Large intestine carcinoid, also known as argentaffinoma, originates from the argentaffin cells (Kulchitsky cells) of the intestinal mucosal glands, which exhibit chromaffin properties. Since it often arises from the lower part of the mucosal layer and early extends into the submucosa, it was once classified as a submucosal tumor. This disease is a rare, low-grade malignant tumor that mostly grows locally and invasively with few metastases. Carcinoids commonly occur in the appendix, small intestine, rectum, stomach, duodenum, and small intestine diverticula.

bubble_chart Clinical Manifestations

1. Appendiceal carcinoid accounts for more than 80% of appendiceal tumors and is the most common site of gastrointestinal carcinoids. Most carcinoids are located at the tip of the appendix, with small tumor size and rarely cause symptoms; when larger tumors cause mechanical obstruction, they may present with clinical manifestations of acute appendicitis.

2. Rectal carcinoid can occur in any part of the rectum, more frequently on the anterior wall than the posterior wall. The tumor diameter ranges from a few millimeters to several centimeters. It is generally believed that carcinoids larger than 2 cm are often accompanied by metastasis, with regional lymph nodes and liver being the most common metastatic sites. Those smaller than 1 cm usually show no metastasis. Since carcinoids often grow into the submucosal layer, the surface mucosa may remain intact.

3. Colonic carcinoid mostly occurs in the cecum and ascending colon. The tumor size is larger than in other locations, and metastasis is more frequent. This may be related to the large lumen of the colon, where early-stage symptoms are often absent, making early detection difficult.

1. Appendiceal carcinoid Preoperative diagnosis of appendiceal carcinoid is difficult, and most cases are incidentally confirmed during surgery for appendicitis. However, in rare cases where the carcinoid infiltrates the root of the appendix and the cecal mucosa, colonoscopy may reveal changes around the appendiceal orifice or a single small elevated lesion. Deep biopsy can provide pathological confirmation.

2. Rectal carcinoid Digital rectal examination may reveal a submucosal mass that is firm, with clear and smooth margins. Colonoscopy may show a broad-based, elevated, firm, and smooth mass in the rectum, resembling a hyperplastic polyp. Definitive diagnosis relies on histopathological examination, so the diagnostic rate is directly related to biopsy technique. For smaller tumors, deep biopsy should be attempted, and if necessary, electrocoagulation excision should be performed for full-tumor biopsy.

3. Colonic carcinoid Endoscopically, carcinoids appear as yellowish or grayish-white, hemispherical, sessile polypoid lesions with smooth surfaces, often featuring a central dimple. Larger carcinoids may develop surface ulcers, making them difficult to distinguish from colorectal cancer.

The definitive diagnosis of carcinoid depends on proper tissue sampling and pathological biopsy. Histologically, characteristic features include small, uniform, round tumor cells with small, regular nuclei, arranged in nests or pseudorosettes. Notably, carcinoids are often associated with synchronous or metachronous tumors, so endoscopic examination should not stop at detecting a single lesion. Careful preoperative, intraoperative, and postoperative follow-up examinations are necessary for timely diagnosis and treatment.

When a colon carcinoid forms a large surface ulcer, it is difficult to distinguish from intestinal cancer at this time.