Lipoma

Variant: Myolipoma of Soft Tissue

Composed of mature fat and bland smooth muscle
- Muscle predominates in most cases
  - Usually evenly interspersed
  - Focal smooth muscle overgrowth in one reported case
  - Muscle in short fascicles
- Both components lack any atypical features
  - No cytologic atypia or lipoblasts
  - No floret cells
  - No necrosis
Stroma may be sclerotic or hyalinized
- Occasionally myxoid
- No prominent vascular component
Occasional features (none affect behavior)
- Mitotic figures up to 1/10 HPF
  - No atypical mitoses
- Focal hypercelllularity of spindle cells
- Degenerative atypia
- Round cell morphology
- Hemosiderin
- Metaplastic cartilage and bone
- Prominent eosinophils

Spindle cells positive for smooth muscle actin, desmin, ER
- CD34, HMB45 negative
HMGA2 60% positive
MDm2, CDK4 infrequent

Angiomyolipoma	Myolipoma
Abnormal large vessels	No abnormal vascular component
Epithelioid perivascular cells	No epithelioid component
HMB45, MelanA positive	HMB45, MelanA negative

Atypical Lipomatous Tumor with Smooth Muscle Metaplasia	Myolipoma
Atypical cells present	No atypical cells
Fibrous septa frequent	Fibrous septa absent
Floret cells or lipoblasts frequent	No floret cells or lipoblasts

Spindle Cell Lipoma	Myolipoma
Restricted to back of neck, upper back, shoulders	Predominantly pelvic
Scant cytoplasm	Tapered cytoplasm in spindle cells
Ropey collagen frequent	No ropey collagen
CD34 positive	CD34 negative
Actin negative	Actin positive

Low Grade Leiomyosarcoma Infiltrating Fat	Myolipoma
Infiltrative pattern	Circumscribed, encapsulated
Fat is overrun (asymmetrically involved)	Fat is integral to lesion (throughout)
May have atypia, mitotic figures	No atypia or mitotic figures rare

Rarely reported
Age 28-94
Vast majority female
Sites of involvement
- Females: pelvic, retroperitoneal, suprapubic,inguinal
- Males: inguinal, abdominal wall, back
- Other sites include eyelid, spinal cord and pericardium
No known association with tuberous sclerosis
No recurrences or metastsis reported

Takahashi Y, Imamura T, Irie H, Tanaka F, Fukushima J, Fukusato T, Harasawa A, Shiga J. Myolipoma of the retroperitoneum. Pathol Int. 2004 Jun;54(6):460-3.
Guillou L, Coindre JM. Newly described adipocytic lesions. Semin Diagn Pathol. 2001 Nov;18(4):238-49.
Michal M. Retroperitoneal myolipoma. A tumour mimicking retroperitoneal angiomyolipoma and liposarcoma with myosarcomatous differentiation. Histopathology. 1994 Jul;25(1):86-8.
Meis JM, Enzinger FM. Myolipoma of soft tissue. Am J Surg Pathol. 1991 Feb;15(2):121-
Fukushima, Mana MD; Schaefer, Inga-Marie MD; Fletcher, Christopher D.M. MD, FRCPath.Myolipoma of Soft Tissue: Clinicopathologic Analysis of 34 Cases.
American Journal of Surgical Pathology. 41(2):153-160, February 2017..