Neuroendocrine Tumour Imaging and Theranostics

Somatostatin-receptor PET/CT, ¹⁸F-FDG in high-grade disease, and ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy

Overview

Neuroendocrine tumours (NETs) are a heterogeneous group of neoplasms arising from cells of the diffuse neuroendocrine system, most commonly in the gastroenteropancreatic (GEP) tract and lungs. Most well-differentiated NETs overexpress somatostatin receptors (SSTRs, especially subtype 2) on the cell surface. This single biological feature makes nuclear medicine central to the disease: the same receptor can be imaged with a radiolabelled somatostatin analogue and treated with a therapeutic radionuclide bound to the same peptide — the paradigm of theranostics.

Nuclear medicine contributes at every step: staging and restaging with ⁶⁸Ga-labelled SSTR positron-emission tomography/computed tomography (PET/CT), selecting candidates for peptide receptor radionuclide therapy (PRRT), and delivering ¹⁷⁷Lu-DOTATATE PRRT itself. ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET/CT complements SSTR imaging by flagging aggressive, dedifferentiated disease.

The clinical importance is practical: SSTR PET frequently changes stage and management, and PRRT is one of few therapies with randomised evidence of prolonged progression-free survival (PFS) in advanced NETs [2,4].

Core concepts and pathophysiology

Well-differentiated NETs are graded by the Ki-67 proliferation index and mitotic count (World Health Organization, WHO): G1 (Ki-67 <3%), G2 (3–20%), G3 (>20%). Poorly differentiated neuroendocrine carcinomas (NEC) are by definition high-grade. As grade rises, SSTR expression tends to fall and glucose metabolism rises — the biological basis for combining SSTR and ¹⁸F-FDG imaging.

DOTA-conjugated peptides (DOTATATE, DOTATOC, DOTANOC) bind SSTR2 with high affinity. The chelator DOTA can hold a diagnostic radiometal (⁶⁸Ga, ⁶⁴Cu) or a therapeutic one (¹⁷⁷Lu, ⁹⁰Y), so imaging predicts and monitors therapy. After receptor binding, the peptide–receptor complex is internalised, retaining the radionuclide intracellularly — essential for delivering a therapeutic radiation dose in PRRT.

Clinical indications

Established (SSTR PET/CT): - Localisation of the primary in a suspected NET, including occult primary with a metastatic NET. - Staging/restaging of well-differentiated GEP and bronchial NETs. - Selecting and monitoring candidates for somatostatin-analogue therapy or PRRT [1].

Established (¹⁷⁷Lu-DOTATATE PRRT): - Progressive, SSTR-positive, well-differentiated GEP-NETs (approval based principally on NETTER-1 in midgut NETs; the FDA label covers SSTR-positive foregut, midgut and hindgut GEP-NETs in adults and, since 2024, in patients aged ≥12 years) [2,3]. - First-line therapy for advanced grade 2–3 GEP-NETs is now supported by NETTER-2 [4].

Selected/conditional: - ¹⁸F-FDG PET/CT in G2–G3 or discordant/aggressive disease for prognosis and to detect SSTR-negative lesions. - SSTR imaging in phaeochromocytoma/paraganglioma, medullary thyroid carcinoma, meningioma, and SSTR-expressing lung disease.

Emerging/investigational: - SSTR antagonists (e.g. ⁶⁸Ga/¹⁷⁷Lu-DOTA-JR11/satoreotide) for higher tumour uptake. - ⁹⁰Y or combination/“tandem” PRRT and PRRT re-treatment; alpha-emitter PRRT (²²⁵Ac).

Unlikely to help: SSTR PET in poorly differentiated NEC with absent receptor expression (¹⁸F-FDG is preferred); routine SSTR imaging of non-neuroendocrine tumours.

Radiopharmaceuticals

⁶⁸Ga-SSTR PET has largely replaced ¹¹¹In-pentetreotide single-photon emission computed tomography (SPECT) and ¹²³I-metaiodobenzylguanidine (MIBG) for most NETs because of superior spatial resolution, sensitivity, and shorter examination time. ¹⁷⁷Lu’s imageable gamma emissions allow post-therapy SPECT/CT dosimetry — a theranostic advantage over pure beta emitters.

