History: Adolescent Male with knee pain.
This is a case of a solitary or unicameral bone cyst. A solitary bone cyst (also called a juvenile bone cyst, unicameral bone cyst, essential bone cyst, or simple bone cyst) is a benign lytic bone lesion that is fluid filled. The most common location that solitary bone cysts occur in is the calcaneus in adults and the proximal humerus in children. They arise in the metaphysis close to the physis, as seen in the images above. On MRI T1 weighted imaging, unicameral bone cysts are homogeneous low to intermediate signal, and on fluid-sensitive sequences they show high signal (image 2 above). High signal in the marrow indicates marrow edema.
The differential diagnosis for this appearance is as follows:
1. Solitary or Unicameral Bone Cyst
3. Aneurysmal bone cyst
5. Langerhans Cell Histiocytosis
Some have used the mnemonic FOG MACHINES or FEGNOMASHIC for lytic bone lesions, which can be further broken down by age group and other various features such as absence of pain. You can read about it here.