Can you identify this bone tumor

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Unicameral bone cysts (UBC), also known as a simple bone cysts, are common benign non-neoplastic lucent bony lesions that are seen mainly in childhood and typically remain asymptomatic.

Epidemiology

They usually found in children in the 1stand 2nd decades (65% in teenagers), and are more common in males (M:F ~ 2-3:1) 2, 6.

Clinical presentation

These lesions are usually asymptomatic and found incidentally, although pain, swelling and stiffness of the adjacent joint also occur. The most frequent complication is pathological fracture, and this is frequently the cause of presentation 1, 2, 6.

Pathology

When uncomplicated by fracture the cysts contain clear serosanguineous fluid surrounded by a fibrous membranous lining. It is thought to arise as a defect during bone growth which fills with fluid, resulting in expansion and thinning of the overlying bone.

During the active phase the cyst remains adjacent to the growth plate. As the lesion becomes inactive it migrates away from the growth plate (normal bone is formed between it and the growth plate) and it gradually resolves 3, 5.

Location

They are typically intramedullary and are most frequently found in the metaphysis of long bones, abutting the growth plate 1. Locations include 1, 2, 5:

• proximal humerus: most common 50-60%
• proximal femur: 30%
• other long bones
• occurrence UBC everywhere else is relatively uncommon, and usually  occurs in adults
  • spine: usually posterior elements
  • pelvis: only 2% of UBC 1

UBC's can be rarely seen in adults in unusual locations such as in the talus, calcaneus, or the iliac wing.

Radiographic features

Plain radiograph

UBCs are well defined geographic lucent lesions with narrow zone of transition, mostly seen in skeletally immature patients, which are centrally located and show sclerotic margin in majority of cases with no periosteal reaction or soft tissue component. They sometimes expand the bone with thinning of the endosteum without any breach of the cortex unless there is a pathologic fracture. Prominent ridges of bone can appear as pseudotrabeculation on x-ray but in fact UBC is made of one contiguous cystic space. Rarely, they are truly multiloculated 3.

If there is fracture through this lesion a dependent bony fragment may be seen, and this is known as the fallen fragment sign.

CT and MRI

CT and MRI add little to the diagnosis, but are however helpful in eliminating other entities that can potentially mimic a simple bone cyst (see differential diagnosis below).

MR signal characteristics for an uncomplicated lesion include:

• T1: low signal
• T2: high signal

Usually there no fluid-fluid levels unless there has been a complication with haemorrhage.