Nonossifying fibroma presenting as an aneurysmal bone cyst: a case report
© Sakamoto et al.; licensee BioMed Central Ltd. 2012
Received: 3 August 2012
Accepted: 31 October 2012
Published: 29 November 2012
Nonossifying fibroma is a common fibrous bone lesion in children that occurs in the metaphysis of the long bones of the lower extremities. The lesion rarely leads to aneurysmal bone cyst, which is characterized as a blood-filled space.
We present the case of a 13-year-old Japanese boy with a complaint of discomfort in the thigh and a small, well-defined, osteolytic lesion with cortical thinning located in the medullary space of the distal diaphysis of the femur. At 10-month follow-up, the size of the lesion had increased. Gadolinium-enhanced magnetic resonance imaging failed to detect any solid area. Curettage and bone graft were performed and confirmed a blood-filled cystic lesion. The pathological diagnosis of the cyst wall was that of nonossifying fibroma, suggesting aneurysmal bone cyst as a secondary change. An aneurysmal bone cyst is rarely found secondary to nonossifying fibroma, and the diaphyseal location is atypical for nonossifying fibroma, both of which made diagnosis challenging.
The current case is a reminder to clinicians that, although rare, nonossifying fibroma can be associated with aneurysmal bone cyst, and both can occur in the diaphysis of long bones.
Nonossifying fibroma (NOF) is a common type of benign fibrous lesion that occurs in the metaphysis of the long bones of the lower extremities[1, 2]. On plain radiographs, NOF appears as a small, cortically based osteolytic lesion with a thin sclerotic rim. Histologically, NOF is composed of spindle-shaped fibroblasts, multinucleated giant cells, and foamy histiocytes. NOF is typically asymptomatic and the lesion is found incidentally. NOF is considered a developmental bone defect, not a neoplastic lesion, because it is usually self-limiting[1–3].
In contrast to NOF, aneurysmal bone cyst (ABC) is a locally destructive bone lesion. Primary ABC typically occurs during the first two decades of life, and most frequently in the metaphyses of long bones, and less frequently in the spine or flat bones. On plain radiographs, ABC appears as an eccentric and expansile lytic lesion. ABC is considered a reactive process secondary to various precursor conditions, including benign and malignant bone neoplasms[2, 5].
We report a rare case of NOF associated with ABC that occurred at the diaphysis of the femur.
The pathogenesis of ABC is presumably a local circulatory abnormality that causes increased venous pressure and results in dilation of the vascular network[2, 5]. Primary ABC has a different genetic pathogenesis from secondary ABC, in spite of their morphological similarities, because primary ABC has been reported to have rearrangements of CDH11 or USP6 that are not seen in secondary ABC.
Typically, the diagnosis of NOF can be easily made based on images and clinical findings[3, 7]. The natural course of NOF is self-limiting involution and, thus, NOF has come to be known as a ‘don’t touch’ lesion because more aggressive diagnostics or treatment are unnecessary. It has been reported that NOF can be found in approximately 30% of young individuals within the first and second decade of life. ABC change secondary to NOF appears to be extremely rare, although the frequency is difficult to assess. Of interest, this case of NOF was located at the femoral diaphysis, but the typical location of NOF is the metaphyseal region. According to Brenner and colleagues, NOF can be divided into three phases on the basis of the intensity of uptake on bone scan: active, healing, and inactive. The low uptake, combined with the diaphyseal location, may suggest the current case was a long standing lesion in the inactive phase. The diaphyseal location made it difficult to arrive at the correct diagnosis of NOF based on the imaging results.
In conclusion, this report presents a rare case of NOF associated with ABC. In addition, the diaphyseal location was atypical for NOF. We present this case report as a reminder to clinicians to remain aware that, although rare, NOF can be associated with ABC, and both can occur in the diaphyses of long bones.
Written informed consent was obtained from the patient’s legal guardian for publication of this manuscript and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
Aneurysmal bone cyst
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