A 2-year-old male presented with a distal radius tumor on the left side that had been growing for 8 months. He presented with a pathological fracture, which was treated conservatively and healed 4 weeks after the injury. The mass had been noticed a couple of weeks before the fall but was not investigated until the patient came to the emergency department due to his fall. The family history was negative for consanguineous parents; however, it revealed that the mother had pre-eclampsia during her pregnancy. Upon physical examination, he had an obviously palpable, tender lesion with no erythema. He had no fever, no lymphadenopathy, and no upper respiratory symptoms. Basic laboratory investigations were within normal ranges, and the lactate dehydrogenase level was normal. Both oral and written informed consents were obtained from the legal guardians regarding the case study and future publications. X-ray imaging showed the osseous origin of the mass with an extraosseous component ( and ). It revealed a 5 cm craniocaudally expansile, osteolytic, bubbly intramedullary lesion of the diaphysis of the left radial bone, extending to the metaphysis. The lesion displayed slow-growing features such as septations and a non-destructive cortical invasion. There were some cortical erosions and disruption, but no significant periosteal reaction. Proximally, there was a wide zone of transition; however, distally, only a narrow transition zone was present. It was difficult to determine whether the lesion was benign or malignant. The opacity between the radius and the ulna suggested extraosseous infiltration, which prompted consideration of a malignant etiology. An MRI was consequently ordered to further investigate the soft-tissue component. MRI images showed an aggressive, destructive bone lesion with a low signal on T1 () and a high signal on STIR ( and ), involving the mid and distal radius with an associated soft-tissue component measuring 5.3 × 2.3 × 2.1 cm. The images showed restricted diffusion with an associated adjacent periosteal reaction. A dense enhancement within the bone lesion, along with heterogeneous enhancement, suggested non-liquefied necrosis or possible fibrosis within the extraosseous soft-tissue component (). Both a CT and a PET scan of the chest were negative for metastasis. The differential diagnosis, based on clinical and radiological parameters, included Ewing sarcoma, rhabdomyosarcoma, Langerhans cell histiocytosis, infantile myofibromatosis, and infantile FS. A CT-guided biopsy was performed. Histologically, the sections revealed a proliferation of spindle cells with elongated nuclei arranged in fascicles, separated by numerous blood vessels (). Mild nuclear atypia was present, and few mitoses (<5 HPF) were observed; rare giant cells were also present (). The immunohistochemical study showed proliferating cells positive for vimentin and SMA (). CD95, CD34, S100, and BCL2 were negative. Ki67 labeled 10–15% of the nuclei. The prominent plexiform architecture and myofibroblastic and histiocytic immunophenotypes (positive for histiocytic markers and smooth muscle actin) introduced plexiform fibrohistiocytic tumor into the differential diagnosis (). However, this differential is atypical given the tumor size, location, aggressive features, and the patient’s young age. Considering this atypical presentation, a discussion with musculoskeletal oncology experts was initiated, and the biopsy specimen was sent for additional molecular studies. Following next-generation sequencing results—which enable sequencing of the entire genome within a day [], offering enhanced sequencing output for a concrete insight into the biological context of the disease mechanism []—a low-grade infantile fibromyxoid sarcoma of the bone emerged as the most likely diagnosis. Considering the location and invasion of the soft-tissue extension, along with the encroachment of the neurovascular bundle, the patient underwent surgical resection in the operating room. Radical resection, referring to the extensive surgical removal of a bone tumor—in this case, infantile FS—aimed to eliminate the entire tumor and some surrounding healthy tissue, referred to as the “margin,” to ensure no malignant cells remained. This involved performing two osteotomies or surgical cuts into the bone. The first osteotomy was executed 5 cm proximal to the growth plate, with an additional centimeter allowed to ensure free margins, thus minimizing the risk of leaving cancerous cells near the growth plate—a critical area for growing bone in pediatric patients. The second osteotomy, distal to the first and 1 cm proximal to the growth plate, served to bracket the tumor and its associated tissues for complete removal ( and ). Following the osteotomies, the resection concluded with the total removal of the tumor and its soft-tissue components, including muscle, fat, and connective tissues (). To further stabilize the surgical site and preserve the function of the radiocarpal joint, central translocation of the ulna was performed, with fixation achieved using K-wires, followed by the application of a long arm cast ( and ). This comprehensive, multi-step surgical approach aimed to eradicate all potential sources of cancerous cells, stabilize the affected area, minimize the risk of recurrence, and maximize the chances of a complete cure. The patient’s post-operative course initially unfolded smoothly; however, the presence of positive surgical margins necessitated additional treatment. Consequently, the patient underwent adjuvant chemotherapy and radiotherapy. Four months post-resection surgery, the K-wires were removed due to suspected osteomyelitis. An 8-week treatment regimen of meropenem and teicoplanin was administered to address the infection. The oncological treatment encompassed a total of six chemotherapy cycles and 30 radiation therapy sessions. Specifically, the chemotherapy regimen included ifosfamide (Holoxan) at a dose of 3000 mg/m^2 IV every 24 hours and doxorubicin (Adriamycin) at a dose of 30 mg/m^2 IV every 24 hours, administered daily with the exception of weekends. Two years post-radical resection and subsequent treatments, a deformity developed in the patient’s wrist. Imaging revealed irregular fusion across the distal radioulnar articulation between the distal ulna and the remaining distal radial metaphysis and epiphysis. Malunion and angulation were also observed (-). These issues were successfully addressed through an osteotomy of the distal radius, followed by the application of an Ilizarov fixator. Post-Ilizarov fixation imaging indicated improved alignment, although evaluation was constrained due to the overlying metallic hardware (). The patient is currently faring well and has been discharged from the hospital in stable condition.