Neuroendocrine tumors (NETs) are abnormal growths originating from neuroendocrine cells, which share characteristics with both nerve and hormone-producing endocrine cells. A Rectal Neuroendocrine Tumor (RNET) is a specific type of NET that develops in the rectum, the final section of the large intestine. Although RNETs are relatively uncommon, their reported incidence is rising due to the widespread use of routine endoscopic screening procedures. RNETs are often slow-growing and frequently discovered when they are small and localized.
The Nature and Classification of Rectal Neuroendocrine Tumors
The behavior and potential for spread of an RNET are determined by its size and cellular aggressiveness, or grade. Tumor size is a prognostic factor: growths smaller than one centimeter typically have a very low risk of metastasizing. RNETs larger than two centimeters carry a higher risk of involving nearby lymph nodes or spreading to distant sites. Tumors between one and two centimeters require assessment using additional microscopic factors.
Cellular aggression is determined by a grading system (G1, G2, or G3) based on the mitotic rate (cell division speed) and the Ki-67 proliferation index. Grade 1 (G1) is the most common type of RNET, characterized by a very low mitotic count and a Ki-67 index below three percent. These tumors are typically indolent and slow-growing.
Intermediate-grade tumors, or Grade 2 (G2), show a higher rate of cell division, with a Ki-67 index ranging from three to 20 percent. These tumors have increased potential for aggressive behavior and metastasis. High-grade tumors, or Grade 3 (G3), are classified as Neuroendocrine Carcinoma (NEC). G3 NECs are poorly differentiated, exhibit a Ki-67 index above 20 percent, and are highly aggressive, requiring intensive treatment.
Recognizing Symptoms and Diagnostic Testing
RNETs are frequently discovered incidentally during routine colonoscopies because most, especially when small, do not cause noticeable symptoms. When symptoms occur, they are often non-specific and may include changes in bowel habits, mild rectal pain, or blood in the stool.
Diagnosis begins with an endoscopic procedure, such as a colonoscopy, where the tumor is visually identified and biopsied. The biopsy is examined microscopically to confirm the neuroendocrine origin of the cells and determine the tumor’s grade (G1, G2, or G3) and size. Once confirmed, further testing determines the tumor’s stage, checking for spread beyond the rectum.
Staging involves various imaging techniques. Computed tomography (CT) scans and magnetic resonance imaging (MRI) visualize the local extent of the tumor and check for spread to nearby lymph nodes or organs. Specialized functional imaging, such as a Gallium-68 DOTATATE PET scan, uses a radioactive tracer that binds to somatostatin receptors highly expressed on NET cells, identifying small areas of distant disease. Blood tests measure levels of the protein Chromogranin A (CgA), which can be used to monitor disease burden and track response to treatment over time.
Treatment Strategies Based on Tumor Stage
Treatment for an RNET is customized based on its size, grade, and spread. For the most common presentation—a small, low-grade RNET (typically less than one centimeter and G1)—local excision is often the only treatment required. This procedure, performed endoscopically or transanally, removes the tumor entirely without major abdominal surgery. Techniques like Endoscopic Mucosal Resection (EMR) or Endoscopic Submucosal Dissection (ESD) achieve complete removal for these early-stage tumors.
If the RNET is larger (over two centimeters), or if the biopsy shows high-grade changes (G2 or G3) or deep invasion, a more radical surgical approach is necessary. This involves a surgical resection, such as a low anterior resection, to remove the affected section of the rectum and surrounding lymph nodes. This ensures all potential areas of spread are removed, which is important for tumors with a higher risk of lymph node involvement.
When the disease has spread to distant sites (metastatic disease), systemic therapies become the primary focus. For well-differentiated, lower-grade tumors expressing somatostatin receptors, treatments include somatostatin analogs to stabilize the tumor and manage symptoms. Peptide Receptor Radionuclide Therapy (PRRT) is an advanced option that delivers targeted radiation directly via these receptors. High-grade Neuroendocrine Carcinomas (G3 NECs) are treated aggressively with chemotherapy regimens similar to those used for small-cell lung cancer.
Life After Treatment and Surveillance
The prognosis for individuals diagnosed with an RNET is favorable, especially when the tumor is small, localized, and completely removed. For patients without lymph node involvement, the long-term survival rate is high. After treatment, a structured surveillance plan monitors for potential recurrence or new tumors.
The follow-up schedule involves imaging, blood tests, and repeat endoscopic examinations, with frequency depending on the tumor’s initial size and grade. Periodic blood tests check Chromogranin A levels to monitor for biochemical signs of residual or progressing disease. Imaging studies, such as CT or MRI scans, are performed at regular intervals to screen for distant spread, particularly to the liver. Repeat endoscopies visually inspect the site of the original tumor and the rest of the colon.
This comprehensive surveillance is maintained over several years to ensure issues are detected early. Long-term survivorship management also addresses lingering side effects from surgery or systemic therapy, ensuring a high quality of life.