A 20-year-old man comes to the office due to pain in the left thigh. Three months ago, a horse kicked him in the left midthigh, resulting in deep pain and swelling in the thigh muscles; x-ray was normal, and, over the next several days, the symptoms resolved with rest and ice packs. However, a few weeks later, the patient gradually developed recurrent pain and swelling in the same area. Vital signs are normal. Examination shows a solitary, hard, mobile, oval mass of approximately 4 × 5 cm in the anterior aspect of the left thigh. Excision of the mass is performed. Histologic examination is likely to show which of the following?

Question

A 20-year-old man comes to the office due to pain in the left thigh.  Three months ago, a horse kicked him in the left midthigh, resulting in deep pain and swelling in the thigh muscles; x-ray was normal, and, over the next several days, the symptoms resolved with rest and ice packs.  However, a few weeks later, the patient gradually developed recurrent pain and swelling in the same area.  Vital signs are normal.  Examination shows a solitary, hard, mobile, oval mass of approximately 4 × 5 cm in the anterior aspect of the left thigh.  Excision of the mass is performed.  Histologic examination is likely to show which of the following?

Quizplus · Accepted Answer

11ec160b_3a5d_a656_9249_d538ec055870_MD0001_00 This patient likely has myositis ossificans (MO), characterized by the formation of lamellar bone in extraskeletal tissues (ie, heterotopic ossification). MO is typically triggered by severe (eg, fracture, muscle contusion) or recurrent trauma that initiates the expression of bone morphogenic proteins, which promote in-migration of spindle stem cells. These cells differentiate to fibroblasts (which produce collagen and extracellular matrix) and subsequently to chondrocytes and osteoblasts (which are responsible for production of osteoid and mineralization). MO typically presents as a painful, firm, mobile mass. It is most common in the quadriceps and brachialis muscles but can occasionally occur in other muscles. X-ray typically shows intramuscular calcification with radiolucent zones (eggshell calcification). Histologic examination often shows a zonal pattern with outer areas of relatively mature bone (benign metaplastic bone tissue) and inner regions of fibroblastic proliferation (without mitotic atypia) and variable amounts of collagen and osteoid. (Choice B) Closely packed, small, round, uniform neoplastic cells are a typical finding in Ewing sarcoma, which presents with bone pain and swelling, often accompanied by fever. Although it may occur in young adults, it is more common in children and is usually fixed to the bone, not mobile. (Choice C) Mature adipocytes with large, atypical hyperchromatic cells are characteristic findings in liposarcoma. These tumors can develop within the lower extremity muscles but are rare, typically occur at age >50, and are painless until they cause compression of surrounding structures. (Choice D) Pleomorphic neoplastic spindle cells and new woven bone indicate osteosarcoma. Although this occurs in patients age ≤20 years, the tumor usually arises in metaphysis (eg, distal femur, proximal tibia), and the mass is not mobile. In addition, this patient had no tumor 2 months ago on x-ray, and it is very unlikely for osteosarcoma to develop and progress within 2 months. (Choice E) A ganglion cyst is an outpouching of synovial structures (eg, joint capsule, tendon sheath) filled with clear, gelatinous fluid. It presents as a painless, rubbery nodule, most commonly at the wrist or dorsal foot. The thigh muscle is not a common location. Educational objective: Myositis ossificans is characterized by the formation of lamellar bone in extraskeletal tissues (ie, heterotopic ossification), often triggered by trauma. It typically presents as a painful, firm, mobile mass within a muscle (eg, quadriceps). Histologic examination shows benign metaplastic bone and proliferating fibroblasts without mitotic atypia.

A 20-Year-Old Man Comes to the Office Due to Pain