James Ewing was the first to report Ewing's sarcoma (EWS), and theorized that EWS originated from the vascular endothelium. In 2002, the World Health Organization classified EWS and peripheral primitive neuroectodermal tumor (pPNET) as a single pathological entity. EWS/pPNET is the second most common primary bone tumor after osteosarcoma in adolescents and children, which usually occurs in the long bones of limbs and pelvis and most often appears in the second decade of life. There are no significant gender differences[8, 9]. Primary intracranial locations are extremely rare, with only approximately 80 cases of primary intracranial EWS/pPNET reported in English, with a relatively high number of male patients. The clinical presentation of primary intracranial EWS/pPNET is varied and is usually determined by the tumor’s site, location and size. In the reported cases, headache and vomiting are the most common clinical symptoms related to increased intracranial pressure (ICP), followed by cranial nerve damage, seizures, and a cranial mass[10, 11]. In this case, the main symptoms were headache and vomiting, which are common signs of raised ICP.
Previous studies have demonstrated that the tumor commonly exhibits iso-to-hypointense signals on T1WI scan and iso-to-hyperintense signals on T2WI scan. As far as enhancement is concerned, it can be expressed as moderate, heterogeneous, and intense enhancement, with heterogeneous enhancement being the most common. The performance of this case is consistent with the literature report. However, due to the non-specific and variable radiological characteristics, it is difficult to make any confident radiological diagnosis; it is easily misdiagnosed as rhabdomyosarcoma hemangiopericytoma or metastatic tumors, especially malignant meningioma. Hence its final diagnosis depends on immunohistochemistry and gene detection.
Previous studies have reported that the MIC2 gene product (CD99) immune expression is important for EWS/PNET diagnosis . The sensitivity of CD99 is 93%, and the specificity is 80%. A central primitive neuroectodermal tumor (cPNET) that is negative for the MIC2 gene and CD99 can be used in the differential diagnosis. However, CD99 can be found in other small blue round cell tumors, including lymphoma, neuroblastoma, and rhabdomyosarcoma [14, 15]. Thus at present, the final diagnosis must be confirmed by the translocation of EWS gene. The most common translocation is t (11, 22)( EWS-FLI-1), which accounts for >90% of EWS/pPNET. The second is ESW/ERG t(21, 22) (EWS-ERG), with less common translocations including EWS-ETV1, EWS-E1AF, and EWS-FEV, with an incidence of each case being less than 1%.
Primary intracranial EWS/pPNET and cPNE are both primitive ectoderm tumors. However, the cell-of-origin is different, cPNET originates from granular cells in the outer cerebellum, subventricular stromal cells of the fourth ventricle, or pineal precursor cells. Primary intracranial EWS/pPNET is mainly derived from the dura mater, and the cells of its origin are not yet clear. It is theorized that it originates from neural crest cells (neural crest cells are involved in meningeal embryonic development). It is important to distinguish between primary intracranial EWS/pPNET and cPNET as treatment and prognosis are entirely different. The reported disease-free survival period of cPNET is short, and the long-term disease-free survival period of primary intracranial EWS/pPNET is nine months to eight years, much longer than cPNET.
Due to the rarity of primary intracranial EWS/pPNET there is a lack of understanding of treatment options for primary intracranial EWS/pPNET. The current treatment recommendation from the National Comprehensive Cancer Network (NCCN) is local treatment (surgery and/or radiotherapy) plus chemotherapy[16, 19]. A gross total resection (GTR) can improve patient quality of life, improve long-term survival and reduce local recurrence. The prognosis of patients who did not receive GTR was worse than those who received GTR [4, 10]. Chen et al.  reported that patients treated with partial resection had a shorter median survival time than those treated with GTR. Chemotherapy is one of the most important treatments for primary intracranial EWS/pPNET. A previous study showed that adjuvant chemotherapy improved the 5‑year survival rate from 5 to 10% to >65%[17, 20]. Current systemic chemotherapy drugs include vincristine-doxorubicin-cyclophosphamide and dactinomycin alternating with ifosfamide-etoposide. Local radiotherapy is generally used for tumors that cannot be removed by surgery or those with residual tumors. Chen et al.  reported that adjuvant radiotherapy can improve the patients’ 1-year and 2-year survival rate (from 60.0% to 88.9%, 0 to 66.7%, respectively). We have observed that patients undergoing surgery, radiotherapy and chemotherapy may have the best prognosis. Michael et al.  reported that a young female patient that received GTR, systemic chemotherapy, and focal radiation had a survival time of 10.5 years. Our case has received surgery, radiotherapy, chemotherapy, and thus far, no indication of recurrence or metastasis.
The risk factors associated with worse prognosis in primary intracranial EWS/pPNET include tumor location, surgical margins, radiotherapy, chemotherapy, and with or without metastasis. Distant metastasis are a poor prognostic factor, with extrapulmonary metastases having the worst prognosis. However, age, gender, and the occurrence of symptoms does not influence the prognosis [4, 10-12].