Case #6

• Pay attention to the patient's age, skeletal location, and the presence of joint effusion, which may complicate epiphyseal lesions.

• Plain radiograph showed an irregular, but circumscribed, lytic epiphyseal lesion surrounded by reactive bone sclerosis. There was no evidence of bone expansion, and the cortex was intact. The growth plates were open.

• Curettage material showed a highly cellular tumor consisting of sheets of round to polygonal cells. Multiple small foci of immature bluish-pink chondroid were present giving a vaguely lobular appearance to the tumor. Many multinucleated giant cells were scattered throughout the lesion.

• The cells were of intermediate size with folded, or clefted, nuclei, fine chromatin pattern and occasional inconspicuous nucleoli. Mitotic activity was low.

• The cytoplasmic borders were very distinct with multiple foci of "chicken-wire" calcification (calcified reticulin network around individual tumor cells).

• Few areas showed secondary cystic change.

Diagnosis: Chondroblastoma

• Chondroblastoma is a rare benign neoplasm occurring in 75% of cases in the second decade of life, when the growth plates are still open. This is one of two neoplasms of incompletely differentiated cartilage. The other neoplasm is chondromyxoid fibroma.
• Most common locations are the long bones (70% arise in the proximal humerus and at the knee), and flat bones, including pelvis. In the long bones, the tumor almost invariably occurs in the epiphysis.
• The neoplastic round and polygonal cells are chondroblasts positive for S-100. Multinucleated giant cells are of different cell line and stain positive for histiocytic markers (CD68). "Chicken-wire" calcification is virtually pathognomonic of chondroblastoma.
• Secondary cystic change in the form of aneurysmal bone cyst is very common 20% - 30% of cases). This is most often seen in tarsal bones. Other types of cystic change include formation unilocular or multilocular cystic spaces filled with serous fluid.
• Differential Diagnosis. Unlike chondroblastoma, Giant Cell Tumor (GCT) occurs in skeletally mature individuals, lacks chondroid matrix, and shows negative staining with S-100. Chondromyxoid fibroma is centered in the metaphysis and lacks calcification. Clear cell chondrosarcoma, epiphyseally located malignant neoplasm, occurs in patients older than 40 years and shows clearly malignant chondrocytes and the characteristic large cells with clear cytoplasm.
• Clinical behavior. Chondroblastoma is a benign neoplasm, which can be successfully treated with curettage. Recurrence rate is about 10% within the bone or in the adjacent soft tissue. Similar to GCT, chondroblastoma can occasionally produce "benign", clinically non-progressive lung implants . They can be successfully managed with surgery. The term "aggressive chondroblastoma" is applied to large, locally destructive lesions, which may erode through the cortex and invade the soft tissue. This behavior is relatively more common in pelvic lesions.