Bone island

Enostoses, also known as bone islands, are common benign sclerotic bone lesion that usually represent incidental findings. They constitute a small focus of compact bone within cancellous bone. Enostoses can be seen on radiographs, CT, and MRI, and are considered one of the skeletal “don’t touch” lesions.

Diagnosis

Diagnosis is usually made on the basis of radiographic features. CT can sometimes provide superior detail of the 'brush-like' periphery of enostoses. 

Pathology

The aetiology of enostoses is not definitively known. They are most likely congenital or developmental in nature and are thought to represent either hamartomatous lesions or failure of osteoclastic activity during bone remodelling ¹.

Although they have at times previously been referred to as benign bone tumorstumours (in part due to occasional increased activity on bone scintigraphy), enostoses have been removed from the WHO classification of bone tumorstumours ².

Location

Bone islands may occur anywhere in the skeleton, although there is some predilection for the pelvis, long bones, spine, and ribs.

Associations

• osteopoikilosis: multiple bone islands

Radiographic features

Plain radiograph / CT

Enostoses are commonly seen as small round or oval foci of homogeneously dense bone within the medullary space. They do not cause cortical destruction or periosteal reaction. The appearance of radiating spicules - "thorny radiation" or "fingers" - at the margins that blend with the surrounding trabeculae is classic, giving a "brush-like" or "stellate" periphery. This is usually best appreciated on CT.

In diaphyseal bone, the long axis of a bone island typically parallels the long axis of the involved bone. In the metaphysis, and other regions where trabeculation is not as linearly organised, the bone islands are typically more spherical.

The size of a bone island is typically <1 cm, although large bone islands may occur, particularly in the pelvis, and when greater than 2 cm in maximal dimension are called giant bone islands ^4,8. Sometimes a slow increase in size is seen in bone islands over time.

Enostoses may be solitary or multiple (monostotic or polyostotic). The term osteopoikilosis is applied when there are multiple enostoses, usually concentrated in the epiphyses or metaphyses. 

One study ² has suggested that a sclerotic bone lesion can be diagnosed as a bone metastasis versus bone island if its mean attenuation is <885 HU and maximum attenuation is <1060 HU with a 95% sensitivity and a 96% specificity for both values.

MRI

LowEnostoses are low signal intensity on all sequences (compatible with compact bone), with no surrounding oedema. They do not enhance following contrast administration. 

Nuclear medicine

There is usually no technetium-99m MDP uptake, and a normal bone scan can exclude an osteoblastic metastasis or osteosarcoma. However, low-grade scintigraphic activity has sometimes been reported in histologically proven enostoses, particularly if the lesion is >1 cm ³.

Treatment and prognosis

Consider biopsy if the lesion grows 50% in one year or 25% in six months ⁴.

Differential diagnosis

In the vast majority of cases, bone islands have a pathognomonic appearance. Larger lesions may sometimes pose a diagnostic dilemma, particularly in the setting of known malignancy.  

Imaging differential considerations include:

• osteoblastic metastasis: uptake on bone scan, and may exhibit aggressive features
• osteoma
• osteoid osteoma
• low-grade osteosarcoma
• benign osteoblastoma ⁷

See also

• solitary sclerotic bone lesion