Definition/General
Introduction:
Colorectal leiomyoma is a benign smooth muscle tumor arising from the muscularis propria or muscularis mucosae of the colorectum
It is the most common mesenchymal tumor of the colorectum
Represents 2-3% of all colorectal tumors
Shows excellent prognosis with complete surgical excision.
Origin:
Arises from smooth muscle cells of the muscularis propria or muscularis mucosae
Most commonly from the inner circular layer of muscularis propria
Can also originate from the longitudinal muscle layer
Less frequently from muscularis mucosae
Shows clonal proliferation of smooth muscle cells.
Classification:
Classified as intramural leiomyoma (most common)
Intraluminal leiomyoma (polypoid growth)
Extramural leiomyoma (subserosal growth)
WHO classification: Benign smooth muscle tumor
No grading system applied
Size criteria help differentiate from leiomyosarcoma.
Epidemiology:
Peak incidence in 5th-6th decades
Female predominance (2:1 ratio)
More common in rectum than colon
Cecum and sigmoid colon are common colonic sites
Rare in Indian population
Most are asymptomatic and incidental findings.
Clinical Features
Presentation:
Asymptomatic in 70% cases (incidental finding)
Rectal bleeding (most common symptom)
Change in bowel habits
Abdominal pain or discomfort
Palpable mass on digital rectal examination
Tenesmus in rectal location
Bowel obstruction in large tumors.
Symptoms:
Rectal bleeding (30-40% cases)
Constipation or diarrhea
Abdominal cramping
Feeling of incomplete evacuation
Mucus discharge
Weight loss (uncommon)
Anemia due to chronic bleeding.
Risk Factors:
Female gender (hormonal influence)
Age >40 years
Hormonal factors (estrogen, pregnancy)
Genetic predisposition (rare familial cases)
Previous pelvic radiation
Inflammatory bowel disease (controversial association)
No clear environmental factors identified.
Screening:
No specific screening recommended
Colonoscopy for symptomatic patients
CT scan for large masses
MRI for rectal lesions (staging)
Endoscopic ultrasound for submucosal lesions
Biopsy not always diagnostic due to submucosal location.
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Gross Description
Appearance:
Well-circumscribed smooth muscle mass
Gray-white to tan colored cut surface
Firm consistency with rubbery texture
Whorled pattern on cut surface
No necrosis or hemorrhage typically
Smooth external surface.
Characteristics:
Encapsulated or pseudo-encapsulated
Homogeneous cut surface
Fascicular arrangement visible grossly
No calcification typically
Intact overlying mucosa in submucosal lesions
Clear margins from surrounding tissue.
Size Location:
Size ranges from 0.5-10 cm (usually <2 cm)
Rectum most common site (60%)
Sigmoid colon (20%)
Cecum (15%)
Other colonic sites (5%)
Submucosal location most frequent
Intramural in muscularis propria.
Multifocality:
Usually solitary (95% cases)
Multiple leiomyomas rare (<5%)
No associated adenomas typically
Synchronous carcinoma rare
Bilateral distribution not applicable
Metachronous occurrence uncommon.
Microscopic Description
Histological Features:
Fascicles of smooth muscle cells with interlacing pattern
Elongated spindle cells with eosinophilic cytoplasm
Cigar-shaped nuclei with blunt ends
No nuclear atypia
Minimal mitotic activity (<2/10 HPF)
No necrosis.
Cellular Characteristics:
Uniform spindle cells with abundant eosinophilic cytoplasm
Oval to elongated nuclei with smooth chromatin
Inconspicuous nucleoli
No pleomorphism
Longitudinal fibrils in cytoplasm
Intercellular collagen between fascicles.
Architectural Patterns:
Intersecting fascicles at right angles
Whorled pattern in some areas
Hyalinized stroma between fascicles
Intact basement membrane
No infiltrative growth
Well-demarcated borders.
Grading Criteria:
No grading system for benign leiomyoma
Mitotic count important for diagnosis (<2/10 HPF)
Size criteria: <2 cm favors leiomyoma
Nuclear atypia absent
Necrosis absent
Cellularity moderate to low.
