Background and Importance
Primitive neuroectodermal tumor (PNET) was first introduced by Hart and Earle as a rare small round cell malignant tumor originating from neuroectoderm1. It is a member of Ewing’s sarcoma (ES) family and mostly develops in children and young adults. Based on differential classification, two subtypes of peripheral PNET (pPNET) and central PNET (cPNET) have been introduced. Evidence suggests that cPNET arises from precursor cells of the subependymal matrix of the central nervous system (CNS) or external granular layer of the cerebellum, pinealocytes, and subependymal cells of the ventricles. In contrast, pPNET originates from the neural crest located outside the CNS2,3.
pPNET is responsible for about 4% of all soft tissue tumors4,5 and can appear in uncommon sites6. Cutaneous localization of pPNET is very rare in adults thus can easily be misdiagnosed7. No definitive diagnostic clue on radiologic findings or laboratory tests is known yet2,8.
As the tumor occurs at young age, long-term survival remains a challenge in patients management1,2,9. While a five-year survival is expected in about 70-80% of patients with localized PNET, it drops by less than 30% in patients with a metastatic disease10. Therefore, an early and accurate diagnosis is crucial for successful management of the patients.
The two subtypes of PNET are morphologically identical5,11. The differential diagnosis of PNET includes other similar round blue cell tumors with the same histomorphological characteristics. Therefore, Immunohistochemical study is mandatory for definite diagnosis12.
Differentiation markers such as CD99 (MIC2) when a positive membranous staining is seen, can distinguish PNET from other round blue cell tumors12,13.
As PNETs are extremely invasive, they do not usually have a clear rim with the adjacent tissues14. According to previous studies, PNET mostly shows mixed isointense to hypointense signals on T1-weighted imaging and isointense to hyperintense signals on T2-weighted imaging, with significant enhancement on contrast magnetic resonance imaging (MRI). However, definite diagnosis cannot be confirmed radiologically, indicating the necessity of other confirmative tests, such as pathological studies1,5,6.
Chest computed tomography (CT) scan and MRI of the primary site, bone scan and other metastasis investigations are strongly recommended for tumor staging10,15. We present a rare case of an axillary fossa mass with the diagnosis of schwannoma in the primary needle biopsy following the diagnosis of PNET in the surgical excision sample.