Patient Guide · NET Awareness

Understanding neuroendocrine tumors and their symptoms.

What NETs are, why they are often diagnosed late, the symptoms by tumour location, carcinoid syndrome, the diagnosis pathway, and where theranostics fits in modern care. A patient-friendly guide grounded in WHO 2019 classification and ENETS/NANETS clinical guidelines.

Dr. Ishita B. Sen, MDDirector & Chief, Nuclear Medicine · FMRI

Published 13 May 2026

Reviewed by Dr. Dharmender Malik

14 min read

Last reviewed by Dr. Dharmender Malik on 13 May 2026 · this article reflects the published primary literature and current clinical practice at FMRI Gurugram.

Introduction

Neuroendocrine tumors (NETs) are a heterogeneous group of rare cancers arising from neuroendocrine cells — the body's diffuse hormone-producing system distributed across the gastrointestinal tract, pancreas, lungs, and other organs^[1]. Roughly 60-70 percent originate in the gut (gastrointestinal NETs), 25 percent in the lungs, and the remainder across the pancreas, thymus, and other sites^[2]. The annual incidence in published cancer-registry data is approximately 6.98 per 100,000 in the U.S., having increased roughly 6-fold since the 1970s — partly reflecting better imaging and pathology recognition rather than a true rise in disease^[3]. This guide covers what NETs are, why symptoms vary so widely, the most common presenting patterns by location, and how diagnosis is made today.

What is a neuroendocrine tumor?

AI Overview · short answer

A neuroendocrine tumor is a cancer that arises from neuroendocrine cells — specialised cells with features of both nerves and hormone-producing glands. NETs can produce hormones (functional NETs, causing distinct syndromes) or be silent (non-functional NETs). Most NETs grow slowly compared to typical cancers; some are aggressive. The WHO 2019 classification grades them by proliferation rate from G1 (slowest) to G3 (fastest) and as neuroendocrine carcinomas (NEC) for the most aggressive subset^[4].

The WHO 2019 grading framework is the foundation for modern NET treatment decisions:

Grade	Ki-67 proliferation index	Mitotic count	Typical behaviour
G1 (well-differentiated)	< 3%	< 2 / 10 HPF	Slow growth; PRRT, somatostatin analogues, surgery
G2 (well-differentiated)	3-20%	2-20 / 10 HPF	Intermediate; PRRT, targeted therapies, surgery
G3 (well-differentiated NET)	> 20%	> 20 / 10 HPF	Aggressive but somatostatin-receptor positive; PRRT may apply
NEC (poorly-differentiated)	> 20%	> 20 / 10 HPF	Highly aggressive; platinum chemotherapy is standard

Why are NETs diagnosed late?

Median diagnostic delay for NETs is approximately 5-7 years from first symptoms to diagnosis in many published series^[5]. Three factors explain this:

Symptoms are non-specific — abdominal pain, diarrhoea, flushing, and weight loss are common to many conditions, and the slow growth of most NETs means they don't produce the dramatic presentations of more aggressive cancers.
Functional symptoms can be subtle — carcinoid syndrome features (flushing, diarrhoea, wheezing) are often initially attributed to allergies, IBS, or menopause^[6].
Conventional imaging is insensitive — small NETs are often missed on standard CT or ultrasound. The introduction of Ga-68 DOTATATE PET-CT has substantially improved NET detection — sensitivity 90-95 percent versus approximately 50-60 percent for octreotide scintigraphy^[7].

Clinical note · when to investigate further

Persistent unexplained symptoms — diarrhoea lasting more than 4-6 weeks, flushing that is unrelated to typical triggers, recurrent abdominal pain, or weight loss without obvious cause — particularly when occurring together, should prompt consideration of NET workup. Plasma chromogranin A and 24-hour urine 5-HIAA are reasonable first-line tests; Ga-68 DOTATATE PET-CT and endoscopy / cross-sectional imaging follow if indicated^[8].

