Definition
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
Original posting:: December 1, 2007
Supplemental studies
Immunohistology
Smooth muscle actin and muscle specific actin positive
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
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
Clinical
Rare when strictly defined
Benign but may recur non-aggressively
Most deep soft tissue tumors are in extremities of young adults
Report
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
Lists
Neoplasms with predominant smooth muscle differentiation
Bibliography
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.
