Imaging diagnosis of pseudotumors and nonmalignant masses in athletes.

Athletes are commonly imaged for focal swelling, persistent pain, or a new palpable abnormality after training or injury. These imaging findings are frequently not neoplastic, although several benign processes can appear sufficiently mass-like or aggressive to raise concern for sarcoma or an aggressive lesion. Such "pseudotumors" are more common in routine musculoskeletal practice than primary malignancy. This review approaches the problem as it is encountered in practice: first, the palpable abnormality that prompts imaging, and second, the incidental or symptomatic injury-related lesion that appears worrisome on radiographs, ultrasound, CT, or MRI. Radiographs should be obtained as a first line of imaging. Both radiographs and CT remain essential when evaluating mineralization, ossification, periosteal reaction, cortical injury, or stress fracture. Ultrasound is particularly useful for imaging evaluation of superficial lesions, fluid collections, vascular abnormalities, and dynamic processes such as muscle herniation. MRI is the principal problem-solving modality for deep or indeterminate lesions because it shows compartment of origin, tissue plane affected, internal architecture, marrow involvement, and enhancement patterns. In athletes, the most useful question is often whether the finding fits the story. A benign injury-related lesion should occur in a plausible location, involve the expected tissue plane, and change over time in a predictable way. Discordant features, including progressive enlargement, unexplained deep location, infiltrative margins, nodular internal enhancement, bone destruction, neurovascular encasement, or failure to evolve as expected, should prompt further imaging evaluation to exclude a more ominous diagnosis.