Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum

Differential Diagnosis

In the deep soft tissue:

Leiomyoma (requires all below)	Smooth Muscle Tumor of Uncertain Malignant Potential (used for any of below)	Leiomyosarcoma (requires any one of below)
Cytologically bland		Cytologic pleomorphism or atypia
<1 mitotic figure / 50 HPF	Bland but 1-4 mitotic figures / 50 HPF	>4 mitotic figures / 50 HPF
No tumor cell necrosis	Multiple recurrences but lacking other atypical features	Coagulative tumor cell necrosis

Malignant Fibrous Histiocytoma	Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
Pleomorphic, histologically undifferentiated	Elongate or spindled eosinophilic cytoplasm
Lacks abundant desmin and caldesmon	Typically desmin or caldesmon positive

MFH may be actin positive
The distinction between MFH and high grade leiomyosarcoma is not clinically important and may be one of convention

Dedifferentiated Liposarcoma	Leiomyosarcoma of Soft Tissue
Evidence of concurrent or antecedent atypical lipomatous tumor	No evidence of atypical lipomatous tumor

Foci of smooth muscle cells may be present in dedifferentiated liposarcoma

Inflammatory Myofibroblastic Tumor	Leiomyosarcoma of Soft Tissue
Usually <14 years, rare >30 years	Rare under 12 years
Chronic inflammation usual	Infrequent inflammation

Rare inflammatory leiomyosarcoma exhibits a regular fascicular pattern

Gastrointestinal Stromal Tumor (GIST)

Tumors arising in the muscularis propria of the GI tract and expressing CD117 should be designated GIST

Parasitic Leiomyoma

Parasitic leiomyoma must be clearly attached to the uterus or other organ
In such cases, the criteria for smooth muscle tumors of that organ must be applied

Mesenteric Fibromatosis	Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
Desmin infrequent	Desmin or caldesmon usually positive
Beta catenin 90% positive	Beta catenin negative
Scant cytoplasm	Elongate eosinophilic cytoplasm
Cytologically bland	May be cytologically atypical
Stroma diffusely myxoid	Stroma infrequently myxoid

Angiomyolipoma/PEComa	Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum
May have fatty component	Lacks intrinsic fat
May have abnormal vascular component	Lacks distinctive large abnormal vessels
HMB45, MelanA positive	HMB45, MelanA negative