“Don’t Touch” Lesions

Posttraumatic Lesions

Myositis ossificans is an example of a lesion on which a biopsy should not be performed because its aggressive histologic appearance can often mimic a sarcoma. Unfortunately, radical surgery has been performed based on the histologic appearance of myositis ossificans when the radiologic appearance was diagnostic. The typical radiologic appearance of myositis ossificans is circumferential calcification with a lucent center ( Fig. 4.1 ). This is often best appreciated on computed tomography (CT) exam ( Fig. 4.2 ). A malignant tumor that mimics myositis ossificans will have an ill-defined periphery and a calcified or ossific center ( Fig. 4.3 ). Periosteal reaction can be seen with myositis ossificans or with a tumor. Occasionally the peripheral calcification of myositis ossificans can be difficult to appreciate; in such cases, a CT scan should help, or delayed films a week or two later are recommended. Biopsy should be avoided when myositis ossificans is a clinical consideration. Magnetic resonance imaging (MRI) in myositis ossificans can be misleading because the peripheral calcification may not be conspicuous and often has marked soft tissue edema surrounding it ( Fig. 4.4 ).

Another posttraumatic entity in which a biopsy can be misleading is an avulsion injury. These injuries can have an aggressive radiographic appearance, but because of their characteristic location at insertion sites (e.g., antero–inferior iliac spine or ischial tuberosity), they should be recognized as benign ( Figs. 4.5 and 4.6 ). Again, delayed films of several weeks will usually allow the problem case to become more radiographically and clinically clear. Biopsy can lead to the mistaken diagnosis of a sarcoma and should therefore be avoided. Any area that is undergoing healing can have a high nuclear–chromatin ratio and a high mitotic figure count, thereby occasionally simulating a malignancy.

A cortical desmoid is a process considered by many to be an avulsion off the medial supracondylar ridge of the distal femur. It occasionally simulates an aggressive lesion radiographically and on biopsy can look malignant. In many instances, biopsy has led to amputation for this benign, radiographically characteristic lesion ( Figs. 4.7 and 4.8 ). Cortical desmoids occur only on the posteromedial epicondyle of the femur. They may or may not be associated with pain and can have increased radionuclide uptake on bone scan. They may or may not exhibit periosteal new bone and usually occur in young people. Biopsy should be avoided in all cases. They are often seen as incidental findings on MRI and have a characteristic appearance ( Fig. 4.9 ).

Trauma can lead to large, cystic geodes or subchondral cysts near joints that can be mistaken for other lytic lesions, and thus a biopsy is performed. Although the biopsy specimen is not likely to mimic a malignant process, it is nevertheless avoidable. Because geodes from degenerative disease almost always are associated with additional findings, such as joint space narrowing, sclerosis, and osteophytes, a diagnosis should be made radiographically ( Figs. 4.10 and 4.11 ). However, on occasion, the additional findings are subtle and can be missed ( Fig. 4.12 ). Geodes can also occur in the setting of calcium pyrophosphate dihydrate crystal disease (also known as CPPD or pseudogout), rheumatoid arthritis, and avascular necrosis.

An entity that is often confused with metastatic disease to the spine is discogenic vertebral disease. It can mimic metastatic disease radiographically and clinically, and unless the radiologist is familiar with this process, it can lead to an unnecessary biopsy. Discogenic vertebral disease most often is sclerotic and focal ( Fig. 4.13 ). It is adjacent to an end plate, and the associated disc space should be narrow. Osteophytosis is invariably present. It represents a variant of a Schmorl’s node and should not be confused with a metastatic focus. On occasion it can be lytic or even mixed lytic–sclerotic. The typical clinical setting is a middle-aged woman with chronic low back pain. Old films often confirm the benign nature of this process. In the setting of disc space narrowing and osteophytosis, a biopsy of focal sclerosis adjacent to an end plate should not be performed.

Occasionally a fracture will be the cause of extensive osteosclerosis and periostitis, which can mimic a primary bone tumor ( Fig. 4.14 ). Lack of immobilization can result in exuberant callus, which can be misinterpreted as aggressive periostitis or even tumor new bone. A biopsy in such a case might resemble a malignant lesion. Therefore any case associated with trauma should be carefully reviewed for a fracture.

Another traumatic process that can be misdiagnosed radiologically, leading to inappropriate treatment and morbidity, is a pseudodislocation of the humerus ( Figs. 4.15 and 4.16 ). This results from a fracture with hemarthrosis, which causes distention of the joint and migration of the humeral head inferiorly. An axial or transscapular view shows that it is not anteriorly or posteriorly dislocated (the usual forms of shoulder dislocation), but merely inferiorly displaced. On an anteroposterior view, it can mimic a posterior dislocation. Often attempts are made to “relocate” the humeral head, which are both fruitless (because it is not dislocated) and painful. A fracture is invariably present, and if not seen on the initial films, it should be sought after with additional views. A transscapular or an axillary view is the key to making the diagnosis of a pseudodislocation. With either of these views, the humeral head can be seen to be normally positioned in relation to the glenoid, although it may appear somewhat inferiorly displaced. If necessary, the joint can be aspirated to confirm the presence of a bloody effusion and to show the normal position of the humeral head with no fluid in the joint.

Costochondritis, or Tietze syndrome, can cause a bulbous swelling of a rib ( Fig. 4.17 ) owing to periostitis, which can mimic a rib lesion. This condition is very painful and usually easily diagnosed clinically; however, with a bony lesion seen on radiographs, many clinicians may want a biopsy to rule out a malignant process. This would be a mistake, as with any posttraumatic or rapidly healing lesion, it can be difficult to categorize histologically. Because Tietze syndrome is a short-lived process, watchful waiting with repeat film in 2 to 3 weeks if the patient is not improved is probably indicated.

Sacral insufficiency fractures (see Chapter 5 ) are occasionally mistaken for an aggressive process such as metastatic disease, and should be easily recognnized and not biopsied or irradiated ( Fig. 4.18 ). Another fracture that can be mistaken for metastatic disease is a supra-acetabular insufficiency fracture. These can resemble a blastic metastatic process on plain films, but an MRI will show a fracture line that typically is curvilinear and parallel to the acetabulum ( Fig. 4.19 ).