Lipoma
Variant: Intermuscular Lipoma
Diagnostic Criteria
- Circumscribed lipoma between muscles in deep soft tissue
- Composed of mature adipose tissue
- No cytologic atypia
Differential Diagnosis
| Intramuscular Lipoma | Intermuscular Lipoma |
|---|---|
| Within skeletal muscle | Between muscles |
| May be infiltrative or circumscribed | Circumscribed |
| Recurrence rate 0-20% | Virtually no recurrences |
Lipoma vs Atypical Lipomatous Tumor (Well Differentiated Liposarcoma)
- Diagnosis and behavior depends to a great extent on location
- Dermis and subcutis of posterior neck, shoulders and upper back
- Virtually all are lipomas
- Most are ordinary lipomas or spindle cell lipomas
- Mature fatty neoplasms with cytologic atypia or lipoblasts are designated pleomorphic lipomas in these locations only
- Dermis and subcutis of all other sites
- The vast majority are ordinary lipomas or angiolipomas
- No cytologic atypia
- Rare mature fatty neoplasms with cytologic atypia or lipoblasts are seen in these locations
- Infrequent recurrences
- Dedifferentiation is very rare
- Natural (untreated) history is unknown
- We designate such neoplasms as atypical lipomatous tumors but with a comment that aggressive behavior is rare in tumors arising in those sites
- Deep soft tissues
- Intramuscular, intermuscular and periosteal
- Lipomas are well described in these locations
- Must not have cytologic atypia or lipoblasts
- Intramuscular lipoma may recur if incompletely excised
- Rare large intramuscular lipomas have recurred with features of atypical lipomatous tumor
- Genetic study for MDM2 amplification may be indicated in large or aggressively recurring intramuscular lipomas even in the absence of atypia
- Rare large intramuscular lipomas have recurred with features of atypical lipomatous tumor
- Atypical lipomatous tumor can occur in these locations
- Infiltration is not relevant to the diagnosis
- Requires cytologic atypia or lipoblasts
- Frequent recurrences, rarely uncontrollable
- Dedifferentiation rare
- Retroperitoneum, spermatic cord, mediastinum
- Vast majority of mature fatty neoplasms are atypical lipomatous tumors
- (Most cord “lipomas” are merely pre-peritoneal fat squeezed through the inguinal ring, and not neoplasms)
- Frequent recurrences, may be uncontrollable
- May dedifferentiate and metastasize
- Rare lipomas have been documented in the retroperitoneum (Macarenco)
- Requires extensive sampling to exclude cytologic atypia and lipoblasts
- Immunohistochemical and/or genetic studies are recommended before making this diagnosis
- Myelolipoma frequently occurs in or around the adrenal
- Megakaryocytes should not be confused with atypical cells of ALT
- We consider atypical lipomatous tumor and well differentiated liposarcoma to be equivalent terms and primarily use the former, although both are acceptable (see discussion of these terms)
- Some use WDL for deep central tumors and ALT for those in other sites
- Immunohistochemistry and/or in situ hybridization may be of use at any site
- Lipomas are reported to be MDM2, CDK4, p16 negative
- ALT are generally MDM2, CDK4, p16 positive
- See subtypes of lipomas for specific differential diagnoses
Clinical
- Virtually no recurrences
Bibliography
- Fletcher CD, Martin-Bates E. Intramuscular and intermuscular lipoma: neglected diagnoses. Histopathology. 1988 Mar;12(3):275-87.
