Collision tumor of the rectum: a case report of metastatic gastric adenocarcinoma plus primary rectal adenocarcinoma

Abstract

Collision tumors are thought to arise from the accidental meeting and interpenetration of two independent tumors. We report here a highly unusual case of a 61-year old man who had a unique tumor that was composed of a metastatic adenocarcinoma from the stomach to the rectum, which harbored a collision tumor of primary rectal adenocarcinoma. The clonalities of the two histologically distinct lesions of the rectal mass were confirmed by immunohistochemical and molecular analysis. Although histologic examination is the cornerstone in pathology, immunohistochemical and molecular analysis can provide evidence regarding whether tumors originate from the same clone or different clones. To the best of our knowledge, this is the first reported case of such an occurrence.

Figure 1

Histological features of rectal tumor showing the interface of a tubular adeonicarcinoma component of primary rectal cancer (right) and a poorly-differentiated adenocarcinoma component metastasized from the stomach (left) (hematoxylin and eosin; x 40) (A), immunohistochemical analysis showing positive MUC2 in metastatic gastric carcinoma and negative MUC2 in primary rectal carcinoma (x 200) (B), and negative CK7 in metastatic gastric carcinoma and positive CK7 in primary rectal carcinoma (x 200) (C).

Figure 2

Microsatellite instability phenotype analysis showing no microsatellite instability in the primary rectal tumor (A), the metastatic gastric tumor (B) and the primary gastric tumor (C). Blue and green line: Normal tissue; Black and red line: Tumor tissue.