Clinical history: 70-year-old male admitted for acute pancreatitis. An incidental mass in the terminal ileum was found on imaging.
What is the diagnosis?
Neuroendocrine tumour arising from the terminal ileum
Epidemiology: Carcinoids represent about one third of neoplasms of the small intestine, with a mean age of diagnosis around 60 years. While duodenal carcinoids are slightly more common in men, jejunoileal carcinoids affect males and females equally. Carcinoid syndrome (diarrhea, flushing, heart failure, etc.) occurs in 5-7% of cases of jejunoileal carcinoids, and almost never occurs with duodenal carcinoids. Patients with jejunoilieal carcinoids present with abdominal pain, small bowel obstruction, or metastases.
Pathogenesis: Small intestinal carcinoids derive from either the foregut (duodenum) or the midgut (jejunum and ileum), with roughly equal incidence in the duodenum and ileum. Most duodenal carcinoids are gastrinomas, the second most common type being somatostatinomas. One third of gastrinomas are functional, causing Zollinger-Ellison syndrome. In the distal small intestine, carcinoids predominantly occur in the ileum, and most are enterochromaffin cell carcinoids which produce serotonin.
Gross: In the duodenum, carcinoids are usually small polypoid submucosal lesions that measure less than 2 cm, sometimes with ulceration of the overlying mucosa. In the jejunum and ileum, carcinoids occur in the deep mucosa or submucosa, are more often multicentric and tend to be larger. Here, they are often locally advanced with involvement of the deep bowel wall and mesentery, leading to bowel obstruction and sometimes intestinal ischemia.
Histology: Carcinoid tumours are composed of small monotonous polygonal cells, with moderate cytoplasm and round nuclei with salt and pepper chromatin. Nuclear pleomorphism and increased mitotic activity are uncommon. The cell of origin of carcinoids cannot be determined based on morphology (immunohistochemistry is required). Carcinoids most commonly grow in an insular or nested, trabecular or acinar patterns, and multiple patterns may be present within one tumour. Malignant and benign carcinoids are distinguished clinically by the presence of metastases rather than by histology.
Immunohistochemistry: Chromogranin and synaptophysin are typically positive. Although immunohistochemistry can identify specific cell types, this information is usually not clinically significant, as it does not correlate with the functional status of the tumour.
Differential diagnosis: The primary differential diagnoses include primary or metastatic adenocarcinomas and other metastatic lesions, such as lobular breast carcinoma and melanoma.
Natural history: While duodenal carcinoids are generally indolent (4% mortality), jejunoileal carcinoids have a worse prognosis (21% mortality). In duodenal carcinoids, poor prognostic factors include invasion beyond the submucosa and lymph node or distant metastases. In jejunoileal carcinoids, poor prognostic factors include distant metastases, multiple tumours, mitotic rate, and invasion beyond the submucosa.