Unusual radiologic presentation of monostotic fibrous dysplasia

Fibrous dysplasia is a benign fibro-osseous lesion that can occur as an isolated skeletal lesion (monostotic form) or affect multiple skeletal sites (polyostotic form). In addition, fibrous dysplasia may be associated with single or multiple endocrinopathies, or with precocious puberty and cutaneous hyperpigmentation in McCune-Albright syndrome. Fibrous dysplasia usually appears as a well-defined radiolucent medullary lesion that is irregular, mildly expansive, and with a hazy opacity classically described as "ground glass" in appearance. In the long tubular bones, fibrous dysplasia may cause expansion of the bone contour with cortical thinning and endosteal scalloping. The shaft is typically involved, but the metaphysis may also be affected. These changes are usually recognizable on plain radiographs, but in cases where the lesion is difficult to visualize computed tomography may be beneficial. However, fibrous dysplasia may present with radiographic features that may mimic other benign fibro-osseous lesions, may be associated with other lesions, and even be confused with certain types of malignancies. We present a case of monostotic fibrous dysplasia in the proximal tibia of an 11-year-old girl with the unusual radiologic features of multifocal lytic lesions and cortical disruption. While most often the clinical presentation, laboratory findings, and imaging studies permit the diagnosis of fibrous dysplasia, in certain cases this may pose a diagnostic challenge. The diagnosis was confirmed by open biopsy.