Carcinoid syndrome — the classic functional presentation

Carcinoid syndrome is the most recognised functional NET presentation, occurring in approximately 20-30 percent of patients with metastatic midgut NETs^[9]. It results from systemic release of serotonin and other vasoactive substances by tumour cells that have metastasised to the liver (bypassing the first-pass hepatic inactivation of serotonin). The core features:

Flushing — typically face and upper chest, lasting minutes; often triggered by meals, alcohol, stress, or hot drinks. Occurs in approximately 85 percent of carcinoid syndrome patients^[9].
Diarrhoea — secretory and persistent; affects approximately 80 percent of patients^[9].
Wheezing or bronchospasm — approximately 10-20 percent of patients.
Carcinoid heart disease — right-heart valvular fibrosis (tricuspid regurgitation, pulmonary stenosis) developing in approximately 20 percent of patients with carcinoid syndrome over years of exposure^[10].

Carcinoid syndrome management is built around somatostatin analogues (octreotide, lanreotide), which reduce serotonin secretion and provide symptom control in most patients^[11]. For refractory disease, telotristat ethyl (a tryptophan hydroxylase inhibitor) and PRRT are options^[12].

Pancreatic NETs — six functional syndromes

Pancreatic NETs account for approximately 7 percent of all NETs but produce some of the most distinctive functional syndromes^[13]:

Tumour	Hormone	Classic features
Insulinoma	Insulin	Whipple triad — fasting hypoglycaemia, low glucose, relief with glucose
Gastrinoma (Zollinger-Ellison syndrome)	Gastrin	Severe peptic ulcer disease, diarrhoea, oesophagitis
VIPoma (Verner-Morrison syndrome)	Vasoactive intestinal peptide	Severe watery diarrhoea, hypokalaemia, achlorhydria (WDHA)
Glucagonoma	Glucagon	Necrolytic migratory erythema, weight loss, new-onset diabetes
Somatostatinoma	Somatostatin	Diabetes, cholelithiasis, steatorrhoea
Non-functional pancreatic NET	None (or inactive)	Mass effect, incidental finding, jaundice

The majority of pancreatic NETs (approximately 60 percent) are non-functional and present as incidental imaging findings or with mass-effect symptoms — abdominal pain, jaundice, weight loss^[13]. Functional pancreatic NETs are individually rare but produce striking clinical syndromes that can take years to diagnose if not specifically considered.

Lung NETs and other locations

Pulmonary NETs account for approximately 25 percent of all NETs and include four subtypes ranging from typical carcinoid (most indolent) through atypical carcinoid, large-cell neuroendocrine carcinoma, and small-cell lung cancer (most aggressive)^[14]. Symptoms include:

Persistent cough, wheeze, or post-obstructive pneumonia (from airway tumours).
Haemoptysis (coughing up blood) in some cases.
Atypical carcinoid syndrome — functional lung NETs can produce ACTH (causing Cushing's syndrome) or growth hormone-releasing hormone.

Other locations include thymic NETs (approximately 5 percent), Merkel cell carcinoma (cutaneous), and rarely the prostate, breast, or kidney. PPGL (pheochromocytoma and paraganglioma) are technically neuroendocrine tumours of chromaffin cell origin but are usually classified separately; see our PPGL guide for that context.

How NETs are diagnosed today

Modern NET diagnostic workup has four pillars:

Biochemical markers — plasma chromogranin A is the most widely used general NET marker (elevated in approximately 60-80 percent of NETs)^[8]. 24-hour urine 5-hydroxyindoleacetic acid (5-HIAA) is specific for serotonin-producing NETs. Functional pancreatic NETs require targeted hormone testing (insulin/proinsulin, gastrin, VIP, etc.).
Cross-sectional imaging — CT or MRI with contrast remains the foundation for anatomic assessment and surgical planning. Multi-phase imaging is preferred for liver lesion characterisation^[15].
Molecular imaging — Ga-68 DOTATATE PET-CT is now the gold standard for NET staging, with sensitivity 90-95 percent versus 50-60 percent for older octreotide scintigraphy^[7]. The Krenning score (semi-quantitative grading of uptake) is also used to assess PRRT eligibility.
Histology and grading — biopsy with WHO 2019 grading (Ki-67 proliferation index and mitotic count) defines G1/G2/G3 vs NEC and shapes all subsequent treatment decisions^[4].

