An 8 yr old child is having fever with pain and swelling in mid thigh. On Xray lamellated appearance and Codman’s triangle is present. Histopathologic examination shows small round cells positive for MIC-2. What is the most likely diagnosis?
**Core Concept**
The question is testing the ability to diagnose a specific type of bone tumor based on its radiographic and histopathological features. The key concept here is the identification of a malignant bone tumor with characteristic radiographic features and immunohistochemical markers.
**Why the Correct Answer is Right**
The presence of a lamellated appearance and Codman's triangle on X-ray is suggestive of a malignant bone tumor. Codman's triangle is a characteristic radiographic feature of osteosarcoma, which is a type of bone cancer that produces immature bone. The histopathologic examination shows small round cells, which is a characteristic feature of Ewing's sarcoma, but the positivity for MIC-2 (CD99) is a specific marker for Ewing's sarcoma. MIC-2 is a cell surface antigen that is expressed in most cases of Ewing's sarcoma, making it a useful diagnostic marker.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the radiographic features described (lamellated appearance and Codman's triangle) are more suggestive of osteosarcoma rather than osteochondroma. Osteochondroma is a benign bone tumor that typically presents with a characteristic "bony spur" on X-ray.
**Option B:** This option is incorrect because the histopathologic features described (small round cells) could be seen in other types of malignant bone tumors such as Ewing's sarcoma, but the positivity for MIC-2 is specific for Ewing's sarcoma. Other types of small round cell tumors, such as neuroblastoma or rhabdomyosarcoma, do not typically express MIC-2.
**Option C:** This option is incorrect because the radiographic features described (lamellated appearance and Codman's triangle) are not typical of osteoblastoma, which is a type of benign bone tumor that typically presents with a well-defined radiolucent lesion.
**Option D:** This option is incorrect because the histopathologic features described (small round cells) could be seen in other types of malignant bone tumors such as Ewing's sarcoma, but the positivity for MIC-2 is specific for Ewing's sarcoma. Other types of small round cell tumors, such as lymphoma or leukemia, do not typically express MIC-2.
**Clinical Pearl / High-Yield Fact**
Ewing's sarcoma is a malignant bone tumor that typically affects children and young adults. It is characterized by a distinctive radiographic appearance and immunohistochemical markers, including positivity for MIC-2 (CD99). Early diagnosis and treatment are critical for improving outcomes in patients with Ewing's sarcoma.
**Correct Answer:** C.