Collision carcinoma of the rectum involving neuroendocrine carcinoma and adenocarcinoma: A case report

BACKGROUND Collision carcinoma is a rare histological pattern, and includes two or more different types of tumors coexisting in the same organ as one neoplasm. Different to the combined type, the two adjacent tumors of collision carcinoma are histologically distinct. Collision carcinoma may occur from any origin or organ, including the cecum, liver, cervix, thyroid, stomach, kidney, and esophagus. In the rectum, adenocarcinoma is the most common pathological type, the combined type is rare, and collision tumors are even rarer. To date, only a limited number of collision carcinoma cases originating from the rectum have been reported. Due to the scarcity of rectal collision carcinoma, more cases need to be reported to fully understand the clinico-pathological features and biological behavior of the tumor. CASE SUMMARY Here we report a 40-year-old female who presented with the chief complaints of persistent changes in bowel habits and hematochezia for 10 d. She underwent Miles' operation which revealed a collision carcinoma of the rectum, showing a "side by side" pattern, composed of a high grade neuroendocrine carcinoma, (small cell carcinoma) and moderately differentiated adenocarcinoma, based on its clinico-pathological features and biological behavior. The patient remained disease-free at 12 mo follow-up. We also focused on the related literature and expert opinion. CONCLUSION Collision carcinoma is a rare tumor with ambiguous biological behavior. Greater attention should be paid to its clinico-pathologic diagnosis. Regular and adequate follow-up is essential to help rule out metastasis and assess the prognosis.