Where theranostics fits in NET care

The most recognised theranostic application in oncology is in NETs: Ga-68 DOTATATE PET-CT is paired with Lu-177 DOTATATE PRRT (Lutathera) — using the same somatostatin-receptor-targeting ligand for both imaging and treatment. The diagnostic scan confirms eligibility for the therapy^[16].

PRRT is established treatment for:

Metastatic well-differentiated G1-G2 midgut NETs — supported by the NETTER-1 phase III trial showing 28.4-month median PFS versus 8.4 months with high-dose octreotide^[17].
Earlier-line use in selected patients — the NETTER-2 trial (published 2024) extended evidence to higher-grade (Ki-67 10-55%) G2-G3 NETs in earlier treatment lines^[18].
Pancreatic, lung, and other SSTR-positive NETs — extensive cohort evidence, though not the original NETTER-1 population^[19].

For more on what PRRT involves see our eligibility guide; for side effects and recovery see our sourced side-effects guide and recovery time guide.

The bottom line — and when to seek a NET specialist

NETs are heterogeneous and often slow-growing, which is why they are frequently diagnosed late (median delay 5-7 years from first symptoms).
Functional NETs produce distinct syndromes — carcinoid syndrome (midgut), Zollinger-Ellison (gastrinoma), Whipple triad (insulinoma), etc. — that are recognisable once specifically considered.
Modern diagnostic workup combines biochemical markers, cross-sectional imaging, Ga-68 DOTATATE PET-CT, and histology with WHO 2019 grading.
Theranostics is the most established personalised-medicine approach in NETs — Ga-68 DOTATATE PET-CT for diagnosis and Lu-177 DOTATATE PRRT for treatment use the same molecular target^[16].
NETs are best managed in multidisciplinary teams at experienced centres — outcomes are meaningfully better at high-volume institutions^[20].

Important

This article is general patient education. If you have symptoms suggesting NET — particularly the combination of flushing, diarrhoea, and weight loss that persists for weeks — please discuss with your physician for individualised assessment. Diagnosis of NET requires biochemical, imaging, and pathology workup; treatment decisions require multidisciplinary input.

"The most common pattern I see in our NET clinic is a patient who has spent 3-5 years being told they have IBS, anxiety, or peri-menopausal flushing — only to be diagnosed with metastatic NET on a CT done for an unrelated reason. Carcinoid syndrome is not subtle once you specifically consider it, but it is easily missed when not considered. The single most useful question for a clinician facing unexplained chronic diarrhoea and flushing is — could this be a NET?"

Dr. Ishita B. Sen, MD · Director & Chief, Nuclear Medicine, FMRI

NET workup consultation · second opinion welcome

If you have persistent unexplained symptoms — chronic diarrhoea, flushing, recurrent abdominal pain, or weight loss — and have not had a NET workup, FMRI's nuclear medicine team can review your case and recommend appropriate next steps including biochemical markers and Ga-68 DOTATATE PET-CT if indicated.

Discuss NET workup · WhatsApp +91 8800 988936

For patients & referring clinicians

Frequently asked questions

Q01 What is a neuroendocrine tumor?

A neuroendocrine tumor (NET) is a cancer arising from neuroendocrine cells — specialised cells with features of both nerves and hormone-producing glands distributed across the GI tract, pancreas, lungs, and other organs. NETs can produce hormones (functional NETs) or be silent (non-functional NETs). The WHO 2019 classification grades them G1/G2/G3 (well-differentiated NET) or NEC (poorly-differentiated neuroendocrine carcinoma) based on the proliferation rate [4].

Q02 What are the main symptoms of a neuroendocrine tumor?

Symptoms vary widely by location and whether the tumour produces hormones. Common general symptoms: persistent abdominal pain, unexplained diarrhoea, weight loss, flushing, fatigue. Functional NETs produce specific syndromes — carcinoid syndrome (flushing + diarrhoea + wheezing for serotonin-secreting midgut NETs), Whipple triad (insulinoma), severe peptic ulcers (gastrinoma), watery diarrhoea (VIPoma). Median diagnostic delay is 5-7 years because early symptoms are non-specific [5].

Q03 What is carcinoid syndrome?

