Synovium as a widespread pathway to the adjacent joint in undifferentiated high-grade pleomorphic sarcoma of the tibia

RATIONALE Undifferentiated high-grade pleomorphic sarcoma (UPS), originated from bone, is a rare tumor, accounting for 2% to 5% of all primary maligment bone neoplasms. Skip lesion can be found in undifferentiated high-grade pleomorphic sarcoma of bone (UPS-B). However, the direct invasion across the articular synovium to bone has not been reported previously. PATIENT CONCERNS We report an unusual case of a 65-year-old man complained of a year history of pain, swelling, and limitation of activity in the left knee joint. At the proximal tibia, there was extensive invasion of articular synovium, which provides a direct anatomic pathway for the tumor invasion to the adjacent bone, including patella and femoral condyle. DIAGNOSES Magnetic resonance imaging was important in defining the marrow involvement and joint invasion, including the thickening articular synovium. Subsequent pathological examination confirmed the diagnosis of UPS. INTERVENTIONS The patient underwent an extensive resection of the knee joint, except for the patellar. OUTCOMES After operation, routine chemotherapy was performed. Unfortunately, half a year later, soft tissue swelling of whole thigh was found. Then this patient came our hospital again. Positron emission tomography imaging showed there was recurrence of UPS with lung metastasis. A week later, this patient died. LESSONS In contrast to frequent infiltration pathway, the articular synovium as a media for this tumor spread is rare. This study adds a better understanding of this direct invasion way to the medical literature.