FINAL DIAGNOSIS: PIGMENTED VILLONODULAR SYNOVITIS
On the basis of the morphologic features, a diagnosis of pigmented villonodular synovitis was favored; atypical osteoblastoma was also considered, especially in view of the location and the radiological findings. The case was referred to Dr. K.K. Unni and Dr. C.Y. Inwards at the Mayo Clinic, Rochester, Minnesota, who confirmed the diagnosis of PVNS of the spine.
We report a case of pigmented villonodular synovitis (PVNS) of the thoracic spine occurring in a 13 year old girl. PVNS is a condition of unknown etiology which usually occurs in the synovial membrane of tendon sheaths and joints of children and young adults. The knee is most commonly affected (1). It rarely arises in the spinal column where it is thought to originate from vertebral articular facet joints; only 26 cases of PVNS located in the spine have been reported. The most complete review of PVNS in this location was published in 1996 by Giannini et.al. who added 12 cases to the previously reported 10 (2). In this location PVNS may affect adults in a wide range of ages with no gender predilection (2). All segments of the spine can be involved; however, most frequently, the lesion involves the posterior aspect of the cervical and the lumbar spine (2). The site of origin is thought to be the synovial membrane of the articular facet joints (2).
Histopathologically, PVNS is usually a cellular lesion composed of round to polygonal cells with round to oval nuclei exhibiting minimal pleomorphism, which some investigators consider to represent synovial cells (2). The growth pattern varies. Lipid-laden macrophages, osteoclast-like giant cells and hemosiderin deposition are additional characteristic features (1,2,3). These features were present in the lesion of our patient. However, less typical areas in the specimen and the low likelihood of occurrence at this site made PVNS an unusual diagnosis. Its etiology is uncertain; both a reactive and a neoplastic nature of the lesion have been suggested. There is cytogenetic evidence of clonality in at least two cases of PVNS (16).
The radiologic differential diagnosis included osteoblastoma and the solid variant of aneurysmal bone cyst. These entities are much more common in the spine and have a predilection for the posterior elements (17). However, osteoblastoma is typically extremely well-circumscribed (17). The lesion of our patient showed ill-defined circumscription. Furthermore, no well-formed bony trabeculae or loose fibrovascular stroma were present. The solid variant of aneurysmal bone cyst usually reveals a stroma composed of spindle cells (17). In our case, round to polygonal cells predominated.
Giant cell tumor of the spine, which was also in the radiologic and histopathologic differential diagnosis, usually occurs within the vertebral body. This entity is characterized by giant cells with numerous nuclei (between 40-60). Additionally, there is morphological similarity between these nuclei and the nuclei of the mononuclear cells (17). The number of nuclei in the giant cells of our case was generally lower and there was a marked difference in the appearance of the multinucleated giant cells and the mononuclear cells. Furthermore, in our case, no zones of necrosis were seen, a feature which is quite commonly encountered in giant cell tumor. Lipid-containing macrophages can occur in both giant cell tumor and PVNS and are, therefore, not helpful in the differential diagnosis (2). Osteosarcoma can be readily ruled out due to the absence of atypical features including cytologic atypia. The radiologic differential diagnosis also included neoplasms of neural origin as well as malignancies, e.g. Ewing's sarcoma, but these entities were not compatible with the histopathologic appearance.
In summary, the characteristic histopathological features in combination with clinical history and radiologic imaging enabled a diagnosis of PVNS. Even if rare, it is important to include this entity in the differential diagnosis of spinal lesions because of its tendency to recur locally (local recurrence rate 18%) (2). Surgical resection with gross total removal of the lesion is the recommended treatment (2).
Contributed by A.K. Bruecks, R.J.B. Macaulay, K.A. Tong, and G. Goplen