What would your DDx be? Is this neoplastic, infectious or inflammatory? What would you biopsy?

Can leukemia / lymphoma infiltrate the tendon sheath like this? Would you consider sarcoid? Other?

Femoral lymphadenopathy

Subcutaneous, intermuscular and intramuscular edema

Adenopathy with subcutaneous and intramuscular edema

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.

40-year-old woman with melanoma of the right knee

A teaching case from MD Anderson Cancer Center

Behrang Amini

40-year-old woman with right knee melanoma

Wide local excision 6 years ago with negative sentinel lymph node biopsy

Liver and mesenteric mets 1 year ago

Started therapy with dabrafenib and trametinib 2 months ago

Most recent PET was negative

Presents with FDG PET/CT below: 
 

Note multiple sites of new uptake in the lower limbs, more numerous on the left (primary was in the right knee)

Axial fused PET/CT shows subcutaneous location of lesions

Axial CT image shows uptake localizing to subtle increased attenuation

Initial instinct is to call this metastatic disease and change therapy.

This is BRAF inhibitor-induced paniculitis.

Currently three BRAF inhibitors are approved in Europe and US for the treatment of patients with BRAF-mutant advanced melanoma: 

Vemurafenib

Dabrafenib

Encorafenib

20-year-old woman with hip pain

AP view of the right hip

Frog-leg lateral view of the right hip

Zoomed-in view of the frog-leg lateral view

Coronal T1-WI

Coronal T2-WI with FS

Axial T1-WI with FS and contrast

Diagnosis: Plexiform neurofibroma with intra- and extra-articular components

H&E stain showing neurofibromas (*)