Answer: Leiomyosarcoma and colorectal adenocarcinoma (collision tumor)

Histology: The large bowel resection shows an infiltrating high grade adenocarcinoma involving the full thickness of the colon wall. Adjacent to the adenocarcinoma is a large spindle cell lesion composed of intersecting fascicles of spindled cells with blunt-ended nuclei and abundant mitotic figures. No tumoral necrosis is identified. The spindle cell lesion appears to ulcerate the colonic mucosa.

Discussion: The differential diagnosis of spindle cell lesions involving the gastrointestinal tract includes tumors arising from the tubular gastrointestinal tract as well as spread of tumors from the retroperitoneum and adjacent organs. Leiomyosarcomas in the colon, like those of other sites, are characterized by fascicles of spindled cells that intersect at right angles (perpendicularly) to each other. The individual cells have blunt-ended nuclei, resembling benign smooth muscle cells. The tumors, like other sarcomas, are graded on the basis of nuclear pleomorphism, mitotic rate and the presence or absence of necrosis. By immunohistochemistry, leiomyosarcomas are positive for smooth muscle markers (actin and desmin), and are negative for cytokeratins c-KIT (CD117), CD34 and S100. This patient has a collision tumor of a leiomyosarcoma and a colonic adenocarcinoma (in situ adenoma component seen elsewhere in specimen).

Sarcomatoid carcinomas demonstrate at least focal positivity for cytokeratins or p63 in the spindled cells. Gastrointestinal stromal tumors (GISTS) are typically positive for c-KIT, DOG1 and CD34 and may also show actin positivity. Fibromatoses (desmoid tumors) are bland spindle cell lesions with nuclear beta-catenin labeling. Finally, melanomas of the gastrointestinal tract can have spindle cytology, may display melanin pigment, and may be variably positive for a combination of S100, melanA, and HMB45.

Reference(s):
– Miettinen M, Sarlomo-Rikala M, Sobin LH, Lasota J. Gastrointestinal stromal tumors and leiomyosarcomas in the colon: a clinicopathologic, immunohistochemical, and molecular genetic study of 44 cases. Am J Surg Pathol. 2000 Oct;24(10):1339-52.