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  • Surgical Pathology Criteria

    Leiomyoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum


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

    Diagnostic Criteria

    • 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

    Original posting:: December 1, 2007

    Supplemental studies


    • Smooth muscle actin and muscle specific actin positive
      • Actin is not specific
    • Desmin or caldesmon positive but may be less extensive than actin
    • CD34 occasionally positive, usually weakly
    • Keratin and EMA may show occasional, usually patchy or dot-like staining
    • S100 has been reported in smooth muscle tumors of various sites
      • The combination of actin negative and S100 positive should strongly suggest nerve sheath tumor

    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

    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

    Separation from myofibroblastic and fibrohistiocytic tumors is based on identification of smooth muscle differentiation:
    • Elongate, eosinophilic cytoplasm
    • Actin is not specific
      • Desmin or caldesmon, if extensive, is more specific in this context



    • Rare when strictly defined
    • Benign but may recur non-aggressively
    • Most deep soft tissue tumors are in extremities of young adults


    • Grading and Staging not applicable

    The surgical pathology report should contain or address the following:

      • Location
      • Type of resection or biopsy
      • Histologic diagnosis
      • Managerial category
        • Ia for leiomyoma (Local recurrence do occur but are not destructive; Never metastasizes)
        • IIc for smooth muscle tumor of uncertain malignant potential (Local recurrence common, Metastasis can rarely occur)
      • Size
      • Results of supplementary studies if performed
      • Relationship to other specimens from the same patient
      • Comment that the rarity of such tumors makes behavior difficult to predict


    Neoplasms with predominant smooth muscle differentiation


    • Kempson RL, Fletcher CDM, Evans HL, Henrickson MR, Sibley RS. Tumors of the Soft Tissues, Atlas of Tumor Pathology, AFIP Third Series, Fascicle 30, 2001
    • Fletcher CDM, Unni KK, Mertens F. Pathology and Genetics of Tumours of Soft Tissue and Bone, World Health Organization Classification of Tumours 2002
    • Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors, 4th edition, 2001
    • Kilpatrick SE, Mentzel T, Fletcher CD. Leiomyoma of deep soft tissue. Clinicopathologic analysis of a series. Am J Surg Pathol. 1994 Jun;18(6):576-82.
    • Hornick JL, Fletcher CD. Criteria for malignancy in nonvisceral smooth muscle tumors. Ann Diagn Pathol. 2003 Feb;7(1):60-6.
    • Weiss SW. Smooth muscle tumors of soft tissue. Adv Anat Pathol. 2002 Nov;9(6):351-9.
    • Billings SD, Folpe AL, Weiss SW. Do leiomyomas of deep soft tissue exist? An analysis of highly differentiated smooth muscle tumors of deep soft tissue supporting two distinct subtypes. Am J Surg Pathol. 2001 Sep;25(9):1134-42.

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