Enostosis (Bone Island)

Definition

Benign focus of compact (cortical) bone located within cancellous bone (medullary cavity)

 

Best diagnostic clue

Homogeneous lesion with characteristics of cortical bone, occurring within marrow space

 

Location

Pelvis, long bones, ribs, spine are most frequent

 

Size

• ·         Usually small (< 1 cm); may be giant (several cm)
• ·         Size may change over time: May enlarge, remain stable, or decrease/disappear

 

Morphology

Round or oval, oriented along long axis of bone

 

Radiographic Findings

• ·         Homogeneously dense, fading at periphery, and merging into normal marrow
• ·         Periphery described as brush-like; may appear somewhat stellate
• ·         No associated cortical destruction
• ·         May be multiple in same bone or polyostotic
• ·         If multiple and concentrated in metaphyseal region, termed osteopoikilosis

 

CT Findings

• •        Sclerotic lesion follows radiographic appearance
• ·         Peripheral extensions into normal adjacent bone (brush border) best seen on CT

 

MR Findings

• •        Low signal on all sequences; faintly higher signal than surrounding bone with fat saturation
• •        Periphery fades into adjacent marrow, as on radiograph and CT
• •        No enhancement with contrast

 

Nuclear Medicine Findings

• •        Bone scan may be either positive or normal Depends in part on lesion size; often some ↑ uptake
• •        seen if lesion > 1 cm in diameter

 

DIFFERENTIAL DIAGNOSIS

 

Metastatic Disease (Sclerotic)

• •        May have nearly identical appearance
• •        Metastatic focus may not be as homogeneous throughout, allowing differentiation
• •        Metastases generally show some enhancement on MR

 

Fibroxanthoma (Nonossifying Fibroma)

• •        Generally heal during teenage years, with slightly sclerotic bone forming from periphery → center of lesion
• •        Eventually replaced by normal bone or leave faint trace of homogeneous sclerosis
• •        Cortically based rather than central

 

Osteoma

• •        Dense focus; usual location is paranasal sinus or skull
• •        If located peripherally, arises on outer cortex of bone, not within marrow
• •        Generally homogeneously sclerotic, though may have regions of inhomogeneity

 

Osteoid Osteoma

• •        Usually have lytic nidus ± central sclerotic focus
• •        Nidus may be obscured on radiograph by homogeneous sclerotic bone reaction: Only appearance that could be confused with bone island
• •        Lytic nidus always seen either with CT or MR

 

Cement & Bone Fillers

• •        Cement usually has peripheral lucent halo
• •        Bone graft seen as multiple sclerotic foci, which gradually merge as healing occurs

 

PATHOLOGY

 Etiology

• •        Likely developmental
• •        Normal cortical bone, which fails to resorb during growth process of endochondral ossification

 

CLINICAL ISSUES

Presentation

•         incidental finding

Demographics

• Age

  Seen in adults far more frequently than children

• Epidemiology

  Incidence reported at 14%; may be higher

Natural History & Prognosis

• No associated morbidity or mortality

 

DIAGNOSTIC CHECKLIST

Consider

• •        Only difficulty in diagnosis occurs in elderly, who are at risk for sclerotic metastases
• •        Polyostotic feature slightly favors metastatic disease
• •        Bone scan usually shows ↑ uptake in both enostosis and sclerotic metastasis
• •        T1 and T2WI MR may be identical; metastases usually enhance, at least peripherally
• •        Spectral CT may be helpful differentiating sclerotic metastasis from bone island, particularly using standard deviation of CT value on high-energy, virtual monochromatic spectral images
• •         Rarely, biopsy is required to differentiate

 

 

AP radiograph shows a typical large bone island The sclerosis is regular and shows brush-like edges, fading into normal bone rather than a distinct sclerotic edge.

Coronal T1 MR in the same patient shows low signal intensity throughout the lesion, similar to that of cortical bone. Particularly at the superior edge of the lesion, one sees the brush-like border of the bone island melding into normal bone, a typical feature of bone island.

Sagittal T2 MR, same patient, shows the homogeneous low signal intensity of the lesion to be identical to that of cortical bone. This signal intensity feature is typical, and maintains without contrast enhancement.

Sagittal T2 FS MR, in the same site as the prior T2 image, shows that there is minimally higher

signal intensity in the bone island than in the intensely low signal fat-saturated surrounding bone. This is typical of bone island and should not be misinterpreted as representing a metastasis.