A rectal neuroendocrine tumor (NET) is a rare cancer originating from specialized neuroendocrine cells in the rectum. These tumors differ from more common rectal cancers. Understanding their characteristics and management is important.
Understanding Rectal Neuroendocrine Tumors
Neuroendocrine cells share characteristics of both nerve cells and hormone-producing endocrine cells. They are present throughout the body, including the digestive system, where most neuroendocrine tumors originate. In the rectum, these cells can undergo abnormal changes in their DNA, leading to uncontrolled growth and tumor formation.
Rectal neuroendocrine tumors are classified into two main types: neuroendocrine tumors (NETs) and neuroendocrine carcinomas (NECs). NETs are well-differentiated, resembling normal neuroendocrine cells, and exhibit slow to moderate growth. NECs are poorly differentiated, appearing abnormal under a microscope, and tend to grow rapidly.
Classification also involves grading, which indicates how quickly tumor cells are dividing and growing. This is determined by assessing the mitotic count and the Ki-67 index, a measure of cell proliferation. NETs are graded as G1 (slow-growing), G2 (moderately growing), or G3 (rapidly growing), while NECs are inherently considered high-grade (G3).
Rectal NETs are rare, making up less than 2% of all colorectal cancers. While many are slow-growing and small, they can spread to nearby lymph nodes or distant organs. The risk of metastasis increases with tumor size, depth of invasion, and higher grades.
Detecting Rectal Neuroendocrine Tumors
Rectal neuroendocrine tumors often cause no early symptoms, leading to incidental discovery during routine medical examinations. When symptoms appear, they can be vague, such as rectal bleeding, changes in bowel habits, abdominal or rectal pain, a feeling of incomplete bowel emptying, or unexplained weight loss.
The diagnostic process begins with an initial examination, followed by a colonoscopy or sigmoidoscopy. These procedures allow a doctor to visually inspect the rectum and colon and obtain biopsies from any suspicious areas. Rectal NETs often appear as small, yellowish polyps in the submucosal layer during an endoscopy.
Pathology plays a key role in confirming the diagnosis and characterizing the tumor. A pathologist examines biopsy samples under a microscope to determine the tumor’s type (NET or NEC) and grade (G1, G2, or G3). Immunohistochemical tests using markers like chromogranin A and synaptophysin confirm the neuroendocrine origin of the cells.
Once a rectal neuroendocrine tumor is diagnosed, staging procedures determine the extent of spread. This involves imaging techniques such as CT scans and MRI of the abdomen and pelvis to check for regional lymph node involvement or distant metastases. Specialized scans like somatostatin receptor scintigraphy, such as Ga-68 DOTATATE PET/CT, are also used because most NET cells express somatostatin receptors, allowing these scans to locate tumors and their spread.
Treatment Options
Treatment for rectal neuroendocrine tumors depends on factors like tumor size, grade, depth of invasion, and whether it has spread. A multidisciplinary team of specialists, including gastroenterologists, surgeons, oncologists, and pathologists, collaborates to determine the most suitable approach for each patient.
Surgical resection is the primary treatment for localized tumors. For very small, low-risk tumors (less than 10 mm and confined to the submucosa), endoscopic removal techniques like endoscopic mucosal resection (EMR) or endoscopic submucosal dissection (ESD) are sufficient. These methods involve removing the tumor through a flexible tube inserted into the rectum, aiming for complete local excision.
For larger tumors, or those that have invaded deeper into the rectal wall or spread to nearby lymph nodes, more extensive surgical approaches are considered. These can include local excision, transanal endoscopic microsurgery (TEM), or formal resections like low anterior resection with total mesorectal excision. The goal of these surgeries is to remove the tumor and any involved lymph nodes to achieve clear margins.
For advanced or metastatic disease, systemic therapies control tumor growth and manage symptoms. Somatostatin analogs (SSAs), such as octreotide and lanreotide, are a first-line treatment for well-differentiated tumors, as they help control symptoms and inhibit tumor growth. Targeted therapies, like everolimus or sunitinib, are also employed.
Chemotherapy is reserved for more aggressive, poorly differentiated neuroendocrine carcinomas (NECs), or for tumors that do not respond to other treatments. Peptide Receptor Radionuclide Therapy (PRRT) uses radioactive medicine to target and destroy cancer cells with somatostatin receptors, and is an effective option for select patients with advanced well-differentiated tumors. In specific cases of small, low-grade tumors, active surveillance is considered as an initial approach.
Managing and Monitoring
After treatment for a rectal neuroendocrine tumor, regular follow-up is necessary to monitor for recurrence and assess the long-term outlook. The frequency and type of follow-up appointments, including physical exams, blood tests, and imaging, are tailored to the individual, considering factors like the original tumor’s size, grade, and whether it had spread.
The long-term outlook for individuals with rectal neuroendocrine tumors varies considerably. It is influenced by the tumor’s characteristics at diagnosis, including its grade, size, and the extent of any spread. Tumors that are small, low-grade, and confined to the rectum have a favorable prognosis, with high survival rates.
Even after successful treatment, recurrence is possible, either locally in the rectum or as distant metastases. Some low-grade tumors, even those under 10 mm, have been reported to recur after many years, highlighting the importance of extended surveillance. Regular follow-up appointments help in the early detection of recurrence, allowing for timely intervention.
Supportive care also plays a role in managing the long-term journey for patients. This can involve addressing physical symptoms, emotional well-being, and ensuring access to information about their condition. Maintaining strength and overall well-being, including good nutrition, is encouraged to support recovery and quality of life.