Fat necrosis right buttock

Case contributed by Dr. Chris O'Donnell

Presentation

Painless lump in the right buttock. No history of trauma or buttock injections

Patient Data

Age: 35
Gender: Female

Irregular spiculated mass in the fat of the right buttock adjacent to the gluteus muscle. Diffuse enhancement but no T1 hyperintensity to suggest blood or liquid fat.

Irregular mass in the fat of the buttock at the site of MRI findings with biopsy needle in situ - mass was very "hard" at biopsy. Pathology typical of fat necrosis with numerous "foamy macrophages".

Early minimal thallium accumulation in the right buttock mass suggesting that is it minimally cellular.

Case Discussion

The most likely and actual diagnosis in this case is fat necrosis. What is unusual is the absence of any trauma history or importantly any buttock injection. Nonetheless the mass has spiculated margins and seems to draw in the adjacent gluteus maximus muscle consistent with a fibrotic process. The buttock is an uncommon site for this pathology being more commonly seen in the breast.

The major differential diagnosis in this case is desmoid tumor (a.k.a. aggressive fibromatosis). It arises from fibroblasts that have undergone genetic mutation (in a gene called beta catenin). They can arise in virtually any part of the body and occur mostly in women in their 30's, but can occur in anyone at any age. They tend to be slow growing but can be extremely aggressive. They do not usually metastasize. Rarely they are associated with familial (colonic) polyposis syndrome i.e. Gardner's Syndrome.

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