Patient selection and preparation

History and work-up: grade/differentiation (Ki-67), prior somatostatin analogue (SSA) therapy, chromogranin A and hormone panels, cross-sectional imaging, renal and hepatic function.

SSTR PET preparation: - No fasting required for SSTR PET. - SSA timing (to avoid receptor blockade): discontinue short-acting SSAs ≥12 h before imaging; schedule long-acting (LAR/depot) imaging just before the next scheduled dose, or ~3–4 weeks after the last injection [1]. Practice varies — some evidence suggests stable long-acting dosing does not require interruption; document local policy. - Pregnancy/breastfeeding: relative contraindication; defer or justify; interrupt breastfeeding per local rules.

¹⁸F-FDG preparation: fast ~4–6 h; target blood glucose typically <8–11 mmol/L (150–200 mg/dL; protocol-dependent); avoid strenuous exercise.

PRRT eligibility: - SSTR-positive disease in all target lesions: uptake at least equal to normal liver on planar ¹¹¹In-pentetreotide (Krenning ≥2; the NETTER-1 criterion) or, on SSTR PET, uptake exceeding liver [2,10]. - Adequate renal function (glomerular filtration rate [GFR] commonly ≥50–60 mL/min — threshold institution-dependent), adequate marrow (e.g. haemoglobin, white cells, platelets above defined minima), and adequate hepatic reserve. - Contraindications: pregnancy; caution with severe renal impairment or marrow compromise. - Consent must cover renal/haematological toxicity, nausea, fatigue, rare myelodysplasia/leukaemia, and radiation-safety precautions.

Protocols in chronological order

⁶⁸Ga-DOTATATE/DOTATOC PET/CT [1]: 1. Confirm SSA timing and pregnancy status; intravenous access. 2. Administer activity: ⁶⁸Ga-DOTATATE ~2 MBq/kg up to ~200 MBq (5.4 mCi); ⁶⁸Ga-DOTATOC ~148 MBq (4 mCi), range 111–185 MBq (3–5 mCi). ⁶⁴Cu-DOTATATE ~148 MBq (4 mCi). 3. Uptake period, patient resting: ~40–90 min for DOTATATE, ~55–90 min for DOTATOC (~45–90 min for ⁶⁴Cu). 4. Void before imaging (intense renal/bladder activity). 5. Acquire vertex/skull-base to mid-thigh PET with diagnostic or low-dose CT; contrast-enhanced CT or PET/magnetic resonance imaging (MRI) where indicated (MRI adds sensitivity for liver and bone). 6. Iterative reconstruction with attenuation, scatter and (time-of-flight/point-spread) corrections; report maximum standardised uptake value (SUVmax) for quantitation.

¹⁷⁷Lu-DOTATATE PRRT [2,5]: 1. Confirm eligibility, baseline labs, antiemesis. 2. Renal protection: commence a positively charged amino-acid infusion (lysine ± arginine, e.g. ~25 g each in ~1–2 L) starting ~30 min before and continuing during and for ≥3 h after the ¹⁷⁷Lu-DOTATATE infusion (~4 h in total) [10], to competitively reduce proximal-tubular reabsorption of the peptide. 3. Administer ¹⁷⁷Lu-DOTATATE 7.4 GBq (200 mCi) per cycle, intravenously over ~30–40 min (not as a bolus). 4. Standard course: 4 cycles at ~8-weekly intervals (interval may extend for toxicity). 5. Post-therapy ¹⁷⁷Lu SPECT/CT (~24 h, timing variable) confirms tumour uptake and enables dosimetry. 6. Monitor blood counts and renal function between cycles; radiation-safety discharge instructions.

Normal findings

Physiological SSTR uptake is intense in the spleen (often the most avid normal organ), kidneys and urinary bladder (renal excretion), and liver, with uptake also in the pituitary, adrenal glands, thyroid, salivary glands, and — importantly — the uncinate process/head of the pancreas [1]. Bowel and stomach show variable uptake. These normal patterns anchor semiquantitative scoring: the liver and spleen are the reference organs for the Krenning score.

