Collision tumors are thought to arise from the accidental meeting of two independent tumors. Adenocarcinoma is the most common malignant rectal tumor, while neuroendocrine tumor (NET) is relatively rare. Due to the endoscopy and reporting, the overall incidence of NETs was increasing recently but still less than 1 per 100,000. This means that a combination of an adenocarcinoma and NET is a very rare finding and an actual collision of these tumors even more so. We report here a highly unusual case of a 64-year-old woman who had collision tumors composed of a primary rectal adenocarcinoma and NET showing a "side by side" pattern. Resection margins are free of both the tumors. The postoperative course was uneventful. The patient underwent a protocol CT scan at 3 months after surgery, which did not show any recurrence. Both the malignant adenocarcinoma and the NET would make a great influence in the rest lifetime and a follow up will be continued, although the CT did not show any recurrence until now. To the best of our knowledge, this is the first reported case of such an occurrence.
Keywords: Adenocarcinoma; Collision tumor; Neuroendocrine tumor; Rectum.