Fibrous dysplasia (FD) is a noninherited bone disease in which abnormal differentiation of osteoblasts leads to replacement of normal marrow and cancellous bone by immature bone and fibrous stroma. It is usually an incidental imaging finding, generally not requiring further investigation. This disease is categorized as either monostotic or polyostotic and may occur as a component of McCune–Albright syndrome or the rare Mazabraud syndrome. Malignant degeneration is rare. Classically, on radiographs, FD lesions are intramedullary, expansile, and well defined with varying degrees of hazy density with a ground-glass quality, although some may be almost completely radiolucent or sclerotic. Endosteal scalloping may be present, but a smooth cortical contour is typically maintained. The lesions usually show nonspecific increased radiotracer uptake on bone scans. The MRI characteristics of FD are variable, typically showing signal intensity that is intermediate to low on T1-weighted images and intermediate to high on T2-weighted images, with heterogeneous enhancement after administration of gadolinium-based contrast agent (Fitzpatrick et al. 2004; Remotti and Feldman 2012). Single cases have been reported in which FD is present in patients undergoing FDG–PET scanning for oncological reasons, and FD can appear with and without increased FDG uptake (Stegger et al. 2007). Biopsy and surgical removal of FD lesions is generally reserved for equivocal cases or those with complications (Figs. 8.8 and 8.9).

Langerhans cell histiocytosis (LCH), formerly known as histiocytosis X, is a rare group of disorders of unknown etiology which can occur in any age group with a wide spectrum of clinical presentation causing local or systemic effects. The hallmark of LCH is the proliferation and accumulation of a specific histiocyte, the Langerhans cell. LCH includes three classic clinical syndromes that are considered to be clinical variations of the same disease: eosinophilic granuloma, Hand–Schuller–Christian disease, and Letterer–Siwe disease. Eosinophilic granuloma, the most common form, is isolated to the bone, often monostotic, with potential involvement of any bone. Radiographically, early-phase lesions are osteolytic with an aggressive appearance, poorly defined margins, variable aggressive-appearing periostitis, and difficult to differentiate from malignancy, such as Ewing sarcoma, lymphoma, leukemia and metastasis, osteoblastoma, or atypical infection. In the flat bones and in the late phase, the lesions are typically better defined and may have sclerotic margins suggestive of spontaneous healing.

CT and MRI are useful in providing detailed cross-sectional anatomic detail of the involved bone, including the bone marrow and the adjacent soft tissues (Fig. 8.10), and bone scintiscans are useful in the detection of multifocal involvement. On MRI, the lesions typically show decreased signal on T1-weighted images, increased signal on fluid-sensitive sequences, and marked enhancement on contrast-enhanced fat-suppressed T1-weighted sequences which decreases with healing. Prominent perilesional edema may be present. FDG–PET shows radiotracer uptake in active lesions as opposed to healed lesions. Patients with skeletal LCH may be asymptomatic or present with pain, swelling, and soft-tissue mass. Biopsy is frequently needed for diagnosis. Although the natural history for most lesions is of gradual healing, curettage and bone grafting are sometimes performed to accelerate the healing process. Internal fixation for stability is occasionally necessary. Chemotherapy is used for multisystemic disease, and radiotherapy is no longer used (Hoover et al. 2007; Arkader et al. 2009; Remotti and Feldman 2012).

Enchondroma is a common benign bone tumor which is characterized by formation of abundant mature hyaline cartilage. This tumor can be seen in any age group. Any bone can be involved and majority of the lesions are asymptomatic. Radiographs show central, well-defined, possibly expansile osteolytic lesions with variable endosteal scalloping. Periosteal reaction or frank cortical breakthrough is an ominous sign. Chondroid matrix calcifications may be seen. When noncalcified, lesions are often radiographically occult and identified first on MRI, where they are characterized by lobulated T1-hypointense signal, lobulated hyperintense signal on fluid-sensitive sequences, and variable enhancement on the contrast-enhanced T1-weighted sequences (Fig. 8.11). On bone scintigraphy, enchondromas typically show normal or only slightly increased radiotracer uptake. Markedly increased activity of enchondroma with pain is most often associated with pathologic fracture or malignant degeneration (Murphey et al. 1998; Azouz et al. 2005; Hommadi et al. 2009; Wootton-Gorges 2009). FDG–PET may help in distinguishing benign from malignant chondroid matrix lesions (Kim et al. 2003).

