55-year-old woman with MGUS and right hip pain

Conventional radiographs show some osteoarthritis.

CT not that impressive

 T1-WI shows fat-containing lesions in bone.

T2-WI with fat suppression shows increased signal within lesion

Due to presumed progression of MGUS to symptomatic myeloma, FDG PET/CT was done

What do you think?

This ended up being sarcoidosis. The PET pattern of symmetric mediastinal and hilar nodes is highly suggestive. Liver involvement would be atypical, but not unheard of for myeloma. Bone disease is certainly typical, but finding fat on MRI in untreated myeloma lesions is not.

Liver was biopsied, showing sarcoid.

Skeletal sarcoidosis:

• Occurs in 1-13% of patients with Sarcoidosis
• More common in Black patients 
• Isolated skeletal involvement is rare
• 80-90% have concurrent pulmonary involvement
• Hands and feet are typically most often affected
• Can be lytic or sclerotic on radiography
• Can be T1 hyper- or hypo-intense
• Typically T2-hyperintense
• Enhances and typically FDG-avid

It's difficult to differentiate skeletal sarcoid from metastasis on imaging. Some features that can help when present are a brush (fuzzy) border characteristics and intralesional lesion fat, features that have near 100% specificity but poor sensitivity (14.3% and 0%, respectively).

Reference:
Moore SL, Kransdorf MJ, Schweitzer ME, Murphey MD, Babb JS. Can sarcoidosis and metastatic bone lesions be reliably differentiated on routine MRI? AJR Am J Roentgenol. 2012 Jun; 198(6):1387-93.