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.