Giant cell tumor of the tendon sheath (GCT-TS) is a benign soft tissue tumor of the tendon sheath and synovium (1). GCT-TS is the second most common type of tumor of the hand, and gnalgion cysts are the most common (2). The majority of GCT-TS cases occur in the fingers and toes, however, rare cases of GCT-TS occur in the knee, exhibiting a nodular pattern of growth (2). GCT-TS usually occurs in individuals between the ages of 30 and 50 years, with a predominance for females, and exhibit the capacity for local recurrence following surgical resection (1). Marginal excision is the standard treatment for of GCT-TS. However, despite its benign character, the rate of local recurrence following excision has been reported to range between 10 and 20% (1). The high local recurrence rate may be due to the fact that complete excision may be difficult, as the mass is frequently associated with the tendon sheath or synovial joint. The current report presents an unusual case of GCT-TS arising from the patellar tendon sheath, extending into the knee joint and involving the tibia. Information provided by the present report may assist clinicians in establishing a more informed diagnosis and administering the correct treatment. Written informed consent was obtained from the patient.

In December 2012, a 41-year-old male with a 15-year history of a slow-growing, painless mass in the left knee and no history of trauma was referred to Shinshu Ueda Medical Center (Ueda, Japan) by the patient’s doctor.

Physical examination revealed a mass, 8 cm in diameter, in the anterior aspect of the left knee (Fig. 1). No localized warmth, redness or tenderness was observed in the left knee, however, there was minimal effusion. The range of motion was 0–125° degrees in the left knee compared with 0–140° in the right knee. The patient displayed normal gait and no neurovascular deficit was observed.

Plain lateral radiographs identified bone erosion in the proximal tibia (Fig. 2), however, no bone lesions were observed in the distal femur and patella. Magnetic resonance imaging (MRI) identified a well-localized mass that extended from the subcutaneous tissue in the anterior aspect of the knee to the infrapatellar fat pad, and deep into the patellar tendon and tibia, exhibiting a homogenous low signal intensity on T1-weighted [repetition time (TR)/echo time (TE), 540/12 msec] and T2-weighted (TR/TE, 4000/84 msec) images and diffuse enhancement (Fig. 3A and B). An axial image revealed that the tumor was wrapped around the patellar tendon (Fig. 3C).

Histopathological examination of the biopsy specimen revealed prominent histiocytes, as well as a variable number of foamy cells, hemosiderin-laden macrophages and multinucleated giant cells (Fig. 4). No mitotic activity or malignant features were observed, thus, the preoperative diagnosis was determined as GCT-TS. Although the most appropriate surgical treatment of GCT-TS is marginal resection, in the present case, en bloc marginal resection could not be achieved; therefore, a piecemeal resection was performed and the patellar tendon remained intact.

The postoperative recovery period was uneventful. The patient regained full range of motion of the knee joint and at the one-year follow-up the patient was asymptomatic. Repeat MRI scans revealed no evidence of recurrence.

The diffuse forms of pigmented villonodular synovitis (PVNS) and GCT-TS are classified as fibrohistiocytic tumors by the World Health Organization (3). GCT-TS occurs more frequently in the digits of the hands and feet compared with in the larger joints; whereas PVNS (also termed diffuse-type giant cell tumors) infiltrate and grow as diffuse tumors in large joints, such as the knee, elbow and ankle (3–5). While patients exhibiting diffuse-type PVNS in the knee typically present with hemarthrosis, patients exhibiting nodular-type GCT-TS typically present with a painless mass. However, certain GCT-TS in the knee joint cause continuous anterior knee pain and/or a locking knee (6,7).

The diagnosis of GCT-TS in the digits is considered to be straightforward; patients typically present with a painless mass, the lesion is usually well-circumscribed and localized, and it infrequently erodes or infiltrates adjacent bone (8). However, GCT-TS in large joints may be more difficult to diagnose, as there are few symptoms, which are non-specific (9). In such cases, differential diagnoses include synovial cyst, ganglion, synovial sarcoma, malignant fibrous histiocytoma and lipoma.

The diagnosis of GCT-TS is rarely aided by the use of plain radiographs, although, bone erosion or soft tissue swelling is occasionally observed using this imaging technique (9). In the present case, plain radiographs were used to identify tibial erosion (Fig. 2). MRI is an effective and highly sensitive tool for the diagnosis of GCT-TS and T1- and T2-weighted imaging (T1WI and T2WI) of tumors, which may present with a homogeneous low signal intensity, typically demonstrate dense collagen and hemosiderin-laden macrophages (9,10). MRI is used to determine the extent of GCT-TS by evaluating the longitudinal tumor size and the tumor extent around the phalanx (the degree of circumferential occupation by a tumor around the phalanx on an axial plane) (11). The present case demonstrated 360° tumor involvement around the patellar tendon (Fig. 3C). To the best of our knowledge, previously reported cases of nodular-type GCT-TS were not as large as the tumor investigated in the present report and exhibited less tumor involvement around the patellar tendon (6,7).

With respect to the differential diagnosis of a soft tissue mass around the knee joint, an intra-articular ganglion demonstrates a homogenous high signal intensity on T2WI; and synovial sarcoma and malignant fibrous histiocytoma exhibit a non-homogenous appearance on MRI. However, a giant cell tumor exhibits a combination of homogenous low signal intensity on T1W1 and T2WI (10). Although the present case exhibited the characteristics of a giant cell tumor on plain radiographs and MRI, histopathological determination of the diagnosis was required.

Treatment of nodular-type GCT-TS involves careful and complete local excision regardless of the site. Although GCT-TS is a benign tumor, it has a high incidence of recurrence following resection (~10–20%) (1), thus, careful observation for recurrence was required in the present case. Adequate initial local excision reduces the risk of local recurrence (5) and postoperative radiotherapy has been proposed as an optional adjuvant therapy (12).

In conclusion, the present study highlights the unique features of GCT-TS in a rare case where the tumor extended around the patellar tendon, and invaded the knee joint and tibia.