LAUSR

Rationale: Parosteal osteosarcoma (PO) is relatively rare, accounting for about 1% of primary malignant bone tumors and 4% of all osteosarcomas. Onset is generally at approximately 20 to 40 years of age; the distal posterior surface of femur is the most common site, accounting for more than 60% cases. However, PO in middle-aged or elderly patients is extremely rare and PO occurring in the fibula is even rarer. To our knowledge, this is the first case of PO occurring in the fibula of a middle-aged adult. Patient concerns: We present a case of a 47-year-old female who had a lateral mass on her left knee for 3 months with no obvious cause. Diagnoses: The patient was initially diagnosed with a proximal left fibula tumor via x-ray scan at a local hospital. Subsequently, osteochondroma was highly suspected through computed tomography (CT) and magnetic resonance imaging (MRI) obtained at our hospital. Finally, the postoperative pathological report confirmed PO. Interventions: She underwent a complete resection of the left fibula tumor and received postoperative adjuvant chemotherapy. Outcomes: The patient recovered well and was discharged on the 10th day after admission. Additionally, there was no evidence of recurrence at the 4-year follow-up after surgery. Lessons: In middle-aged or elderly patients, early diagnosis of PO in the fibula is difficult and misdiagnosis or missed diagnosis is common. In clinical practice, PO is uncommon in middle-aged or elderly patients, occurs more rarely in the peroneal bone, and should be carefully identified. CT and MRI are essential for diagnosis, with pathological examination being effective for confirmation. Because of the limited reports of PO of the fibula in middle-aged or elderly adults, the present case raises awareness of this rare condition.