Chondromyxoid fibroma-like osteosarcoma: a case report
Chondromyxoid fibroma-like osteosarcoma (CMF-OS) is an exceedingly rare subtype of low-grade central osteosarcoma (LGCO), accounting for up to 10% of cases and making it difficult to diagnose. CMF-OS is frequently misdiagnosed on a radiological examination and biopsy, even after the initial operation. Its treatment is a controversial issue due to its low-grade classification and actual high-grade behavior.

A 21-year-old female presented with a one-month history of pain and weakness of her left knee. A physical examination (PE) showed obvious swelling and limitations in mobility. Significant lab test results included an elevated alkaline phosphatase (ALP) level of 240 U/L. Computed radiography (CR) showed an osteogenic lesion in the distal femur and an associated soft tissue mass. Computed tomography (CT) findings were similar to those of CR. Magnetic resonance imaging (MRI) demonstrated abnormal signals in the distal femur on T1-weighted and T2-weighted images . The radiological diagnosis was osteosarcoma, which agreed with the biopsy results. She received four cycles of neoadjuvant chemotherapy before surgery, and the necrosis rate of the tumor was only 10%, indicating a poor prognosis. She was confirmed to have CMF-OS and received one cycle of adjuvant chemotherapy after surgery. Six months later, several nodules accompanied by pleural effusion were found in her lung and confirmed as metastatic tumors. The patient received no further treatment and refused further follow-up.

Conclusively, it is important to recognize CMF-OS and distinguish it from CMF, CHS, COS and other LGCOs. CMF-OS has a relatively poor prognosis despite its low-grade classification.