Immunohistochemistry
Positive Markers:
Smooth muscle actin (SMA) diffusely positive
Desmin positive
Calponin positive
h-Caldesmon positive
Muscle-specific actin positive
Vimentin positive.
Negative Markers:
c-KIT (CD117) negative
DOG1 negative
S-100 negative
CD34 negative (except blood vessels)
Cytokeratins negative
EMA negative.
Diagnostic Utility:
Differentiates from GIST (c-KIT negative)
Confirms smooth muscle origin
Rules out schwannoma (S-100 negative)
Excludes epithelial tumors (cytokeratin negative)
SMA and desmin are most reliable markers.
Molecular Subtypes:
No molecular subtypes recognized
No specific mutations identified
Estrogen receptor occasionally positive (hormonal influence)
Progesterone receptor may be positive
Ki-67 low proliferation index (<5%).
Molecular/Genetic
Genetic Mutations:
No specific mutations identified consistently
Chromosomal abnormalities rare
Different from uterine leiomyomas genetically
No MED12 mutations typically
Rare cases show chromosomal rearrangements
Benign karyotype in most cases.
Molecular Markers:
Low Ki-67 proliferation index (<5%)
p53 typically wild-type
No MDM2 amplification
Hormone receptors variably expressed
No specific molecular markers for diagnosis
Cytogenetic studies usually normal.
Prognostic Significance:
Excellent prognosis with complete excision
No malignant potential
No metastatic risk
Rare recurrence after complete excision
Size >2 cm requires careful evaluation for leiomyosarcoma
Long-term follow-up not required.
Therapeutic Targets:
Complete surgical excision curative
No medical therapy required
Hormonal therapy not indicated
No targeted therapy available or needed
Endoscopic resection for small polypoid lesions
Segmental resection for large tumors.
Differential Diagnosis
Similar Entities:
Gastrointestinal stromal tumor (GIST) - c-KIT positive, spindle or epithelioid cells
Leiomyosarcoma - high mitotic rate, nuclear atypia, necrosis
Schwannoma - S-100 positive, Antoni A and B areas
Fibroma - collagenous stroma, fibroblasts
Inflammatory myofibroblastic tumor - inflammatory cells, ALK positive.
Distinguishing Features:
GIST: c-KIT/DOG1 positive, different morphology
Leiomyosarcoma: >2 mitoses/10 HPF, atypia, size >2 cm
Schwannoma: S-100 positive, Verocay bodies
Fibroma: collagen-rich, fibroblastic cells
IMT: myofibroblasts, inflammatory infiltrate, ALK expression.
Diagnostic Challenges:
Small biopsy samples may be non-diagnostic
Submucosal location limits tissue sampling
Distinction from leiomyosarcoma in large tumors
Crush artifact in endoscopic biopsies
Immunohistochemistry essential for accurate diagnosis.
Rare Variants:
Epithelioid leiomyoma - epithelioid morphology, rare
Plexiform leiomyoma - plexiform growth pattern
Symplastic leiomyoma - bizarre nuclei, low mitoses
Cellular leiomyoma - increased cellularity
Myxoid leiomyoma - myxoid stroma.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
Colorectal resection specimen measuring [size] cm, containing a [size] cm submucosal/intramural mass
Diagnosis
Colorectal Leiomyoma
Classification
Benign smooth muscle tumor
Histological Features
Shows fascicles of bland spindle cells with eosinophilic cytoplasm and cigar-shaped nuclei. Minimal mitotic activity (<2/10 HPF). No necrosis or significant atypia.
Size and Extent
Size: [X] cm, confined to [muscularis propria/submucosa]
Margins
Margins are uninvolved with complete excision achieved
Immunohistochemistry
SMA: positive, Desmin: positive, c-KIT: negative, S-100: negative
Special Studies
Immunohistochemistry confirms smooth muscle differentiation
No molecular testing required
Mitotic count: <2/10 HPF
Final Diagnosis
Colorectal leiomyoma, completely excised