Carcinoid syndrome is the classic functional presentation of serotonin-secreting midgut NETs that have metastasised to the liver. It occurs in approximately 20-30% of metastatic midgut NETs. Core features: flushing (~85% of patients), secretory diarrhoea (~80%), wheezing or bronchospasm (10-20%), and carcinoid heart disease (~20% over years) — right-heart valvular fibrosis from chronic serotonin exposure [9][10].

Q04 How is a neuroendocrine tumor diagnosed?

NET diagnosis requires four components: biochemical markers (plasma chromogranin A, 24-hour urine 5-HIAA, targeted hormone testing); cross-sectional imaging (CT or MRI with contrast); molecular imaging (Ga-68 DOTATATE PET-CT, sensitivity 90-95% vs 50-60% for older octreotide scan); histology with WHO 2019 grading (Ki-67 proliferation index) [4][7][8].

Q05 What is the difference between NET and NEC?

WHO 2019 distinguishes well-differentiated NETs (G1/G2/G3) from poorly-differentiated neuroendocrine carcinomas (NEC). NETs grow more slowly, have lower Ki-67 (though G3 NETs can have Ki-67 >20%), tend to be SSTR-positive (making PRRT possible), and respond to somatostatin analogues. NECs grow rapidly, are SSTR-negative, and are treated like small-cell lung cancer (platinum + etoposide chemotherapy) [4].

Q06 Why are NETs often diagnosed late?

Three reasons: (1) symptoms are non-specific (abdominal pain, diarrhoea, flushing — common to many conditions); (2) functional symptoms can be subtle and are often attributed to IBS, allergies, or menopause; (3) small NETs are often missed on standard CT or ultrasound until Ga-68 DOTATATE PET-CT became widely available. Median diagnostic delay is 5-7 years in published series [5][7].

Q07 What is Ga-68 DOTATATE PET-CT?

Ga-68 DOTATATE PET-CT is a molecular imaging test that uses a radiolabelled somatostatin analogue (DOTATATE labelled with Gallium-68) to detect NET cells expressing somatostatin receptor type 2 (SSTR2). It has revolutionised NET staging — sensitivity 90-95% versus 50-60% for the older octreotide scintigraphy [7]. It is also used to confirm eligibility for PRRT, since high uptake correlates with likelihood of PRRT response.

Q08 What is PRRT and when is it used?

PRRT (peptide receptor radionuclide therapy) is a targeted radiation therapy that uses Lu-177-labelled DOTATATE — a somatostatin analogue — to deliver radiation to NET cells. It is established treatment for metastatic well-differentiated G1-G2 midgut NETs (NETTER-1 trial: 28.4 vs 8.4 months median PFS) [17] and is increasingly used in earlier lines and other NET locations. The NETTER-2 trial (2024) extended evidence to higher-grade NETs [18].

Q09 Are NETs cancer?

Yes — NETs are a form of cancer, though they are biologically distinct from most cancers in their slower growth (in G1/G2 disease) and characteristic hormone-producing patterns. The WHO 2019 classification formally defines NETs as malignant tumours. Many NETs can be cured by surgery if detected early; metastatic NETs are usually not curable but are often controllable for many years with somatostatin analogues, PRRT, targeted therapies, and other approaches [4].

Q10 How common are neuroendocrine tumors?

The annual incidence in U.S. cancer-registry data is approximately 6.98 per 100,000 (about 175,000 new cases per year globally based on extrapolation). Incidence has risen approximately 6-fold since the 1970s, partly reflecting improved imaging and pathology recognition rather than a true rise in disease [3]. NETs are individually rare but collectively important — approximately 1.0-1.5% of all cancers.

Q11 Where do NETs most commonly occur?

Approximately 60-70% of NETs are gastrointestinal (small bowel midgut NETs are most common, followed by rectal, gastric, and appendiceal NETs); 25% are pulmonary (carcinoid tumours of the lung); 7% are pancreatic; and the remainder are distributed across the thymus, skin (Merkel cell), and other rare locations [2]. The pancreatic NETs are individually rare but produce some of the most distinctive functional syndromes.

Q12 Should I see a NET specialist?

Yes — outcomes are meaningfully better at high-volume NET centres with multidisciplinary teams. NET care typically involves medical oncology, surgical oncology, nuclear medicine, gastroenterology, endocrinology, interventional radiology, and pathology specialists. Major academic centres and dedicated NET clinics provide this integrated approach [20]. For NET diagnosis, second-opinion review at a specialist centre is reasonable before major treatment decisions.

