Leiomyosarcoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum

Definition

Malignant neoplasm exhibiting smooth muscle differentiation located in the deep soft tissues, retroperitoneum, mesentery and omentum

The definition of malignancy for smooth muscle tumors varies by site
- The features listed here apply only to the stated sites
Smooth muscle differentiation
- Elongate eosinophilic cytoplasm
- Pleomorphic or epithelioid cases may require desmin or caldesmon reactivity to support smooth muscle differentiation
Any one of the following is sufficient to diagnose a smooth muscle tumor as malignant for the stated sites
- Cytologic pleomorphism or atypia
- Tumor cell necrosis
- In retroperitoneum, mesentery and omentum
  - Mitotic rate >4 or >10 per 50 HPF (results from different published series)
- In deep soft tissue
  - Mitotic rate >4 per 50 HPF
Occasional findings
- Epithelioid cytoplasm
  - May be clear
- Multinucleated, osteoclast-like cells
- Inflammatory infiltrate
- Palisading / verocay bodies
- Stroma may range from hyalinized to myxoid
Smooth muscle tumor of uncertain malignant potential criteria vary by site
- In deep soft tissue
  - Bland tumors with 1-4 mitotic figures per 50 HPF
  - Tumors with multiple recurrences
- In retroperitoneum, mesentery and omentum
  - Bland tumors with 1-10 mitotic figures per 50 HPF
Neoplasms with the morophologic features of smooth muscle tumors arising in the bowel wall and expressing CD117 are considered GI stromal tumors

Richard L Kempson MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting:: December 1, 2007