Soft Tissue Pathology

- Most common type of liposarcoma
- Marked prediliction for lower extremities (especially thigh)
- Never in retroperitoneum (if see in this location, call it "Atypical Lipomatous Tumor with Prominent Myxoid Change")

- Few or no mitotic figures :)
- Proliferative lipoblasts at various stages of differentiation
- Prominent anastomosing capillary network (present of delicate network of thin-walled vessels is very important feature!)
- Mucoid matrix rich in hyaluronidase-sensitive acid mucopolysaccharides

t(12;16)(q13;p11) = FUS-DDIT3 (= TLS-CHOP)
t(12;22)(q13;q12) = EWS-DDIT3

[Note: DDIT3 = gene involved in adipocyte differentiation :)]

Myxoid Liposarcoma

- Most common type of liposarcoma
- Marked prediliction for lower extremities (especially thigh)
- Never in retroperitoneum (if see in this location, call it "Atypical Lipomatous Tumor with Prominent Myxoid Change")

- Few or no mitotic figures :)
- Proliferative lipoblasts at various stages of differentiation
- Prominent anastomosing capillary network (present of delicate network of thin-walled vessels is very important feature!)
- Mucoid matrix rich in hyaluronidase-sensitive acid mucopolysaccharides

t(12;16)(q13;p11) = FUS-DDIT3 (= TLS-CHOP)
t(12;22)(q13;q12) = EWS-DDIT3

[Note: DDIT3 = gene involved in adipocyte differentiation :)]

Round Cell Liposarcoma

- Poorly differentiated type of myxoid liposarcoma (same translocation & gene fusion)
- Associated with poorer prognosis :( [even 1-5% component is assoc.d with increased local recurrence risk and poorer disease specific survival)

- (+) S100
- Small cells
- Acidophilic cytoplasm
- Scattered lipoblasts
- More mitoses & less prominent vascular network (vs. myxoid liposarcoma)
-

Pleomorphic Liposarcoma

- highly cellular
- poorly differentiated
- numerous tumor giant cells
- (+++) mitoses
- (+++) necrotic foci

IHC:
- (+) S100 (BUT.....in reality, only seen in 1/3 of cases)
- may also be (+) desmin, (+) SMA (smooth mus. actin)

Epitheloid variant
- looks like undifferentiated carcinoma and even has focal reactivity for keratin!

DDx:
- Malignant Fibrous Histiocytoma
- Pleomorphic Rhabdomyosarcoma

Dedifferentiated Liposarcoma

- when see emergence of "nonlipogenic component" within an atypical lipomatous tumor/liposarcoma.
- commonly seen in recurrent or metastatic foci (but can also be present at original excision)
- more common in retroperitoneal locations (but can see in extremities)

- High grade
- can look like Malignant Fibrous Histiocytoma (MFH)
- (+/-) myxoid features
- (+) Heterologous components, e.g.:
- skeletal muscle (freq)
- cartilage
- blood vessels
- viewed as "divergent differentiation" and has stains by IHC as such.

Other patterns:
- discontinuous micronodular
- whorling (meningothelial/neural-like)

IHC:
- high percentage stain for PPAR-gamma (vs. only +ve in ~1/4 other sarcomas)

Genetics