Chondroblastoma

In contrast to the aforementioned mature hyaline cartilage tumors, this benign lesion consists of round-to-spindled cells resembling immature chondrocytes. The great preponderance involves the epiphyseal region of the distal femur, proximal tibia, and proximal humerus. Both cytology and histology may vary. Most tumor cells will show a sharp cytoplasmic border. The cytoplasm may be lightly eosinophilic to clear, and the round-to-oval nuclei may contain grooves, clefts, or invaginations. Cellular atypia can be identified on occasion. Mitoses are present, and may number as high as 4 per 10 high-power microscopic fields; however, atypical mitoses should be absent. Spindle cells are identified as either separate collections or scattered among the chondroblastic cells. Chondroid matrix may be present and is usually eosinophilic rather than the typical basophilic matrix of hyaline cartilage. Calcifications are either within the matrix or surround individual cells in a "chicken-wire" type configuration (Fig. 11). This is one

FIGURE 11 Chondroblastoma. Plump and polygonal chondroblastic cells with hyperchromatic nuclei. Calcification surrounds cells in "chicken-wire" configuration while an osteoclast-like giant cell is present superiorly (hematoxylin and eosin, original magnification x 40).
FIGURE 12 Chondromyxoid fibroma. Lobulated myxoid and eosinophilic cartilaginous matrix containing spindle and stellate cells. Blood vessels are present within the intervening fibrous septa (hematoxylin and eosin, original magnification x 10).

of the benign tumors, which can contain multinucleated cells and osteoclast-like giant cells (48). The differential diagnosis would include giant-cell tumor and clear-cell chondrosarcoma. Unlike giant-cell tumor, the background mononuclear cells of chondroblastoma resemble chon-droblasts and not histiocytes. A chondroid matrix is noted and an S-100 immunohistochemical stain will be positive (49). Clear-cell chondrosarcoma typically contains sheets of tumor cells with clear or nearly clear cytoplasm, whereas this is usually a focal occurrence in chondroblastoma.

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