Chondroblastoma is a rare benign bone tumor of chondroblast origin  that almost always occurs in a primary or secondary site of ossification, suggesting possible derivation from cartilage growth plates. The official designation of a primary bone tumor as a chondroblastoma depends on the histologic finding of foci of chondroid matrix, and/or chondroblasts, which are typically rounded cells with clearly distinct cell borders separating them from each other and from the matrix.
The following image depicts the epiphyseal location of a chondroblastoma of the femoral head.
Chondroblastomas account for 1-2% of all benign bone tumors.  Although persons of any age can be affected, the tumor is more common in males than in females, and 90% of cases occur in patients between the ages of 5 and 25 years (average age of presentation is 22 y). 
Treatment for chondroblastomas is usually curettage with bone graft. [4, 5] Risk factors for recurrence and metastasis have not been defined. However, the recurrence rate is less than 30%, and metastasis is extremely unlikely. Occasionally radiofrequency ablation can also be used. [6, 7]
Chondroblastoma is almost always found at sites of secondary ossification centers, notably the epiphyses and apophyses of long bones. [8, 9] The epiphyses in the femur, humerus, and tibia (in descending order of frequency) account for about 70% of all chondroblastomas. Less common locations of tumors include the bones of the foot, [10, 11] the scapula, the patella,  , the radius, and the fibula. In the skull, the most common location of chondroblastoma is the temporal bone  ; generally, because the bones at the base of the skull develop from cartilage, they have a greater potential for developing chondroblastoma. 
The most common clinical symptoms of chondroblastomas in the long bones are pain,  lasting from only a few months up to 1 or more years, and edema. The most common sign is edema, often presenting as a joint effusion. Usually the affected bones are somewhat restricted in movement, often manifesting as a limp. An occasional finding is muscle wasting at the affected joint.
The clinical symptoms are, however, nonspecific, quite similar to the signs of most bone tumors.  Pathologic fracture is not a common feature. In the temporal bone, the tumor can present as a nontender mass with varied otologic complaints (in some cases due to swelling into the auditory canal). 
Radiography and MRI
On radiographic images, chondroblastoma is characterized by a well-defined osteolytic lesion involving an epiphysis or secondary ossification center (see the following image).  In 30-50% of cases, internal calcifications will be seen within the lesion. The tumor may make the bone seem somewhat expanded with a sclerotic margin. The average size of the tumor (along its longest diameter) is 4.5 cm.
On T1-weighted magnetic resonance images (MRIs), the tumor can show up as hypointense to intense (see the image below).  T2-weighted images show up as isointense to hyperintense.  On these images, the chondroblastoma may mimic aneurysmal bone cysts, which are also expansile and osteolytic. The observation of fluid levels and septations will distinguish the cyst.
Gross, Microscopic, and Immunohistochemical Features
In rare instances, the sclerotic border of the chondroblastoma will be scalloped. Curettings are characteristically soft, friable, red to gray, or brown and resemble granulation tissue.  Flecks of gritty, calcific material may be observed.  When present, cysts usually comprise a small portion of the tumor. Rarely, the cysts are large, such that the tumor grossly resembles a simple cyst. 
Biopsy and hematoxylin and eosin (H&E) staining under high light microscopy magnification show numerous chondroblastic cells embedded in a highly mesenchymal cartilaginous matrix.  In general, 2 cell types will be observed: chondroblasts and osteoclastlike giant cells. 
The most important feature to observe is areas of chondroid with or without calcific deposits in the stroma.  Although the matrix resembles hyaline cartilage, its tinctorial properties are different, usually with eosinophilia, and frankly regular chondrocytes with well-defined lacunae are very uncommon. Hyaline cartilage present in curettings may sometimes be accounted for by small fragments of growth plate, articular cartilage, or microcallus.
The nuclei of the chondroblasts are round to ovoid, sometimes bean-shaped, and nuclear folding often causes nuclear grooves to be identified. Mitotic features can be present but are sparse among the generally well-defined nuclei. Chicken-wire calcification in a latticelike pattern surrounding the tumor cells is present in 30% of chondroblastomas. When present, this feature is helpful; however, it is not present in the majority of lesions, making it a secondary morphologic criterion. See the following images.
Immunohistochemistry for S100 is positive in chondroblastomas, as shown in the image below. This feature can be helpful, particularly in instances when a core biopsy or fine-needle aspiration (FNA) biopsy has been performed, and the specimen is limited.