Microscopic description and IHC:
The appearance of chondroblastoma histologically shows pink extracellular material resembling cartilage with chondroblasts; chondroblasts have a grooved or ovoid nuclear morphology and clear to eosinophilic cytoplasm. Pericellular calcification in a “chicken-wire” appearance and giant cells can also be seen. S-100 is classically positive; however, in this case, interestingly it is negative. DOG-1 positivity has been seen. CD68 highlight the giant cells.
A secondary aneurysmal bone cyst is seen in the second image. Histologically, these show blood filled cystic spaces and intervening fibrous septa with scattered giant cells adjacent to bone or osteoid.
Discussion:
Chondroblastoma is seen in approximately 1% of all bone tumors, occurring between the ages of 10 and 25 years with a male predominance. Most occur within a single long bone (femur, tibia, humerus) at the epiphyseal and epimetaphyseal regions. There is usually a history of localized pain which can occur for many years. Radiologically, according to the World Health Organization, “chondroblastomas are typically lytic, centrally or eccentrically placed, relatively small lesion (3 to 6 cm), occupying less than one half of the epiphysis and are sharply demarcated, with or without a thin sclerotic border.”
Aneurysmal bone cysts typically occur within the first two decades, but can affect all ages. They typically occur within the metaphysis of long bones (femur, tibia, and humerus) and can also occur within the vertebral bodies. Patients typically present with pain and swelling.
References:
Fletcher, C., Unni, K. & Mertens, F. (2002). Pathology and genetics of tumours of soft tissue and bone. Lyon: IARC Press.
Akpalo, Hana H (06/2012). "Discovered on gastrointestinal stromal tumour 1 (DOG1): a useful immunohistochemical marker for diagnosing chondroblastoma.". Histopathology (0309-0167), 60 (7), p. 1099.