Leiomyoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum

Definition

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

Smooth muscle differentiation
- Elongate eosinophilic cytoplasm
Must fulfill all of the following
- No cytologic pleomorphism and no more than minor focal atypia
- No tumor cell necrosis
- Mitotic rate <1 per 50 HPF
Must not originate in uterus
Must not originate in muscularis propria of GI tract and express CD117
- Such tumors are considered GI stromal tumors
Occasional findings
- Involvement of vessel walls
- Calcification
- Psammoma bodies
- Ossification
- Palisading / verocay bodies
- Epitihelioid morphology
- Fatty differentiation
- Hyalinization
Smooth muscle tumor of uncertain malignant potential criteria vary by site
- In retroperitoneum, mesentery and omentum must fulfill all of above criteria except
  - 1-10 mitotic figures per 50 HPF
- In deep soft tissue
  - Bland tumors with 1-4 mitotic figures per 50 HPF, OR
  - Tumors with multiple recurrences

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