319 Primary sinonasal Ewing sarcoma: A case report Mohd Fariz AMRI 1 , Asma ABDULLAH 2 , Mohd Imree AZMI 3 , Faizah MOHD ZAKI 3 , Suria Hayati MD PAUZI 1 * 1 Department of Pathology, Faculty of Medicine, Universiti Kebangsaan Malaysia Medical Centre, 56000 Kuala Lumpur, Malaysia; 2 Department of Otorhinolaryngology, Head and Neck Surgery, Faculty of Medicine, Universiti Kebangsaan Malaysia Medical Centre, 56000 Kuala Lumpur, Malaysia; 3 Department of Radiology, Faculty of Medicine, Universiti Kebangsaan Malaysia Medical Centre, 56000 Kuala Lumpur, Malaysia Abstract Background: Ewing sarcoma (ES) is an aggressive tumour which is typically skeletal in origin. ES involving the head and neck region is uncommon and can be easily confused with other small round blue cell tumours. We herein present a rare case of ES involving the sinonasal area. Case presentation: A 5-year-old Somalian boy with no known medical illness presented with progressive nasal blockage associated with clear nasal discharge and intermittent spontaneous epistaxis for three months. CT paranasal sinus and neck region revealed poorly enhancing expansile mass in the right maxillary sinus with areas of necrosis within. Initial radiological differential diagnoses were lymphoma and rhabdomyosarcoma. The mass was biopsied and histologically showed diffuse sheets of small round blue cells that was positive to CD99, NSE and vimentin. The muscle and lymphoid markers were negative. Fluorescence in-situ hybridisation (FISH) study revealed the presence of EWSR1 gene rearrangement thus diagnosis of ES was rendered. Conclusions: ES of sinonasal tract is a rare entity and its pathological features significantly overlap with others small round blue cells tumour. Demonstration of EWSR1 gene translocation is recommended for the diagnosis of ES particularly at uncommon sites. Keywords: Sinonasal tract, sinonasal tumour, Ewing sarcoma, EWSR1 gene rearrangement CASE REPORT Malays J Pathol 2021; 43(2): 319 – 325 *Address for correspondence: Suria Hayati Md Pauzi. Address: Department of Pathology, Faculty of Medicine, Universiti Kebangsaan Malaysia Medical Centre, Jalan Yaacob Latif, Bandar Tun Razak, 56000 Cheras, Kuala Lumpur, Malaysia. Telephone: +603-91455364. Fax: +603-91459485. Email: su_hayati@ppukm.ukm.edu.my INTRODUCTION Ewing sarcoma (ES) accounts for 6-8% of primary bone tumour. 1 It is typically seen in the long bones of extremities. Head and neck origin of this tumour is rarely observed (1-4% of all ES) with sinonasal ES forms another uncommon subset. 2 ES of the head and neck may be difficult to diagnose as they display significant histologic overlap with other more common undifferentiated small blue round cell malignancies. Therefore, additional investigations including evaluation of EWSR1 gene rearrangement to confirm the diagnosis of ES are essential particularly at the uncommon sites. Presence of the above gene rearrangement is the hallmark of ES which complement the clinical, radiological, histological and immunohistochemical assessments. CASE REPORT A 5-year-old Somalian boy with no known medical illness presented to a hospital in Somalia with progressive nasal blockage associated with clear nasal discharge and intermittent spontaneous epistaxis for three months. There was no history of foreign body insertion, foul- smelling nasal discharge or visual abnormalities. An initial paranasal sinus computed tomography (CT) scan done at a hospital in Somalia reported as a benign nasal tumour. Subsequently, the parents decided to seek a second opinion at our centre. Further assessment revealed that there was no cervical lymphadenopathy or hepatosplenomegaly. The cranial nerves were intact. Anterior rhinoscopy showed a pink mass covered by slough occupying the whole right nasal cavity with clear secretions