DIAGNOSIS: INTRACRANIAL MESENCHYMAL OSTEOCHONDROMA
Osteochondromas are a benign cartilaginous neoplasm composed of mature hyaline cartilage with focal ossification. They are the most common benign bone tumor and account for 40% of the benign bone lesions in the Mayo Clinic series (1). Extraskeletal (mesenchymal) osteochondromas are known to originate from non-skeletal or non-cartilaginous tissue. Hypotheses advanced to explain this phenomenon include development from heterotopic cartilaginous cells or pluripotential mesenchymal tissue, cartilaginous metaplasia of clones of normal cells or cartilaginous metaplasia of fibroblasts activated by trauma or inflammation (2).
Intracranial osteochondromas are uncommon, solitary slow-growing tumors with a reported incidence of 0.1 to 0.2% of all intracranial tumors (1,2,3,4). The majority arise from the base of the skull originating from residual rests of primordial cartilage in basilar synchondroses entrapped during endochondral ossification of the skull base. It is therefore not surprising that these tumors occur most commonly in the middle cranial fossa where so many sutures converge.
Intracranial osteochondromas of the posterior fossa have been reported but are very rare (5). About 15% of intracranial osteochondromas arise supratentorially, typically attached to the falx in a frontoparietal location (1,2), as in the case presented here. They have also been reported to arise from cranial nerves, walls of the ventricles and from the choroid plexus (2,6). In some cases the tumors have occurred as components of disorders of generalized mesenchymal neoplasias including Maffucci's syndrome and Ollier disease (2,4,7).
Intracranial osteochondromas can occur at any age with a predilection for younger individuals (2,3). Characteristically, the tumor grows slowly over many years and can attain a very large size without clinical symptoms, especially when supratentorial. (2,3,4,8). Similar tumors arising from the base of the skull present earlier.
Typically, the histomorphology resembles mature hyaline cartilage without anaplastic proliferation of chondrocytes or nuclear atypia, with a lobular arrangement of clusters of lacunae containing single chondrocytes (3). The presence of central coagulative necrosis and cystic degeneration as seen in this case appear consistent with the tumor out-growing the vascular supply and undergoing central degeneration.
Although a meningioma might be suspected from the imaging, the principal pathological differential diagnosis is an osteochondroma versus a low-grade osteochondrosarcoma. The absence of cellular pleomorphism, nuclear atypia and binucleate chondrocytes makes this diagnosis unlikely. Intracranial mesenchymal osteochondromas exhibit a benign clinical course. Complete surgical excision is curative, however the critical location of skull base tumors may prevent total excision. In such situations, delayed growth of the residual tumor has been reported and surgical re-excision where possible is curative (3,4,5,8). Transition to osteochondrosarcoma has rarely been documented (3,4,5,8)
Contributed by Bennet I. Omalu, Clayton A. Wiley, Ronald L. Hamilton