Background: Although rare, the annual incidence of intraosseus sarcomas is up to 2 per 100.000 population with a peak in the sixth decade of life. Due to their slow growth and because they are often asymptomatic for a long period of time, they can be misinterpreted as a benign bony lesion. Particularly, if the tumor shows a high myxoid signal on MRI, the lesion might be mistaken for a bone cyst.
Case presentation: We present the case of a 70-year-old female complaining of pain in her left knee. Initial imaging showed a grade 3 osteoarthritis, according to the Kellgren-Lawrence classification system, and a cystic lesion of the distal femur. The patient received a total knee endoprothesis and five years after the index surgery, she presented again with pain in her left knee after a fall. Radiographs showed an expansile lesion of the distal femur. A biopsy was performed and the lesion proved to be a chondrosarcoma with myxoid pattern. After en bloc-resection, a megaendoprosthesis was implanted in a palliative approach. Four years later, the patient died without local recurrence due to metastatic disease from an adenocarcinoma of unknown origin.
Conclusion: Malignant tumors should be taken into consideration whenever imaging shows an expansile lesion of the bone, even when they are painless, well delimited and slowly or not at all growing. We recommend a biopsy to be performed before performing an arthroplasty in such cases.