DIFFERENTIAL DIAGNOSIS Chordoma, metastasis, plasmacytoma, lymphoma, sarcoma.

DIAGNOSIS Chordoma.

DISCUSSION The diagnosis is suggested by the characteristic location in the midline of the sacrococcygeal region, with asymmetric expansion from the bone into the anterior soft tissues. The radiologic features typical of chordoma in this location are irregular osteolysis and expansion with an associated anterior soft tissue mass. Fifty percent to seventy percent of cases may show amorphous calcification, usually in the periphery of the tumor; some lesions present with sclerosis instead of osteolysis. On T2-weighted MRI, the lesions are typically as bright as hydrated intervertebral discs [1,2]. Chordoma is a locally aggressive lesion of notochordal origin [3]. Chordomas are most common in the sacrococcygeal region (50% to 60% of lesions), in men (2:1 predilection), and between the ages of 40 and 70. Sphenooccipital lesions account for almost all of the remainder of lesions (25% to 40%). Sacrococcygeal lesions typically present with pain and perineal numbness. The lesions usually arise from the vertebral body, but they can extend into the posterior elements.

DIFFERENTIAL DIAGNOSIS Giant cell tumor, aneurysmal bone cyst, lymphoma, sarcoma, chordoma, metastasis, plasmacytoma/myeloma.

DIAGNOSIS Ewing’s sarcoma.

DISCUSSION An expansile, destructive mass arising in the sacrococcygeal region of a young adult raises a somewhat different differential diagnosis than the same in an older adult. Giant cell tumor of bone and aneurysmal bone cyst are possible diagnoses in young adults that would be much less likely after age 50. Chordoma, metastasis, and plasmacytoma/myeloma are unlikely possibilities in a young adult, but these lesions are more likely after age 50. The aggressive imaging features of the case here are not specific for Ewing’s sarcoma, but do indicate the need for a tissue diagnosis. Ewing’s sarcoma may occur in virtually any bone, with about 6% in the sacrum and 4% in other regions of the spine [4]. Of primary malignant bone tumors found in the sacrum in the Mayo Clinic series, 45% were chordoma, 12% were myeloma, 11% were lymphoma, 9% were osteosarcoma, 8% were Ewing’s sarcoma, and 14% were others [4].

DIFFERENTIAL DIAGNOSIS Radiation changes, insufficiency fracture, Paget disease

DIAGNOSIS Radiation changes with insufficiency fracture.

DISCUSSION The irregular marrow sclerosis in this case raises the possibility of Paget disease, but the cortex is not thickened and the bone is not enlarged. The subchondral fracture with detached but nondisplaced fragment is characteristic of fractures that occur in osteonecrosis. As bone remodeling occurs around a region of osteonecrosis, trabeculae are removed and then replaced by creeping substitution, leaving the site vulnerable to fracture. In adults, unintended effects of radiation therapy on bone include radiation osteitis and insufficiency fractures. In one study of pelvic bone complications following radiation therapy for uterine cancer, the authors found that the prevalence of insufficiency fractures, as detected by MRI, increased with time after radiation, from approximately 15% after 1 year to 45% after 5 years [6]. The majority of fractures were located in the sacrum, and most of these involved the sacral ala. Radiation may also have a toxic effect on chondrocytes.

DIFFERENTIAL DIAGNOSIS Lymphoma, leukemia, metastases, infection.

DIAGNOSIS Lymphoma (Richter’s syndrome).

DISCUSSION The images show soft tissue masses involving the paraspinal and gluteus musculature, an appearance that raises the differential diagnosis of lymphoma, leukemia, metastases, and infection. The presence of bone destruction eliminates hematoma as a possibility, and the lack of a single dominant mass lesion is very uncharacteristic of a connective tissue sarcoma.

