A Case Report of Primitive Myxoid Mesenchymal Tumor of Infancy (PMMTI) with BCOR-ITD Genetic Alteration in the Sinonasal Cavity and Review of Literature.

[BACKGROUND] Primitive myxoid mesenchymal tumor of infancy (PMMTI) is a rare fibroblastic/myofibroblastic neoplasm, mostly occurring in the deep soft tissues of infants and young children. This study aims to report the first case of PMMTI arising in the sinonasal cavity with BCOR-ITD genetic alteration and elucidate its clinicopathological and genetic characteristics through a comprehensive literature review.

[METHODS] Clinical data of an 11-month-old male infant with sinonasal PMMTI was collected and analyzed. Imaging examination (MRI) was performed for lesion evaluation. Pathological examination (histomorphological observation) and immunohistochemical staining (for BCOR, SATB2, androgen receptor, cyclin D1, vimentin, CD99, CD10, β-catenin, etc.) were conducted on the resected tumor. Next-generation sequencing (NGS) was used for genetic detection. The patient was followed up for 9 months, and relevant literature was comprehensively reviewed.

[RESULTS] The patient presented with a hemorrhagic and necrotic mass in the right sinonasal cavity causing local bone destruction. MRI showed a cystic-solid lesion expanding into the right nasal cavity and orbit, with homogeneous hyperdensity on T2-weighted imaging (T2WI) and hypodensity on T1-weighted imaging (T1WI). Pathological examination revealed a prominent myxoid stroma with diffuse proliferation of spindle-shaped, round, and stellate cells, interspersed with delicate branching vasculature. Immunohistochemically, nuclear staining was positive for BCOR, SATB2, androgen receptor and cyclin D1; vimentin and CD99 were diffusely positive; CD10 was focally positive; no nuclear staining was observed for β-catenin; Ki-67 proliferation index was approximately 40%. NGS identified an internal tandem duplication (ITD) in exon 15 of the BCOR gene. During follow-up, the patient had tumor recurrence 6 months postoperatively and remained alive with persistent disease.

[CONCLUSIONS] PMMTI is a moderately to low grade malignant tumor. This is the first reported case of PMMTI occurring in the sinonasal region. PMMTI should be included in the differential diagnosis of pediatric sinonasal tumors, and molecular diagnostics should be integrated to confirm BCOR alterations for accurate diagnosis.