Answer: Small cell carcinoma

Histology: The tumor cells show a nested pattern in a desmoplastic stroma. The cells have a somewhat fusiform appearance and demonstrate nuclear molding. Chromatin is hyperchromatic and diffuse throughout, without prominent nucleoli. Necrosis is extensive, and the mitotic rate is greater than 10/10hpf. These are the typical features of small cell carcinoma. By immunohistochemistry, the tumor cells show punctate cytokeratin immunoreactivity, and label for neuroendocrine markers, chromogranin and synaptophysin. CDX-2 is diffusely positive, while WT-1, ER, and PR are negative.

Discussion: The presence of nuclear molding morphologically argues against all the other differential diagnostic possibilities in this case, and strongly suggests a neuroendocrine neoplasm. Malignant lymphoma would show greater discohesion, and thicker chromatin at the nuclear membrane. The punctate immunoreactivity for cytokeratin and labeling for neuroendocrine markers also argues against all the lesions in the differential diagnosis. The immunoreactivity for CDX-2 raises the possibility that this small cell carcinoma is primary to the colon in this site. Hence, the lesion is distinct from the patient’s concurrent endometroid adenocarcinoma. Interestingly, colorectal small cell carcinomas are often right sided, but have intact mismatch repair proteins, suggesting that they develop secondary to chromosomal instability rather than microsatelite instability.

Reference(s):
– Am J Surg Pathol 2006;30:1401-1404