Microscopic description:
The lesion shows densely packed sheets of small, round, uniform, undifferentiated cells with round to oval nuclei, fine granular chromatin, small nucleoli, and pale-to-clear scanty cytoplasm with minimal amount of stroma
IHC stain showed CD99, CD117, synaptophysin (focal), and S100 positivity
Neoplastic cells were negative for WT-2, AE1/AE3, CK7, CK20, PAX-8, CD3, and CA-IX
PAS was positive
FISH EWSR1 translocation
Discussion:
PNET is a small round cell malignant neoplasm with a possible origin from neural crest cells and the primitive neuroendocrine cells
Often seen in bone, but rarely may arise from solid organs containing neuroendocrine cells like kidney, bladder, heart, lungs, parotid glands, and small intestine[1]
Usually observed in young adults and adolescents with an aggressive clinical course, with early metastasis to bone, liver, lung or lymph nodes and poor prognosis [2]
Patients present with malaise, abdominal pain, renal colic, flank pain, hematuria, weight loss and abdominal distension
Grossly, these tumors are solid, lobulated in appearance, and may be associated with necrosis, hemorrhage, cystic change and/or calcification
Microscopically, monotonous small dark round cells arranged in cords, nests or clusters with or without rosettes and pseudorosettes are observed. The hallmark finding is the formation of rosette- like structures by the tumor cells
PNET cells diffusely express CD99, vimentin, NSE, CD56 with focally reactive S-100 protein and synaptophysin
CD99 positivity is also shared by other entities like synovial sarcoma, rhabdomyosarcoma and blastema predominant Wilms tumor
Positive EWSR1 translocation required to confirm the diagnosis
Vascular invasion is commonly seen: inferior vena cava and pulmonary artery tumour thrombus, as well as direct intraspinal
Renal PNET should be seriously considered if a malignant renal neoplasm with renal vein and IVC invasion is found in a young adult or adolescent[3]
References:
Li, T., et al., Primary Ewing's sarcoma/primitive neuroectodermal tumor of the ileum: case report of a 16-year-old Chinese female and literature review. Diagn Pathol, 2017. 12(1): p. 37.
Wada, Y., et al., Primitive neuroectodermal tumour of the kidney with spontaneous regression of pulmonary metastases after nephrectomy. BJU Int, 2003. 91(1): p. 121-2.
Chu, W.C., et al., Primitive neuroectodermal tumour (PNET) of the kidney: a rare renal tumour in adolescents with seemingly characteristic radiological features. Pediatr Radiol, 2008. 38(10): p. 1089-94.