The specific diagnosis depends on the clinical situation and must be confirmed by biopsy. Secondary malignancies develop in approximately 5% of patients with CLL, the most common of which is non-Hodgkin’s lymphoma, followed by Hodgkin’s lymphoma and multiple myeloma [9,10]. The development of non-Hodgkin’s lymphoma in CLL is called Richter’s syndrome. These lymphomas are high-grade malignancies of B-cell origin, associated with abrupt onset of constitutional symptoms, rapidly progressive lymphadenopathy, and rapid clinical deterioration to a fatal outcome. They are resistant to current therapies and may develop while the CLL is in remission. In approximately 60% of cases, the lymphoma evolves from the original leukemic cell clone, but in the remainder it has a different clonal evolution.

A. AP view of pelvis shows a multiloculated lytic lesion causing destruction about the symphysis pubis. There is no periosteal reaction in the adjacent pubic rami.

B. Coronal T1-weighted MRI shows lobulated mass centered at the symphysis and isointense to muscle.

C, D. This mass shows high signal in axial fat-saturated T2-weighted sequence (C) and heterogeneous enhancement with gadolinium on T1-weighted fatsaturated sequence (D).

DIFFERENTIAL DIAGNOSIS Chondrosarcoma, metastasis, plasmacytoma, infection

DIAGNOSIS Chondrosarcoma

DISCUSSION Chondrosarcomas are malignant, cartilagecontaining tumors that arise de novo or from preexisting cartilage-containing lesions such as osteochondromas. In the Mayo Clinic series, approximately 24% of chondrosarcomas were found in the pelvis, and of those, about 24% were found at the symphysis; it is unusual to find chondrosarcoma distal to the ankle or wrist [11]. Chondrosarcomas are the third most common primary malignant bone tumor [12]. Chondrosarcomas tend to present in patients between the ages of 30 and 60, but the age distribution is broad. Lesions in the axial skeleton tend to present in an older age group than lesions in the peripheral skeleton. Lack of reactive bone is ominous for an aggressive, malignant process. Without the characteristic calcification and lobular structure, the diagnosis of chondrosarcoma cannot be made prospectively, and a biopsy is required. Chondrosarcomas follow a slow clinical evolution. They tend to metastasize late in the clinical course, and although the lesions are often large, patients rarely present with metastatic disease [13].

DIFFERENTIAL DIAGNOSIS Osteosarcoma, chondrosarcoma, lymphoma, metastasis.

DIAGNOSIS Osteosarcoma

DISCUSSION An aggressive, destructive lesion centered within bone that has internal mineralization raises the necessity for a tissue diagnosis. Of the other possibilities in the differential diagnosis, metastases tend to be multiple and smaller and frequently have a known primary site, chondrosarcomas tend to have a lobular morphology and rings-andarcs calcifications, and lymphomas tend to be infiltrative. Although the peak incidence for onset of osteosarcoma is in the second decade of life (45% of the osteosarcomas in the Mayo Clinic series), more osteosarcomas actually occur in the third or later decades (50% in the Mayo Clinic series) [14]. Only about 2% of osteosarcomas are found in the sacrum. PET and PET/CT have become important modalities in the imaging evaluation of bone lesions [15]. However, an abnormal PET scan is nonspecific and does not obviate the need for a tissue diagnosis.

DIFFERENTIAL DIAGNOSIS None.

DIAGNOSIS Acetabular dysplasia.

DISCUSSION Acetabular dysplasia is a common cause of early osteoarthritis in adults [18]. The normal acetabulum is nearly a full hemisphere, forming the socket for the femoral head. The dysplastic acetabulum is like a shallow bowl, normally oriented but forming only a shallow socket for the femoral head. As a consequence, both the anterior and posterior articular surfaces are reduced in area, and a smaller proportion of the femoral head is covered by bone. On the radiograph, the roof of the acetabulum normally covers the top of the femoral head and inclines downward at its lateral margin. In acetabular dysplasia, the roof often fails to reach a horizontal orientation at its lateral margin, with much less incline downward. Patients are usually moderately symptomatic when they present at 30 or 40 years old, but will progress inevitably to end-stage osteoarthritis if untreated. An acetabular osteotomy provides more complete bony coverage of the femoral head and can prevent accelerated osteoarthritis [19].

