Chondroblastoma with prominent secondary aneurysmal bone cyst-like changes: The role of H3.3 K36 M.

Secondary aneurysmal bone cyst (ABC)-like changes are a well-recognized phenomenon observed in a variety of primary bone tumors that can pose diagnostic difficulties. We have investigated the role of H3.3 K36 M immunohistochemistry for the diagnosis of patients with chondroblastoma of bone in which extensive secondary ABC-like changes masked the underlying tumor. A total of 25 patients with bone tumors showing equivocal histologic and radiologic features of chondroblastoma were identified in the files of the respective institutions. The patients included 4 women and 21 men, aged 12-36 years (average: 18.8 years); they arose in the femur (8), tibia (4), calcaneus (4), humerus (3), scapula (2), fibula (1), talus (1), ischium (1), and temporal bone (1). Imaging studies showed well-defined radiolucent lesions showing prominent multicystic spaces with fluid-fluid levels; only rare tumors showed internal matrix mineralization. All tumors were treated by curettage. On histology, multiple blood-filled cystic spaces separated by connective tissue septa containing scattered osteoclastic giant cells were present. Additionally, focal microscopic areas containing a monotonous polygonal cell population suggestive of chondroblastoma could also be identified on careful examination. Immunohistochemical staining revealed nuclear positivity for H3.3 K36 M in 24/25 cases (96%), helping to support a diagnosis of chondroblastoma. Immunostaining for H3.3 K36 M can serve as a useful adjunct for the diagnosis of chondroblastoma in cases with prominent ABC-like changes, particularly in small biopsies or bone curettage specimens in which the diagnostic cytomorphological and architectural features of the tumor may not be readily appreciated.