A 27-year-old man comes to the emergency department with progressive right knee swelling and pain. He has no history of trauma to the area. The patient has no other medical conditions and takes no medications. X-rays reveal a large lytic lesion involving the proximal tibia with extensive soft-tissue swelling. After additional confirmatory testing, the patient undergoes a right-sided, above-knee amputation. Histologic examination of the resected mass is shown in the exhibits.

Which of the following is the most likely diagnosis?

Question

A 27-year-old man comes to the emergency department with progressive right knee swelling and pain. He has no history of trauma to the area. The patient has no other medical conditions and takes no medications. X-rays reveal a large lytic lesion involving the proximal tibia with extensive soft-tissue swelling. After additional confirmatory testing, the patient undergoes a right-sided, above-knee amputation. Histologic examination of the resected mass is shown in the exhibits.

Which of the following is the most likely diagnosis?

Quizplus · Accepted Answer

11ec160b_3a55_8ff6_9249_2df5a4a6cce8_MD0001_00 This patient's spindle-shaped tumor cells admixed with bone and osteoid indicates osteosarcoma, the most common primary bone tumor in children and young adults. Most cases arise in the metaphyses of long bones (eg, proximal tibia), the location of the growth plate and site of greatest bone proliferation. Patients typically present with pain and soft-tissue swelling, and x-ray usually reveals a lytic bone lesion. Osteosarcomas arise from a malignant mesenchymal stem cell that generates cartilage, bone, or fibrous tissue. Therefore, the diagnosis is confirmed when histopathology reveals neoplastic spindle-shaped stromal cells admixed with tumor osteoid and thin trabeculae of bone. (Choice A) Unlike osteosarcomas, chondrosarcomas and fibrosarcomas do not produce osteoid or bone. Chondrosarcomas are characterized by neoplastic chondrocytes in a hyaline cartilage matrix, usually with small calcifications. (Choice B) Ewing sarcoma, the second most common primary bone malignancy in young patients, often arises in long bones and causes progressive pain, swelling, and lytic bone lesions. However, histopathology reveals sheets of small, round, cells separated by fibrous septae and patches of necrosis/hemorrhage; no osteoid or bone is produced. (Choice C) Adenocarcinoma of the lung often metastasizes to bone. However, histology would show neoplastic glands lined with mucin-producing cells. (Choice D) Osteoid osteoma is a small, benign, bone-forming tumor that typically occurs in adolescent boys. Histopathology shows irregular patterns of woven bone lined by a single layer of benign-appearing osteoblasts. The presence of a large lytic bone lesion and highly pleomorphic spindle-shaped tumor cells makes this diagnosis unlikely. Educational objective: Osteosarcoma is the most common primary bone tumor in children and young adults and typically arises near the metaphyses of long bones. Patients usually have slowly worsening pain and soft-tissue swelling. X-ray typically reveals a lytic bone lesion, and biopsy classically shows pleomorphic, spindle-shaped tumor cells that generate osteoid and thin trabeculae of neoplastic bone. __________ References: Osteosarcoma. (http://www.ncbi.nlm.nih.gov/pubmed/25070231)

A 27-Year-Old Man Comes to the Emergency Department with Progressive