A 13-year-old, previously healthy boy has had pain in the right leg for the past month. There is no history of trauma or recent illness. On physical examination, there is warmth and tenderness to palpation of the right lower thigh anteriorly, and the circumference of the right thigh is slightly larger than that of the left. His temperature is 39degC. A radiograph of the right leg shows a 6-cm expansile mass in the diaphyseal region of the right lower femur that extends into the soft tissue and is covered by layers of reactive bone. A biopsy of the mass is done, and microscopic examination shows sheets of closely packed primitive cells with small, uniform nuclei and only scant cytoplasm. Karyotypic analysis of the tumor cells shows a t(11;22) translocation. What is the most likely diagnosis?
A 13-year-old, previously healthy boy has had pain in the right leg for the past month. There is no history of trauma or recent illness. On physical examination, there is warmth and tenderness to palpation of the right lower thigh anteriorly, and the circumference of the right thigh is slightly larger than that of the left. His temperature is 39degC. A radiograph of the right leg shows a 6-cm expansile mass in the diaphyseal region of the right lower femur that extends into the soft tissue and is covered by layers of reactive bone. A biopsy of the mass is done, and microscopic examination shows sheets of closely packed primitive cells with small, uniform nuclei and only scant cytoplasm. Karyotypic analysis of the tumor cells shows a t(11;22) translocation. What is the most likely diagnosis?
💡 Explanation
**Core Concept**
The question is testing the student's ability to identify a specific type of bone tumor characterized by a particular genetic translocation, histological features, and clinical presentation.
**Why the Correct Answer is Right**
The patient's presentation of a painless, expansile mass in the diaphyseal region of the femur, covered by reactive bone, and a biopsy showing primitive cells with small, uniform nuclei is consistent with Ewing's sarcoma. The t(11;22) translocation, specifically the fusion of the EWS and FLI1 genes, is a hallmark genetic feature of this tumor. This translocation leads to the aberrant expression of transcription factors that promote cell proliferation and tumor formation.
**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma typically presents as a destructive lytic lesion with a characteristic "sunburst" appearance on radiographs, which is not consistent with this case.
**Option B:** Fibrosarcoma is a type of soft tissue sarcoma that typically arises in the deep soft tissues, not in the bone.
**Option C:** Osteochondroma is a benign bone tumor that typically presents as a bony projection or "cartilage-capped" lesion, which is not consistent with the expansile mass described in this case.
**Clinical Pearl / High-Yield Fact**
Ewing's sarcoma is a highly malignant tumor that requires prompt diagnosis and treatment, as it has a high propensity for metastasis and recurrence.
**Correct Answer:** C. Ewing's sarcoma is a malignant bone tumor characterized by a specific genetic translocation, histological features, and clinical presentation, making it the most likely diagnosis in this case.
✓ Correct Answer: B. Ewing sarcoma
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