Colorectal leiomyoma is a rare benign mesenchymal tumor originating from smooth muscle cells (muscularis mucosae or muscularis propria) of the colon and rectal wall. Approximately 3% of gastrointestinal leiomyomas are found in the colorectum, most commonly in the rectum. Usually small (<3 cm), well-circumscribed, asymptomatic, and incidentally detected. Distinguished from gastrointestinal stromal tumor (GIST) immunohistochemically: leiomyoma is CD117 (c-kit) and DOG1 negative, desmin and SMA (smooth muscle actin) positive. Appears as well-defined, homogeneous, soft tissue density intramural mass on CT and MRI. Does not show necrosis, hemorrhage, or cystic degeneration (unlike GIST). Risk of malignant transformation is extremely low; surgical excision is usually curative.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Rare
Colorectal leiomyoma arises from benign proliferation of smooth muscle cells in the bowel wall. Two types are distinguished based on the layer of origin: those from muscularis mucosae typically present as small (<1 cm) intraluminal polypoid lesions; those from muscularis propria can be larger and show exophytic or intramural growth patterns. Histologically composed of spindle-shaped smooth muscle cells arranged in regular, parallel fascicles with extremely low mitotic activity (<1/50 HPF). The regular cellular arrangement and low cellular atypia result in homogeneous tumor structure, reflected in imaging as homogeneous enhancement and absence of necrosis/hemorrhage. The homogeneous soft tissue density mass appearance on CT is due to the homogeneous tissue composition of smooth muscle cells and collagen matrix. The intermediate-to-low T2 signal on MRI is due to the short T2 relaxation time of smooth muscle fibers.
Well-defined, homogeneous, low-to-moderate enhancing intramural mass — absence of necrosis, hemorrhage, or cystic change is the distinguishing feature of leiomyoma and is critical in differentiation from GIST.
Well-defined, oval or round, homogeneous soft tissue density (30-50 HU) intramural mass in the colon or rectal wall. Nearly isodense to surrounding muscle on non-contrast CT; shows low-to-moderate homogeneous enhancement on contrast-enhanced phases. Contains no necrosis, hemorrhage, cystic degeneration, or calcification. Lesion margins are smooth and sharp without evidence of invasion into surrounding tissues.
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A well-defined, homogeneously enhancing intramural mass measuring approximately ... mm is seen in the rectal/colonic wall without necrosis or cystic degeneration; leiomyoma should be primarily considered.
On T2-weighted images, leiomyoma shows intermediate-to-low signal intensity, reflecting the short T2 relaxation time of smooth muscle tissue. Signal is homogeneous without foci of T2 hyperintensity from internal hemorrhage or necrosis. Lesion margins are well-defined with clean separation from surrounding tissue.
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The intramural mass shows homogeneous intermediate-to-low signal on MRI T2-weighted images, consistent with a benign smooth muscle origin lesion.
On T1-weighted images, leiomyoma shows homogeneous signal isointense to surrounding muscle tissue. Low-to-moderate homogeneous enhancement is observed on contrast-enhanced T1 sequences. No foci of necrosis or hemorrhage are present.
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The mass shows homogeneous signal isointense to surrounding muscle on T1-weighted images with low-to-moderate homogeneous enhancement on contrast-enhanced series.
Appears as a well-defined, homogeneous hypoechoic mass in the submucosal location on endorectal or transabdominal ultrasound. Mucosal layer is intact and stretches smoothly over the mass. Doppler examination shows minimal internal flow with low vascularity. Lesion size is usually <3 cm.
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A well-defined, homogeneously hypoechoic mass is seen in the submucosal location of the rectal/colonic wall on endorectal/transabdominal US with intact mucosal layer; consistent with leiomyoma.
Leiomyoma does not show significant diffusion restriction on DWI; ADC values are in the intermediate-to-high range. This finding reflects low cellularity and good differentiation. Distinguished from highly cellular tumors such as GIST or leiomyosarcoma by this feature.
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The mass does not show significant diffusion restriction on DWI with intermediate-to-high ADC values; consistent with a low-cellularity benign lesion.
On non-contrast CT, leiomyoma appears as an isodense (30-50 HU) well-defined intramural mass relative to surrounding muscle. Calcification is very rare. No fat content (distinguishing from lipoma). Mass usually grows towards bowel lumen (intraluminal) or stays within the wall (intramural); rarely exophytic.
Report Sentence
A well-defined intramural mass isodense to surrounding muscle is seen in the colonic/rectal wall on non-contrast CT.
Criteria
Originating from muscularis mucosae, usually small (<1 cm), intraluminal polypoid lesion. Appears as submucosal mass endoscopically. May be pedunculated or sessile.
Distinct Features
Small size, intraluminal growth, treatable with endoscopic polypectomy. Usually too small to detect on CT.
Criteria
Originating from muscularis propria, can be larger (1-5 cm), showing intramural or exophytic growth. Detectable size on CT and MRI. May require surgical excision.
Distinct Features
Larger size, intramural/exophytic growth pattern, endorectal US can demonstrate origin from muscularis propria layer.
Criteria
Leiomyoma variant with vascular component. Very rare in colorectum. Smooth muscle cells organized around thickened vessel walls. May show more prominent enhancement.
Distinct Features
More intense enhancement (due to vascular component), increased vascularity on Doppler US, extremely rare in colorectum.
Distinguishing Feature
GIST typically shows heterogeneous enhancement, necrosis, hemorrhage, or cystic degeneration with predominantly exophytic growth. Leiomyoma is homogeneous, non-necrotic, and smaller. Definitive distinction by immunohistochemistry (GIST: CD117+/DOG1+; Leiomyoma: desmin+/SMA+).
Distinguishing Feature
Carcinoid tumor shows prominent hypervascular enhancement in arterial phase (leiomyoma shows low-moderate enhancement). Carcinoid typically appears as small submucosal nodule in rectum and is positive on somatostatin receptor scintigraphy.
Distinguishing Feature
Lipoma is pathognomonically recognized at homogeneous fat density (-50 to -100 HU) on CT; leiomyoma is at soft tissue density (30-50 HU) without fat content.
Distinguishing Feature
Adenocarcinoma presents as irregularly marginated, heterogeneously enhancing mass with invasion into surrounding tissues. Leiomyoma is well-defined, homogeneous, and non-invasive. Adenocarcinoma usually shows annular thickening pattern.
Urgency
routineManagement
surveillanceBiopsy
NeededFollow-up
12-monthColorectal leiomyoma is a benign tumor with extremely low risk of malignant transformation. Endoscopic follow-up may be sufficient for small (<2 cm), asymptomatic lesions. Endoscopic or surgical excision is curative for symptomatic or large lesions. Biopsy (immunohistochemistry) is generally required to exclude GIST as imaging alone cannot provide definitive distinction. Recurrence rate is very low.
Benign tumor with very low malignant potential. Small lesions can be followed. Symptomatic or growing lesions are removed endoscopically or surgically. Differentiation from GIST is by immunohistochemistry (c-KIT negative).