Cystic Tumors of Bone

Clinical Features

Simple bone cyst, also known as unicameral or solitary bone cyst, is a unilocular to multilocular, intramedullary cyst, associated with minimal to no bone expansion. Although the pathogenesis is unknown, recent studies suggest it is a neoplastic process.

Virtually any age may be affected, but most (80%) arise during the first two decades of life. There is a male predominance. Most simple bone cysts are asymptomatic and represent incidental findings. However, clinical symptoms may include localized pain, pathologic fracture, soft tissue swelling, and/or joint stiffness. More than 80% of solitary bone cysts occur in the metaphyseal regions of the proximal humerus and proximal femur ( Fig. 23.1A–C ). Simple bone cysts are rare in adults, and show somewhat different localization, more often involving the pelvic bones, especially the ilium, and the calcaneus ( Fig. 23.2 ).

Radiological Features

Simple bone cysts form purely lytic, well-marginated, metaphyseal-centered lesions, often abutting the physis and arising within skeletally immature (open physis) patients ( Figs. 23.1A–C and 23.2A–B ). Frequently the growth plate grows at a faster pace than the cyst, resulting in the appearance that the cyst has “migrated” away from the physis toward the diaphysis ( Fig. 23.1C ). As a general rule, the affected bone appears slightly expanded but does usually exceed the width of the adjacent physis. Although the overlying cortex may be thinned, it is not penetrated. In some cases, the solitary bone cysts may become quite large but remain sharply marginated. In the setting of a pathologic fracture a portion of cortical bone often drops to the bottom of the cyst, a feature referred to as the “fallen fragment sign” ( Fig. 23.1C ). The “rising bubble sign” refers to the presence of a gas bubble in the most non-dependent portion of the cyst. MRI confirms the fluid content of the cyst ( Fig. 23.2B ).

Pathological Features

Simple bone cyst is characteristically a unilocular, intramedullary, cavitary lesion containing clear to slightly yellow fluid. Despite the large size of some lesions, the amount of material generally obtained by curettage rarely exceeds more than a single slide. The cyst lining ranges from thin, single cell strips to broad fibrous tissue septa containing variable numbers of multinucleated osteoclast-type giant cells ( Fig. 23.3A–B ). Frequently, the cyst wall shows some myxoid change ( Fig. 23.4 ). Additionally, aggregates of eosinophilic fibrin-like debris, sometimes calcified, resembling dental cementum, are observed and are quite characteristic of a simple bone cyst ( Fig. 23.5A–B ). Even in the absence of a fracture, calcifications and reactive bone may be evident (rarely extensive) within the cyst wall. Simple bone cysts with secondary fracture may contain small areas indistinguishable from an aneurysmal bone cyst ( Fig. 23.5C–D ).

Ancillary Findings

Recent studies have identified FUS-NFATC2 or EWSR1-NFATC2 fusions in simple bone cysts. USP6 rearrangements, typically present in aneurysmal bone cysts, are absent.

Differential Diagnosis

The differential diagnosis mainly involves an aneurysmal bone cyst (ABC). Radiographically, simple bone cysts are minimally (to non) expansile lesions, typically occur in younger children, often less than 10 years of age, and generate minimal amounts of tissue, rarely occupying more than one tissue block.

Prognosis and Treatment

The therapy of solitary bone cyst remains controversial. Traditional treatment has included bone curettage and packing with bone chips. More recent therapy includes aspiration and removal of cyst fluid followed by steroid injections. Decompression techniques also have been utilized. The overall local recurrence rate is similar for most types of therapy, ranging from 10% to 20%. It appears that the rate of local recurrence is higher among patients younger than 10 years of age. Other factors that may increase the likelihood of local recurrence include the size of the lesion and the presence of a fracture. Spontaneous healing following pathologic fracture occurs in up to 10% of patients.