Atypical Lipomatous Tumor
Differential Diagnosis
- Benign lipomatous tumors
- Malignant lipomatous tumors
- Other lesions containing prominent fat
- Spindle cell lesions that may be confused with ALT
- Other tumors containing atypical cells resembling those of ALT
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
Pleomorphic Lipoma vs Atypical Lipomatous Tumor
- The diagnosis of tumors composed of mature fat with atypia depends upon the location
- In the dermis and subcutis of the posterior neck, upper back or shoulders, it is considered pleomorphic lipoma
- In all other locations, it is considered atypical lipomatous tumor
- Pleomorphic lipoma MDM2 and CDK4 negative, while most ALT are positive
- Ropey collagen and extensive CD34 staining support pleomorphic lipoma
| Hibernoma with predominant mature fat | Atypical Lipomatous Tumor |
|---|---|
| Small, central bland nuclei in vacuolated cells | Lipoblasts, if present, have atypical nuclei |
| Lacks atypical nuclei | Contains atypical nuclei |
| Frequently superifcial | Rarely superficial |
| MDM2, CDK4, p16 negative | MDM2, CDK4, p16 most positive |
| Myxoid Liposarcoma | Atypical Lipomatous Tumor |
|---|---|
| No large atypical nuclei | Large atypical nuclei present |
| Prominent "chicken wire" vascular pattern | Lacks "chicken wire" vascular pattern |
| Rare as a primary lesion in retroperitoneum | Frequently occurs in retroperitoneum |
| MDM2 and CDK4 4% by immunohistology but 1/5 MDM2 by FISH | MDM2 and CDK4 most positive |
| Pleomorphic Liposarcoma | Atypical Lipomatous Tumor |
|---|---|
| Lacks mature fat | Prominent mature fat |
| Highly cellular | Low cellularity |
| Mitotic figures frequent | Mitotic figures rare |
| MDM2 and CDK4 negative | MDM2 and CDK4 most positive |
| Fat Necrosis | Atypical Lipomatous Tumor |
|---|---|
| Small central nuclei | Large eccentric nuclei |
| Nuclei not indented by vacuoles | Vacuoles indent nuclei in lipoblasts |
| Foamy cells frequently fill spaces of dead lipocytes | Lipoblasts scattered |
| Angiomyolipoma | Atypical Lipomatous Tumor |
|---|---|
| Large atypical blood vessels | No large vascular component |
| Large epithelioid cells | No large epithelioid cells |
| HMB45 positive | HMB4 negative |
| MDM2 and CDK4 negative | MDM2 and CDK4 most positive |
Subconjunctival herniated fat and fat adjacent to renal cell carcinomas have been reported to simulate atypical lipomatous tumor
- Awareness of the clinical situation should make the diagnosis apparent
| Low Grade Myxofibrosarcoma | Atypical Lipomatous Tumor |
|---|---|
| No lipoblasts | Lipoblasts frequently present |
| Hypocellular | Moderate cellularity |
| Perivascular grouping of cells | No perivascular pattern |
| May have atypical mitotic figures | No atypical mitotic figures |
| May have marked pleomorphism | Mild to moderate atypia |
MFH and other pleomorphic sarcomas vs Atypical Lipomatous Tumor
- Various neoplasms may have atypical cells resembling those of ALT
- The diagnosis of ALT is made by identifying mature fat integral to the lesion
- This requires appropriate surgical and pathological sampling of fatty areas
- Fatty areas of neoplasm may be confused grossly with normal fat if the level of suspicion is not high
- ALT is positive for MDM2 and CDK4 while MFH is infrequently positive (11 and 3%)
