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Colon tumor

Mesenchymal tumors

Gastrointestinal stromal tumors (GIST) of colon

Reviewers: Charanjeet Singh, M.D. (see Reviewers page)
Revised: 29 September 2014, last major update April 2012
Copyright: (c) 2003-2014, PathologyOutlines.com, Inc.


● Tumors that differentiate along lines of interstitial cells of Cajal, the gut’s pacemaker cells (Mod Pathol 2003;16:366)
● 5-10% of GISTs occur in colon, often in rectum
● Median age 67 years, usually > 50 years (Am J Surg Pathol 2000;24:1339)
● 30-50% are malignant with 5 year survival of 50%

NIH criteria for assessing risk (Int J Surg Pathol 2002;10:81, Hum Pathol 2002;33:459):
High risk: > 1 cm and > 5 MF/50 HPF; also infiltrative border within muscularis propria; most colorectal tumors are high risk; 2/3 develop metastases (Dis Colon Rectum 2006;49:609)
Intermediate risk: 1-5 MF/50 HPF and > 1 cm
Low risk: < 1 cm (often are serosal)

Case reports

● Prostatic stromal sarcoma with rectal GIST (Urology 2006;68:672.e11)
● Man in mid-40’s (Yonago Acta medica 2002;45:117)


● Gleevec (imatinib, STI571), which inhibits tyrosine kinases including CD117 / c-kit and Abl protein in CML

Gross description

● Often large, bulky and intramural masses
● Fish-flesh or tan-brown appearance, hemorrhage, necrosis and cystic softening

Gross images

Tumors from left-hepatic flexure; right-rectum

Leiomyosarcoma (top) vs. GIST (bottom)

Micro description

● Usually transmural, usually plump spindle cells with eosinophilic cytoplasm within variably hyalinized or edematous stroma
● Skenoid fibers (extracellular collagen globules) common
● Muscle infiltration is common but not predictive of behavior
● May have epithelioid morphology
● May resemble leiomyoma or schwannoma
● Incidental GIST tumors in resections are uncommon (Am J Surg Pathol 2008;32:867)
● Rarely has osteoclast-like giant cells (Arch Pathol Lab Med 2004;128:440)
● 1-2 mitoses / 10 HPF

Micro images

Epithelioid cells and osteoclast-like giant cells

Focal calcification

Site unspecified:
Left to right: nuclear palisading; angiomatoid pattern; myxoid stroma

Left to right: perivascular hyalinization; skenoid fibers

Cigar shaped cells (left) and spindled cells (right)

Prominent cytoplasmic vacuoles; pattern of interlacing fascicles

Various stains

Low risk tumor has bland tumor cells and no mitotic activity

Intermediate risk tumor is cellular with higher N/C ratio

High risk tumor is cellular with high N/C ratio and mitotic activity

Positive stains

● CD117, CD34 and vimentin
● Alpha smooth muscle actin (30-40%), S100 (5%)
● Variable keratin (weak)

Negative stains

● Desmin

Molecular description

● 70% have c-kit mutations (most commonly in codon 11), 15-20% have mutations in PDGFRA (most often associated with epithelioid morphology, gastric location and myxoid background), 10% are c-kit wild type and are associated with syndromes such as Neurofibromatosis - type I and Carney’s triad
NOTE: If CD117 is negative, and morphology is suggestive of GIST, mutation analysis is recommended

Electron microscopy description

Predominantly smooth muscle differentiation: long interdigitating cytoplasmic processes, intercellular junctions and dense core granules
Predominant neural differentiation: neuron-like cells with axonal cytoplasmic processes; synapse like structures; dense core neurosecretory granules

Electron microscopy images

Bulbous axon-like processes with empty vesicles and dense core granule

Cellular processes and skenoid fibers

Differential diagnosis

Leiomyosarcoma: marked atypia, necrosis, positive for smooth muscle actin or desmin, CD117-, CD34- and no c-kit mutations
Uterine type leiomyomas: attached to colon without wall involvement, resemble benign leiomyoma, actin+, desmin+ and CD117- (Int J Colorectal Dis 2006;21:84, Int J Gynecol Cancer 2006;16:927)
Fibromatosis: may be CD117+ depending on antibody used (Am J Surg Pathol 2000;24:947, Am J Surg Pathol 2001;25:549)
Inflammatory myofibroblastic tumor: positive for ALK1 but c-kit negative

Additional references

Radiographics 2003;23:283

End of Colon tumor > Mesenchymal tumors > Gastrointestinal stromal tumors (GIST) of colon

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