Citations & references

All clinical numbers above are sourced from the primary literature listed below. Every reference links to the open journal page or the FDA archive — open in a new tab to verify.

[1] Rindi G, Klimstra DS, Abedi-Ardekani B, et al. A common classification framework for neuroendocrine neoplasms: an International Agency for Research on Cancer (IARC) and World Health Organization (WHO) expert consensus proposal. Mod Pathol. 2018;31(12):1770-1786. View source ↗

[2] Yao JC, Hassan M, Phan A, et al. One hundred years after "carcinoid": epidemiology of and prognostic factors for neuroendocrine tumors in 35,825 cases in the United States. J Clin Oncol. 2008;26(18):3063-3072. View source ↗

[3] Dasari A, Shen C, Halperin D, et al. Trends in the Incidence, Prevalence, and Survival Outcomes in Patients With Neuroendocrine Tumors in the United States. JAMA Oncol. 2017;3(10):1335-1342. View source ↗

[4] World Health Organization. WHO classification of tumours of endocrine organs (5th ed). Lyon: IARC; 2019. View source ↗

[5] Modlin IM, Oberg K, Chung DC, et al. Gastroenteropancreatic neuroendocrine tumours. Lancet Oncol. 2008;9(1):61-72. View source ↗

[6] Halperin DM, Shen C, Dasari A, et al. Frequency of carcinoid syndrome at neuroendocrine tumour diagnosis: a population-based study. Lancet Oncol. 2017;18(4):525-534. View source ↗

[7] Geijer H, Breimer LH. Somatostatin receptor PET/CT in neuroendocrine tumours: update on systematic review and meta-analysis. Eur J Nucl Med Mol Imaging. 2013;40(11):1770-1780. View source ↗

[8] Modlin IM, Gustafsson BI, Moss SF, et al. Chromogranin A — biological function and clinical utility in neuro endocrine tumor disease. Ann Surg Oncol. 2010;17(9):2427-2443. View source ↗

[9] Mota JM, Sousa LG, Riechelmann RP. Complications from carcinoid syndrome: review of the current evidence. Ecancermedicalscience. 2016;10:662. View source ↗

[10] Davar J, Connolly HM, Caplin ME, et al. Diagnosing and Managing Carcinoid Heart Disease in Patients With Neuroendocrine Tumors. J Am Coll Cardiol. 2017;69(10):1288-1304. View source ↗

[11] Caplin ME, Pavel M, Ćwikła JB, et al. Lanreotide in metastatic enteropancreatic neuroendocrine tumors (CLARINET). N Engl J Med. 2014;371(3):224-233. View source ↗

[12] Kulke MH, Hörsch D, Caplin ME, et al. Telotristat Ethyl, a Tryptophan Hydroxylase Inhibitor for the Treatment of Carcinoid Syndrome. J Clin Oncol. 2017;35(1):14-23. View source ↗

[13] Falconi M, Eriksson B, Kaltsas G, et al. ENETS Consensus Guidelines Update for the Management of Patients with Functional Pancreatic Neuroendocrine Tumors and Non-Functional Pancreatic Neuroendocrine Tumors. Neuroendocrinology. 2016;103(2):153-171. View source ↗

[14] Caplin ME, Baudin E, Ferolla P, et al. Pulmonary neuroendocrine (carcinoid) tumors: European Neuroendocrine Tumor Society expert consensus and recommendations for best practice. Ann Oncol. 2015;26(8):1604-1620. View source ↗

[15] Sundin A, Arnold R, Baudin E, et al. ENETS Consensus Guidelines for the Standards of Care in Neuroendocrine Tumors: Radiological, Nuclear Medicine and Hybrid Imaging. Neuroendocrinology. 2017;105(3):212-244. View source ↗

[16] Hennrich U, Kopka K. Lutathera®: The First FDA- and EMA-Approved Radiopharmaceutical for Peptide Receptor Radionuclide Therapy. Pharmaceuticals (Basel). 2019;12(3):114. View source ↗

