Neuroendocrine Tumors (NETs): Carcinoid, PNETs, and PRRT Treatment — Overview, Diagnosis & Treatment Options | MyMedicPlus

Neuroendocrine tumors (NETs) are a heterogeneous group of malignancies arising from neuroendocrine cells throughout the body. Common primary sites include the small intestine (ileum, carcinoid), pancreas (PNETs: insulinoma, gastrinoma, glucagonoma, VIPoma, non-functioning), lung (typical and atypical carcinoid), rectum, and appendix. Annual incidence has increased to approximately 8 cases per 100,000. WHO 2022 grades NETs G1-G3 by Ki-67 and mitotic count. Well-differentiated NETs (G1-G2) have indolent to moderately aggressive behavior; NEC (neuroendocrine carcinoma) is highly aggressive.

Most NETs are sporadic without identifiable cause. Hereditary syndromes account for approximately 5-10% of NETs: MEN1 (menin mutation, pancreatic NETs plus pituitary plus parathyroid tumors), MEN2A/MEN2B (RET mutation, medullary thyroid carcinoma), VHL disease (pancreatic NETs plus hemangioblastoma plus clear cell RCC), tuberous sclerosis complex (TSC1/TSC2 mutations), and NF1 (duodenal somatostatinomas). Sporadic pancreatic NETs show mutations in MEN1, DAXX/ATRX, mTOR pathway genes (PTEN, TSC2, PIK3CA), and chromosomal alterations. Small intestinal NETs have characteristic CDKN1B loss and chromosome 18 monosomy.

Functional NETs produce symptoms from hormone hypersecretion. Insulinoma (pancreas): hypoglycemia fasting symptoms (sweating, palpitations, confusion, seizures), Whipple's triad. Gastrinoma (Zollinger-Ellison syndrome): severe peptic ulcers, diarrhea, esophagitis from excess gastrin. Carcinoid syndrome (ileal/midgut NETs with liver metastases): episodic flushing, watery diarrhea, bronchospasm, carcinoid heart disease. Glucagonoma: migratory necrolytic erythema, diabetes, weight loss. VIPoma: profuse watery diarrhea (WDHA syndrome), hypokalemia, achlorhydria. Non-functioning NETs often present late with mass effects: abdominal pain, weight loss, or incidental imaging finding.

Ga-68 DOTATATE PET-CT (Netspot) is the definitive functional imaging modality for somatostatin receptor-positive NETs, with sensitivity exceeding 90%, superseding OctreoScan. It detects sites not visible on CT/MRI and guides PRRT eligibility. CT and MRI define structural disease. Serum chromogranin A (CgA) and urinary 5-HIAA are functional tumor markers. Biopsy with Ki-67 immunohistochemistry and synaptophysin/chromogranin A IHC confirms diagnosis and grade. AJCC 8th edition provides site-specific TNM staging for small intestinal NETs, pancreatic NETs, and appendiceal NETs. WHO 2022 grade is a key prognostic factor independent of staging.

Surgery: curative intent resection for localized NETs; liver-directed surgery and ablation for oligometastatic disease. Somatostatin analogs (SSAs): octreotide LAR (PROMID trial) and lanreotide (CLARINET trial) achieve tumor control and treat functional symptoms in G1-G2 somatostatin receptor-positive NETs. Lu-177-DOTATATE PRRT (peptide receptor radionuclide therapy): NETTER-1 trial showed superior PFS vs octreotide LAR in progressive midgut NETs; 4 infusions 8 weeks apart for SSTR2-positive tumors. Targeted therapy: everolimus (mTOR inhibitor) prolongs PFS in non-functional PNETs and lung/GI NETs; sunitinib for PNETs. Chemotherapy: streptozocin plus 5-FU for PNETs; platinum plus etoposide for NEC.

NETs have highly variable prognosis by grade, site, and stage. Small intestinal NETs (G1-G2): 5-year OS approximately 60-75% even with liver metastases; median OS greater than 10 years for low-grade disease. Pancreatic NETs (G1-G2): 5-year OS approximately 60-70% for localized; 20-40% for metastatic disease. Insulinoma (benign in 90%): excellent prognosis after surgical cure. High-grade NET (G3, Ki-67 20-55%): median OS 18-24 months. NEC (large cell or small cell): median OS 6-12 months despite platinum-based chemotherapy. Lu-177-DOTATATE PRRT has improved outcomes for SSTR2-positive progressive NETs.

Patients with MEN1 syndrome should undergo annual surveillance with serum calcium, prolactin, IGF-1, and fasting gastrin, plus cross-sectional imaging (CT or MRI) every 1-3 years to detect pancreatic NETs at an early, resectable stage. MEN2A/MEN2B patients should have prophylactic thyroidectomy in childhood for medullary thyroid carcinoma prevention. VHL disease patients should have annual MRI surveillance of the pancreas. No prevention strategy exists for sporadic NETs. Screening colonoscopy may identify rectal NETs incidentally. Biochemical and imaging surveillance every 6-12 months is standard after resection of localized NETs to detect recurrence.