summary Lipomas are common benign tumors of mature fat which may be subcutaneous, extramuscular, or intramuscular. The condition is typically seen in patients 40-60 years of age who present with a stable, mobile, and painless mass. Diagnosis is made with MRI studies showing a homogenous lesion with signal intensity matching adipose tissue on all image sequencing. Treatment is observation for asymptomatic lesions. Marginal surgical resection is indicated for symptomatic lesions or lesions that are rapidly growing Epidemiology Demographics slightly more common in men affects predominantly patients between 40-60 years old develops in sedentary individuals Anatomic location superficial/subcutaneous location is common superficial lesions are common in the upper back, thighs, buttocks, shoulders and arms deep lesions are affixed to surrounding muscle, in the thighs, shoulders and arms ~5-10% of patients with a known superficial lipoma, will have multiple lesions Classification Common variants of lipoma include spindle cell lipoma common in male patients ages 45-65 years stain positive for CD34 but negative for S-100 treated with marginal excision pleomorphic lipoma common in middle aged patients may be confused with liposarcomas angiolipoma unique in that it is painful when palpated often present with small nodules in the upper extremity intramuscular lipoma often symptomatic and require marginal resection hibernoma tumor of brown fat affects younger patients (20-40 years old) Symptoms Symptoms usually a painless mass that has been present for a long time exception is the angiolipoma, which is painful when palpated Physical exam palpable, mobile, painless lesion Imaging Radiographs may show a radiolucent lesion in the soft tissues may see mineralization, which should raise concern for synovial cell sarcoma may see calcifications or presence of bone within the lesion CT scan well demarcated lesion lesion looks akin to subcutaneous fat MRI well demarcated lesion homogenous, signal intensity matches adipose tissue on all image sequencing shows well demarcated lesion with same characteristics as mature fat high signal intensity on T1 weighted images high signal intensity on T2 weighted images, entirely suppressed by STIR or fat saturated sequences low signal intensity on STIR image Histology Biopsy often not necessary as diagnosis can be made by imaging (MRI) Gross appearance Lipomas are soft, lobular, may be encapsulated and whitish/yellowish in color Hibernoma are reddish brown because of rich vascular supply in addition to high numbers of mitochondria Histology in general shows bland acellular stroma with neoplastic cells that lack cellular atypia. Histology varies by variant spindle cell lipoma mixture of mature fat cells and spindle cells mucoid matrix with varying number of birefringent collagen fibers pleomorphic lipoma lipocytes, spindle cells, and scattered atypical giant cells angiolipoma mature fat cells with nests of small arborizing vessels intramuscular lipoma pathology shows lipoblasts and muscle infiltration Treatment Nonoperative observation only indications lesion is painless and MRI is determinate for a benign fatty lesion Operative marginal resection (may be intralesional) indications symptomatic lesions mass is rapidly growing tumors located deep to the fascia or in the retroperitoneum deep or retroperitoneal lipomas show a higher likelyhood to be/become atypical lipomatous tumors in the retroperitoneum, referred to as well-differentiated liposarcoma in the extremities, referred to as atypical lipomas spindle cell/pleomorphic lipomas are treated by marginal resection Complications Local recurrence uncommon (< 5%) Prognosis Size typically plateaus after initial growth