Abstract:Background 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. Two subcategories of central and peripheral PNET have been discussed. Central nervous system (CNS) is the usual primary site where PNETs are found, while PNETs of peripheral nerves are uncommon. Ulnar nerve, in particular, is an extremely peculiar origin for PNET and to the best of our knowledge only few well-documented cases have been yet reported.Clinical Presentation: A 30-year-old male presented with progressive paresthesia of right hand’s little finger and painless swelling of right axillary. Magnetic resonance (MR) neurography showed a heterogeneous, high-signal, round mass within the right axillary fossa in proximity to the medial aspect of brachial plexus branches. Clinical and radiological study were unable for accurate diagnosis and surgical resection of the tumor was done. Pathology reported a small, round, blue cell tumor which immunohistochemistricaly was consistent with PNET.Conclusion: Although pPNET is not obviously the first differential diagnosis coming to mind when a rapidly growing mass in the axillary fossa arises from the peripheral nerves, but due to its highly malignant behavior, it is important to be considered in the differential diagnosis of peripheral nerve neoplasms.Running title: Primitive Neuro-Ectodermal Tumor of Ulnar NerveKeywords: Peripheral primitive neuroectodermal tumor, pPNET, Ulnar nerve

Primitive neuroectodermal tumor (PNET) is a highly aggressive tumor and mostly develops in children and young adults. PNETs of peripheral nerves are uncommon. Ulnar nerve, in particular, is an extremely peculiar origin for PNET and to the best of our knowledge only few well-documented cases have been yet reported.