[Chondroblastoma occurring in atypical sites].

Objective: To study the clinical, pathologic and radiologic features of chondroblastoma occurring in sites other than epiphysis and apophysis of long bones, and to investigate possible reasons for misdiagnosis.

Methods: The clinical, pathologic and radiologic data of 18 chondroblastoma cases occurring in atypical sites were collected from 5 major hospitals in Shanghai during the past 12 years. S-100 immunostaining was performed to confirm the cartilaginous differentiation of the tumor cells.

Results: Chondroblastoma occurred in small bones of feet in 10 of the 18 cases (55.6%) studied, being commonest in the talus and calcaneus bones. Mean age of the patients was 27.8 years, with 55.6% over 25 years of age. Radiologic examination revealed expansive, multilocular and well-demarcated radiolucent lesions in most cases. There was local cortical destruction in 5 cases (28%) and soft tissue infiltration in 1 case. In 10 cases (55.6%), the tumor was associated with aneurismal bone cyst or simple bone cyst formation. None of the cases studied was accurately diagnosed clinically before the operation. In 2 cases, the pathology was also misdiagnosed, often being diagnosed as aneurismal bone cyst or giant cell tumor.

Conclusions: Chondroblastoma occurring in atypical sites are often associated with atypical age, radiologic features and pathologic findings at presentation. Thorough understanding of the potential pitfalls is essential in order to avoid misdiagnosis.