[17] Strosberg J, El-Haddad G, Wolin E, et al. Phase 3 Trial of ¹⁷⁷Lu-Dotatate for Midgut Neuroendocrine Tumors (NETTER-1). N Engl J Med. 2017;376(2):125-135. View source ↗

[18] Singh S, Halperin D, Myrehaug S, et al. [177Lu]Lu-DOTA-TATE plus long-acting octreotide versus high-dose long-acting octreotide for the treatment of newly diagnosed, advanced grade 2-3, well-differentiated, gastroenteropancreatic neuroendocrine tumours (NETTER-2). Lancet. 2024;403(10446):2807-2817. View source ↗

[19] Brabander T, van der Zwan WA, Teunissen JJM, et al. Long-Term Efficacy, Survival, and Safety of [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate. Clin Cancer Res. 2017;23(16):4617-4624. View source ↗

[20] Strosberg JR, Halfdanarson TR, Bellizzi AM, et al. NANETS Consensus Guidelines for Surveillance and Medical Management of Midgut NETs. Pancreas. 2017;46(6):707-714. View source ↗

[21] Pavel M, Öberg K, Falconi M, et al. ESMO Clinical Practice Guidelines for Gastroenteropancreatic Neuroendocrine Neoplasms. Ann Oncol. 2020;31(7):844-860. View source ↗

[22] Rinke A, Müller HH, Schade-Brittinger C, et al. Placebo-controlled, double-blind, prospective, randomized study on the effect of octreotide LAR in the control of tumor growth in patients with metastatic neuroendocrine midgut tumors (PROMID). J Clin Oncol. 2009;27(28):4656-4663. View source ↗

[23] Yao JC, Shah MH, Ito T, et al. Everolimus for advanced pancreatic neuroendocrine tumors (RADIANT-3). N Engl J Med. 2011;364(6):514-523. View source ↗

[24] Raymond E, Dahan L, Raoul JL, et al. Sunitinib malate for the treatment of pancreatic neuroendocrine tumors. N Engl J Med. 2011;364(6):501-513. View source ↗

[25] Klimstra DS, Modlin IR, Coppola D, et al. The pathologic classification of neuroendocrine tumors: a review of nomenclature, grading, and staging systems. Pancreas. 2010;39(6):707-712. View source ↗

[26] Pavel M, O'Toole D, Costa F, et al. ENETS Consensus Guidelines update for the management of distant metastatic disease in NETs. Neuroendocrinology. 2016;103(2):172-185. View source ↗

[27] Sansovini M, Severi S, Ianniello A, et al. Long-term follow-up of ¹⁷⁷Lu-DOTATATE in pancreatic NET. Eur J Nucl Med Mol Imaging. 2017;44(3):490-499. View source ↗

[28] Halfdanarson TR, Strosberg JR, Tang L, et al. NANETS 2020 Consensus Paper on Diagnosis and Management of Pancreatic Neuroendocrine Tumors. Pancreas. 2020;49(7):863-881. View source ↗

[29] Yao JC, Fazio N, Singh S, et al. Everolimus for the treatment of advanced, non-functional neuroendocrine tumours of the lung or gastrointestinal tract (RADIANT-4). Lancet. 2016;387(10022):968-977. View source ↗

[30] Cingarlini S, Bonomi M, Trentin C, et al. Profiling of the immune system in patients with NETs. Endocr Relat Cancer. 2018;25(7):R407-R424. View source ↗

[31] Kunz PL, Reidy-Lagunes D, Anthony LB, et al. Consensus guidelines for the management and treatment of NETs. Pancreas. 2013;42(4):557-577. View source ↗

[32] Modlin IM, Drozdov I, Kidd M. The identification of gut neuroendocrine tumor disease by multiple synchronous transcript analysis in blood. PLoS One. 2013;8(5):e63364. View source ↗

About the Author

Dr. Ishita B. Sen

MBBS · MD (Nuclear Medicine) · DNB · Post-doctoral Fellowship, Memorial Sloan Kettering Cancer Center, New York

Director and Chief of Nuclear Medicine at Fortis Memorial Research Institute. Co-founder of Theranostic Physicians Private Limited (TPPL). Two decades of clinical practice in PSMA imaging and PSMA-directed radioligand therapy, with one of the largest Indian institutional experiences in Lu-PSMA.

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