Deep soft-tissue leiomyoma of the popliteal fossa in a 14-year-old girl.

This is the first case report of a deep soft-tissue leiomyoma with intraarticular localization within the knee joint. A 14-year-old girl presented with a painless mass in the dorsal aspect of her knee which she had noticed some 9 months earlier. While on plain radiographs no bony lesions were visible, magnetic resonance imaging (MRI) revealed an intraarticularly located tumor with well-defined margins. Intraoperatively, the tumor was found adjacent to the posterior cruciate ligament. The final diagnosis was established immunohistologically. Although this is a very rare tumor, it has to be considered in the differential diagnosis for soft-tissue swellings of the popliteal fossa.