Fibrocartilaginous Mesenchymoma

Not clinically relevant

Unknown

Fibrocartilaginous mesenchymoma is composed of tan-white fibrous tissue with scattered pale, bluish-grey, glistening areas corresponding to cartilaginous tissue. Resected specimens may show areas of cortical destruction.

Fibrocartilaginous mesenchymoma is characterized by a population of spindle cells, hyaline cartilage nodules, and trabeculae of bone. The hypocellular to moderately cellular spindle cell component contains elongated cells with slightly hyperchromatic, mildly atypical nuclei arranged in bundles or intersecting fascicles. Cellular areas may resemble a low-grade spindle cell sarcoma, but mitotic figures are rare or absent. Benign-appearing cartilaginous nodules of varying size and shape are scattered throughout the tumor. They often contain zones mimicking epiphyseal growth plate characterized by chondrocytes arranged in parallel columns and areas of enchondral ossification. Bone is present as long or short trabeculae. Woven bone rimmed by osteoblasts can be seen emerging from enchondral ossification of the cartilaginous nodules. Both the spindle cell and the cartilaginous components often infiltrate host bone, at times extending into surrounding soft tissues. Clusters of osteoclast-type giant cells may be found.

Immunohistochemistry
Immunohistochemistry is not helpful.

Differential diagnosis
The main differential diagnosis is with fibrous dysplasia containing cartilage (fibrocartilaginous dysplasia). The cartilage in fibrous dysplasia can have a growth plate–like appearance, but in fibrocartilaginous mesenchymoma it is a more prominent feature. Moreover, the spindle cells in fibrous dysplasia contain small bland nuclei, unlike those in fibrocartilaginous mesenchymoma with mild hyperchromasia. Desmoplastic fibroma of bone and low-grade fibrosarcoma lack cartilaginous nodules. Unlike fibrocartilaginous mesenchymoma, dedifferentiated chondrosarcoma is characterized by a high-grade sarcomatous component. Chondromesenchymal hamartoma shows similar cartilaginous nodules, but these occur in the chest wall of infants and usually contain an aneurysmal bone cyst–like component.

Not clinically relevant

No metastatic spread or tumor-related deaths have been reported. Local recurrences tend to occur after intralesional excision, although the literature describes some patients who were disease-free many years after incomplete surgery. Surgical removal with a wide margin is generally the best treatment.

Mutations in GNAS, IDH1, and IDH2 and amplification of MDM2 were absent in the few tumors tested.

Essential: bone tumor with compatible imaging; spindle cells with mild cytological atypia, nodules of hyaline cartilage–containing areas resembling growth plate–like cartilage, and bone trabeculae.

Desirable: young patient (aged under 30 years).