Tumor-Like Lesions of Bone

Simple Bone Cyst

SBC, otherwise known as a solitary bone cyst or a unicameral bone cyst, is a common tumor-like osseous lesion that may present as an incidental finding or more frequently as a pathologic fracture. The annual prevalence of SBC has been reported to be approximately 0.30 per 100,000 population. It is a true intraosseous cyst with a fluid-filled cavity lined by a thin layer of mesothelial-like cells. Most cases present in the first and second decades, with only 15% cases occurring later. There is a male preponderance, with 55% of cases arising in the proximal humeral metaphysis and 25% in the proximal femoral metaphysis ( Fig. 94-1 ). Other sites, with exception of the anterior calcaneus, are uncommon ( Fig. 94-2 ). Radiographically, SBC appears as a central, well-defined, mildly expansile, lytic lesion arising within a metaphysis usually bordering an open growth plate. A typical feature in the presence of a fracture is the “fallen fragment” sign in which a small segment of fractured cortex is seen to settle into the dependent part of the cyst seen in approximately 20% of cases (see Fig. 94-1 ). This sign is characteristic but, as with many signs in imaging, not pathognomonic because it may be seen in any bone lesion with a fracture associated with a prominent cystic component. MRI will show the fluid nature of the lesions that are slightly hypointense on T1-weighted MR images and hyperintense on T2-weighted MR images (see Fig. 94-2 ). Debris and a solitary fluid-fluid level may be seen in the presence of a fracture due to internal hemorrhage. The cyst may heal after fracture alone. Other treatments advocated over the years include injecting corticosteroids and/or Ethibloc and curettage, with or without bone grafting. In time, the cyst will grow away from the physis and appear to migrate down the diaphysis as healthy new bone is laid down in the metaphysis. The precise appearance of the cyst on follow-up radiographs depends on the nature of the initial treatment. It is not unusual, particularly in active young boys, to see a cycle of fracture, partial consolidation, and re-fracture occurring over a period of several years as the SBC migrates down the shaft of the long bone that is frequently associated with a bowing deformity.

Aneurysmal Bone Cyst

ABC is a benign cystic lesion of bone composed of cystic blood-filled spaces divided by connective tissue septa containing giant cells, fibroblasts, and reactive woven bone. The cause of the lesion has long been debated. To date, it has been generally considered a tumor-like lesion, possibly arising due to local hemodynamic problems or perhaps in response to trauma. Interestingly, theories are now going full circle, with increasing recent evidence that ABC is a true neoplasm and may therefore be classified in the future with the other true tumors of bone covered in Chapter 92 . ABC constitutes approximately 4% of tumors and tumor-like lesions of bone. Eighty percent of ABCs occur in individuals younger than 20 years of age. It has a similar annual prevalence to SBC of approximately 0.32 cases per 100,000 population. Seventy percent arise de novo in bone, and 30% occur in association with other osseous tumors (secondary ABC), including giant cell tumor, chondroblastoma, osteoblastoma, and, less commonly, chondromyxoid fibroma and fibrous dysplasia ( Fig. 94-3 ). ABC-like areas may be seen in malignant bone tumors, notably telangiectatic osteosarcoma. It is therefore important to review all of the imaging to see whether there is any evidence of an antecedent bone lesion. Common sites of involvement include the long bones (50%) and posterior vertebral arch (20%). Eighty five percent of cases arise within medullary bone, and 15% occur in a cortical or subperiosteal location. The predominant radiographic features are those of a lytic, eccentric, multiseptated, markedly expansile lesion arising in the metaphysis of the long bone in a child or young adult ( Fig. 94-4 ). There may be a thin-ridged or smooth peripheral shell of periosteal new bone formation around the margins of the lesion, but, if the lesion is in a particularly active phase of growth, the peripheral shell may be absent, with marginal buttresses simulating Codman angles to give the lesion the overall appearance of a more aggressive, potentially malignant process ( Fig. 94-5 ). Subperiosteal ABC can appear as a blowout surface lesion of bone and is more frequently diaphyseal in location (see eFig. 94-1 ). The multicystic nature of ABC is confirmed with either CT or MRI. Fluid-fluid levels can frequently be visualized due to the layering out of blood products within the individual cysts (see Fig. 94-5C ).

