Osteoblastoma is a rare tumor and represents about 0.8% of all bone tumors. The aggressive variant is even rarer and difficult to diagnose. We present a case of a 17 year old male with a tumor of the acetabulum with characteristic epithelioid osteoblasts and other features of an aggressive osteoblastoma. It is important to recognize this entity as it can be clinically confused with an osteosarcoma. Our case not only showed epithelioid morphology but also had vacuolated cytoplasm, which simulated a mucin secreting adenocarcinoma. Since treatment and prognosis for these entities is strikingly different, an accurate diagnosis becomes all the more important. This case emphasizes the need for better co-ordination between the clinician and pathologist and use of histochemical stains and immunohistochemistry apart from routine histopathology for arriving at a final diagnosis. It is essential to have a close follow-up of the patient to look for recurrence of the tumor.
Keywords: Acetabulum; Bone neoplasms; Osteoblastoma.