DIFFERENTIAL DIAGNOSIS Prior traumatic osteoarthritis, femoroacetabular impingement (FAI), prior slipped capital femoral epiphysis (SCFE).

DIAGNOSIS Femoroacetabular impingement.

DISCUSSION In this case, the FAI was secondary to previous SCFE. FAI is early progressive osteoarthritis of the hip joint due to abnormal mechanical relation between proximal femur and acetabulum. Pincer type is due to abnormally shaped acetabulum, whereas cam type is due to abnormal femoral head-neck junction shape [20]. Abnormal femoral headneck junction shape can be seen as a result of prior SCFE, Legg-Calve-Perthes disease, trauma, adult-onset avascular necrosis, or unknown (idiopathic) causes [21]. SCFE occurs most often in overweight boys during their pubescent years. There is posteromedial slip of the femoral epiphysis off the proximal femur.

DIFFERENTIAL DIAGNOSIS Soft tissue sarcoma, intramuscular myxoma, nerve sheath tumor.

DIAGNOSIS Intramuscular myxoma.

DISCUSSION The location of the mass is within the vastus lateralis muscle. Intramuscular myxoma is a benign, soft tissue tumor that presents as a deep mass within skeletal muscle [22]. It is a hypocellular and hypovascular lesion with a gelatinous consistency. On CT, the lesion is spherical or ovoid and is sharply marginated, but with no discernible capsule. Attenuation is typically 10 to 30 Hounsfield units, the lesions are homogeneous, and they do not exhibit enhancement. On MRI, the lesions have low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Typical size at presentation is 6 cm, and the most common location is in the thigh musculature. The possibility of soft tissue sarcoma, particularly sarcomas with myxoid components, can only be definitively excluded by biopsy. CT-guided biopsy may be nondiagnostic if only fine-needle aspiration is used; usually aspiration with a large-bore needle (larger than 20 gauge) is required to sample the gelatinous material. Surgical excision is virtually always curative. The lesion is more common in women (2:1) and is found in adults (mean age 52 years). Association of intramuscular myxomas with fibrous dysplasia of bone is called Mazabraud syndrome [23].

DIFFERENTIAL DIAGNOSIS Burns, trauma, paralysis, diffuse idiopathic skeletal hyperostosis, fibrodysplasia ossificans progressiva (FOP).

DIAGNOSIS Fibrodysplasia ossificans progressiva.

DISCUSSION Ossification in the soft tissues can be recognized and distinguished from calcification by the presence of a cortical and trabecular structure. The most commonly seen soft tissue ossification is that occurring after trauma, not only in the form of myositis ossificans, but more frequently at sites of ligamentous sprains. Burns and paralysis may also be associated with focal or periarticular soft tissue ossification. Diffuse idiopathic skeletal hyperostosis is a condition of soft tissue ossification, but in this condition it is always entheseal (at the attachment of ligaments, capsules, and tendons to bone).

FOP is a rare and catastrophic genetic disorder of progressive heterotopic ossification, with extensive and cumulative disabling extraskeletal ossification [24]. Individuals with FOP appear normal at birth except for great toe malformations that look like congenital bunions. Then starting first decade of life, patients undergo multiple “flare-ups” in response to minor traumas including vaccinations or minor bumps and bruises from sports events. During these “flare-ups,” there is transient inflammatory reaction that leads to new heterotopic ossifications. This rare condition occurs mostly as new mutations, but when inherited, it is transmitted in autosomal dominant fashion [25]. FOP has been recently linked to mutations in the bone morphogenetic protein type 1 receptor gene [26]. Severe disability results from progressive immobilization of the limbs, jaw, and chest wall. Life-threatening complications include restrictive chest wall disease [27,28] and falls [29]. Falls can be particularly catastrophic, and frequently they initiate a painful flare-up that leads to the permanent loss of movement. Intracranial injuries are common and may be severe, probably the result of deficiencies in coordinated gait and protective reflexes. Most patients die of thoracic insufficiency syndrome.