Adjacent to this area, the serosal and mesenteric fat contained several coalescing, firm, cream-colored to gray nodules up to 1 cm in diameter. Spontaneous carcinomas of the small and large intestine are reported to be rare in rats. References disagree regarding the most common site of origin, and the most common site may differ in different strains and stocks. In this Long-Evans rat, there was transmural invasion of the neoplasm through the colonic wall with subserosal abscess formation, but metastases were not observed. Case 5-1 .
Shows subserosal, colonic adenocarcinoma with cyst and abscess formation.
There was considerable variation in the slides distributed for this case. Several slides viewed in conference demonstrated minimal or no invasion of neoplastic cells into the tunica muscularis, and no neoplastic cells were present in the subserosal abscess. This led some participants to a diagnosis of atypical hyperplasia or carcinoma in situ. However, in other slides, invasive growth indicative of malignancy is evident. In nonhuman primates, primary intestinal adenocarcinomas are relatively rare, with the exception of the cotton-top tamarin, in which colonic adenocarcinomas are often coincident with chronic proliferative colitis.