation, but tissue diagnosis remains crucial. Treatment involves surgical resection, chemotherapy, and radiotherapy tailored to individual patients. Conclusion: Although pPNET is not apparently the first differential diagnosis coming to mind when encountering a rapidly growing mass in the axillary fossa with peripheral nerve origin, its highly malignant behavior, makes it crucial to be considered in the differential diagnoses.