Giant cell tumors of the tendon sheath are benign, circumscribed tumors that may either arise from the tendon sheath or from the synovium. These lesions are part of the fibriohistiocytic tumor cluster. Most commonly, they are localized, but may also be diffuse. The diffuse form mainly affects lower extremities, while localized tumors are more often encountered in the hand. Recurrences after excision may occur.
Giant cell tumors of the tendon sheath are described as firm, lobulated masses that may cause no symptoms or patients may present with impaired mobility of the affected segment with distal paresthesia. Pain is uncommon. Lesions usually have a slow progression rate, but faster growth (within weeks) has also been reported. They usually occur in the distal interphalangeal joints rather than larger articulations  , especially on their palmar or plantar side , near the extensor or flexor tendon sheath and may extend and implicate both volar and dorsal parts of the digits. Metastasis has not been described, but recurrence after excision may be as high as 44%. Clinical examination reveals the fact that they are solitary or multinodular masses, adherent to the underlying and overlying structures, or skin may be mobile over the lesion. The examiner should try to determine if the joint capsule, nerves or vessels are compressed by the tumor or if adjacent bone erosion is present. Ankles, elbows, wrists and knees may also be involved . In children upper and lower extremities are equally affected , but the disease more often appears in 30 to 50 years old individuals, slightly more frequently in women. The right hand is more often affected. Transillumination is negative in giant cell tumors of the tendon sheath, making differential diagnosis with cysts easier.
Clinical examination must be followed by an imaging workup in order to establish the diagnosis of a giant cell tumor of the tendon sheath. The first step is to perform a plain radiography, that shows a well circumscribed, sometimes calcified tumor that may cause bone erosion  due to its pressure effect and, not due to direct invasion.
If radiological features are inconclusive then a magnetic resonance imaging scan should be performed. This examination will establish the soft tissue character of the pathological process  and may highlight decreased signal intensity in certain areas, due to hemosiderin accumulation.
Echography offers information about tumor size, internal vascularization- which is often rich, echogenity (hypo  or hyperechoic ), location and relationship with neighboring structures . Due to the fact that tumors are usually superficial, they can be well characterized using this method. Tumor borders are usually clear.
If clinical presentation dictates excision, the diagnosis will also be confirmed by means of histology. Tumors are usually 0.5 to 5 cm in diameter and composed of rounded or polygonal cells, collagen, inflammatory cells histiocytes, giant cells and xanthoma cells, rich in hemosiderin  embedded in a fibrous matrix . A fibrous capsule surrounds this. The tumor has low mitotic activity and necrosis is rarely seen. Mitotic activity is not a predictor for future recurrence.