Non-ossifying fibroma (NOF) (or fibroxanthoma) is a common benign tumor seen in children and adolescents that has typical radiographic and MRI characteristics. On radiographs, these lesions are usually seen in the diametaphyseal regions; they are eccentrically located and show sclerotic margins. They appear lobulated and radiolucent, become sclerotic with healing, and may resolve (Kim et al. 2003; Stacy and Dixon 2007; Remotti and Feldman 2012). When PET or bone scintigraphy reveal metabolically active osseous abnormalities in children who are at risk for bone metastases, benign fibro-osseous lesions should also be considered in the differential diagnosis, and correlation with radiographs should be performed (Feldman et al. 2005). Fractures (stress or acute) through NOFs are particularly confusing and may be mistaken for malignancy on MRI. Radiographic findings are typically reassuring (Goodin et al. 2006) (Fig. 8.12).

Advances in oncological treatments have resulted in improved survival rates, and it is not unusual to see the late effects of radiotherapy in both pediatric and skeletally mature cancer patients (Sakamoto et al. 2008). It is important to recognize the skeletal complications of radiotherapy, as they can cause diagnostic problems, and some are associated with significant morbidity. The effects of radiation on mature bone result in weakening and vary according to the absorbed dose and other factors, such as beam energy and fractionation. The benign radiographic changes constitute a spectrum from mild osteopenia, approximately 1 year after treatment, to osteonecrosis, reflecting bone atrophy, attempted repair, and possibly late vascular changes. On MRI, benign radiation-induced changes may mimic infection and malignancy such as multiple myeloma, recurrent tumor, and metastatic disease (Williams and Davies 2006) (Fig. 8.13). In equivocal cases, tissue diagnosis may be required.

Paget disease affects elderly and middle-aged patients and is characterized by abnormal osteoclastic and osteoblastic activity. In the early “active” phase, osteolytic lesions are observed on radiographs. Involved long bones may have a characteristic “blade of grass” appearance. In the late “inactive” phase, cortical and trabecular thickening is seen. Characteristics of both phases may be present simultaneously. CT imaging parallels the radiographic findings. MRI findings are variable, ranging from normal to abnormal with nonspecific foci of decreased signal intensity on the T1-weighted sequences and increased signal on the fluid-sensitive sequences that can mimic metastatic disease or multiple myeloma. Increased radiotracer uptake is observed on bone scintigraphy (Stacy and Dixon 2007; Marra et al. 2008) (Figs. 8.21 and 8.22).

Melorheostosis is an uncommon mesenchymal dysplasia that affects the appendicular or (rarely) axial skeleton and may have soft-tissue manifestations. Unilateral well-marginated undulating zones of cortical hyperostosis with a “dripping candle wax” appearance, with or without associated soft-tissue masses, occur in the distribution of the sclerotomes (Judkiewicz et al. 2001; Nasu et al. 2002; Smith et al. 2002) (Fig. 8.23). Mineralized or nonmineralized soft-tissue masses adjacent to sites of osseous melorheostosis should be recognized as another manifestation of this disease, as opposed to a more ominous finding, making biopsy unwarranted (Smith et al. 2002). The lesions typically show increased radiotracer uptake on bone scintigraphy (Chanda and Millner 2007; Izadyar and Gholamrezanezhad 2012).

Neuropathic joint or Charcot arthropathy typically does not mimic malignancy. As shown in this chapter, however, proliferative Charcot arthropathy may occasionally resemble an osteosarcoma (Fig. 8.24).

Brown tumors of primary and secondary hyperparathyroidism are now uncommon, with typical earlier clinical diagnosis and effective treatment of hyperparathyroidism. However, brown tumors remain a consideration in the evaluation of patients presenting with multiple lucent or tumor-like lesions on radiographs, CT, MRI, and bone scintigraphy, in which no primary neoplasm is known (Tigges et al. 1995; Meydan et al. 2006; Ollivier et al. 2006; Kirk et al. 2009; Sonoda et al. 2011) (Fig. 8.25). Clinical, laboratory, radiographic, and histologic investigations can lead to a correct diagnosis.