The diagnostic value of identifying fluid-fluid levels has been debated in the literature. Studies have shown that the most common cause of a bone lesion in a child showing multiple fluid-fluid levels is an ABC and that lesions comprising a proportion greater than two thirds of fluid-fluid levels were more likely to be a primary or secondary ABC rather than a malignancy. In contrast, another study correctly identified the nonspecific nature of this sign when looking at a large series of both bone and soft tissue tumors. However, in a more recent study, all lesions with complete fluid-fluid levels were benign with 78% of all lesions being diagnosed as ABC, 8% as benign lesions with secondary ABC change, and 14% as SBC. The role of needle biopsy for focal bone lesions with complete fluid-fluid levels on magnetic resonance imaging remains therefore unclear.

Isolated cases of malignant transformation have been reported in ABC, but the cynical observer might be tempted to suggest that these were ABC-like areas within an existing sarcoma (e.g., telangiectatic osteosarcoma) that took some time to manifest the true malignant nature of the underlying lesion. For the sake of completeness, it should be noted that there are two further rare variants of ABC: the solid ABC and the soft tissue ABC, which are rarely diagnosed before biopsy.

Epidermal Inclusion Cyst

Epidermal inclusion cysts, otherwise known as implantation dermoid cysts, typically involve the distal phalanges of the hand and, to a lesser extent, the foot. They are seen in the skeletally mature and are attributed to penetrating injuries with intraosseous inoculation of skin epithelium. Histologically, the lesions are lined with squamous epithelium and contain keratin debris. The radiographic appearances are those of a well-defined, rounded lytic lesion with a sclerotic border arising within the terminal phalanx ( Fig. 94-6AB ). The lesion may or may not show expansion and is typically relatively asymptomatic unless presenting as a pathologic fracture. Similar appearances at this site may be seen with a glomus tumor (see Chapter 92 ), but this entity is usually painful and tender. Developmental epidermal cysts, as opposed to posttraumatic ones, typically arise in the skull vault of children.

Intraosseous Ganglion

Intraosseous ganglia are benign non-neoplastic bone lesions that are histologically similar to their soft tissue counterparts. They consist of a cavity or cavities of varying size, without an epithelial or synovial lining, containing mucoid viscous material. Originally said to be rare, they are increasingly recognized.

Confusing nomenclature has undoubtedly contributed to the perception that they are uncommon. One article published more than 25 years ago identifies no fewer than 12 different names.

Today, the terms intraosseous ganglion, subchondral cyst , or geode are often applied interchangeably, although the latter two are more frequently used to describe juxta-articular lesions associated with degenerative or inflammatory joint disease.

The etiology remains unknown. Several theories have been postulated to explain the pathogenesis. These include a primary or idiopathic form arising de novo in bone possibly due to intramedullary metaplasia followed by mucoid degeneration or secondary spread of a synovial cyst or intrusion of synovial fluid from an adjacent joint into the bone.

Intraosseous ganglia occur in the skeletally mature individual at all ages, with the peak incidence in the fourth and fifth decades. There is a predilection for the long bones of the lower limb, although the carpal bones are other well-recognized sites. The radiographic appearances are those of well-defined, lytic, oval or round, unilocular, or multilocular lesions located in the epiphysis or metaphysis with or without cortical expansion and soft tissue extension ( Fig. 94-7 ). Periosteal new bone formation is not a feature. The majority of intraosseous ganglia are small, between 1 and 2 cm in diameter, with lesions over 5 cm uncommon. Large lesions may be mistaken as giant cell tumor, chondroma, and, in the older patient, chondrosarcoma, metastasis, and plasmacytoma. A diagnostic feature occasionally seen, more clearly identified on CT, is gas within the cyst that is sometimes referred to as an intraosseous pneumatocyst. Although these lesions tend to behave in an indolent manner, increased activity on bone scintigraphy is frequently seen due to osteoclastic and osteoblastic activity in the surrounding bone. The lesions appear hypointense or isointense to muscle on T1-weighted MR images and hyperintense on T2-weighted and short tau inversion recovery (STIR) images, reflecting the mucoid/cystic nature of the contents (see Fig. 94-7BC ). There is marginal or, less commonly, heterogeneous enhancement with gadolinium (see Fig. 94-7D ). Although there is no epithelial or synovial lining, there is a fibrous membrane of varying thickness, which is presumably the structure showing peripheral enhancement. The heterogeneous enhancement may be due to connective tissue in earlier lesions undergoing myxoid transformation. Surrounding marrow edema may be seen in approximately 50% of cases, suggesting extension of the lesion into surrounding marrow or trabecular microfractures secondary to expansion of the lesion. Fluid-fluid levels have also been described in intraosseous ganglia.