On ¹⁸F-FDG, low-grade NETs are typically not markedly avid; FDG-avidity itself signals higher biological aggressiveness.

Interpretation

Recommended sequence: correlate with anatomy on CT/MRI; assess each target lesion’s uptake relative to liver and spleen; record SUVmax; then integrate with any ¹⁸F-FDG study.

Modified Krenning score (uptake vs reference organs):

Lesions scoring ≥2–3 predict SSTR-directed therapy benefit and support PRRT eligibility [1,5].

SSTR vs ¹⁸F-FDG concordance (prognostic): SSTR-positive/FDG-negative implies indolent, PRRT-suitable disease; SSTR-negative/FDG-positive implies aggressive, dedifferentiated disease better treated by chemotherapy; mixed/discordant patterns identify heterogeneous tumours where high-grade FDG-avid, SSTR-negative sites may escape PRRT. There is no single universally adopted numerical response criterion for SSTR PET; Response Evaluation Criteria in Solid Tumours (RECIST 1.1) on CT/MRI remains the standard structural response tool, with SSTR PET used qualitatively and for PRRT selection.

Differential diagnosis

Pitfalls and artefacts

False positives: physiological uncinate pancreatic uptake mimicking a head NET; accessory spleen/splenosis; inflammation, granulomas, and reactive nodes; post-surgical/radiation change; vertebral haemangioma; adrenal and pituitary physiological uptake.

False negatives: recent long-acting SSA blocking receptors (reduced tumour and altered normal biodistribution); small (<5–6 mm) or dedifferentiated SSTR-negative lesions; lesions adjacent to high-uptake organs (liver, spleen, kidney) obscured by background — hence MRI’s added value for liver.

Physiological variants: prominent uncinate process, adrenal, and pituitary uptake.

Technical: urinary/bladder activity obscuring pelvic disease (void before scan); PET/CT misregistration from respiration causing apparent uptake mismatch; SUV variability across scanners/uptake times limiting cross-study comparison.

Reporting guidance

Checklist: indication and grade/Ki-67; radiopharmaceutical, activity, uptake time; SSA status/last dose; CT technique (contrast/low-dose); primary site; number, location and Krenning score of the most avid and representative lesions; SUVmax of index lesions; presence of any FDG-positive/SSTR-negative discordance if dual imaging performed; comparison with prior; overall disease burden and receptor status conclusion; PRRT-eligibility statement where relevant.

Sample conclusion (not patient-specific): “Multiple somatostatin-receptor–expressing hepatic and nodal lesions with uptake greater than liver (Krenning 3–4), consistent with metastatic well-differentiated NET. Receptor expression is favourable for peptide receptor radionuclide therapy; correlation with grade and, where indicated, ¹⁸F-FDG PET is advised.”

Clinical management impact

SSTR PET frequently alters staging and management versus conventional imaging and older SSTR scintigraphy, by detecting additional metastases, localising occult primaries, and confirming receptor status. It directly gates PRRT and SSA therapy. Dual SSTR/¹⁸F-FDG imaging refines prognosis and steers patients between receptor-directed therapy and chemotherapy.

Comparison with alternatives

Radiation safety and dosimetry

Diagnostic SSTR PET: the effective dose is modest — ⁶⁸Ga-DOTA-peptide PET contributes roughly 0.02 mSv/MBq (about 3–4 mSv for a 150–200 MBq study), plus the CT component; ⁶⁴Cu-DOTATATE is somewhat higher (~4.7 mSv for 148 MBq) [1]. Renal excretion drives bladder dose, so hydration and voiding reduce it.

PRRT dosimetry: ¹⁷⁷Lu-DOTATATE delivers a therapeutic β⁻ dose to tumour while relatively sparing normal tissue; the dose-limiting organs are the kidneys and red marrow. Widely used constraints are a renal absorbed dose of about 23 Gy (adapted from external-beam experience) or a renal biologically effective dose (BED) of about 28 Gy in patients with renal risk factors and 40 Gy in those without, and a red-marrow absorbed dose around 2 Gy, though tolerances vary with renal reserve and prior therapy. Amino-acid co-infusion lowers renal dose. Imageable ¹⁷⁷Lu gamma emissions (113/208 keV) permit post-therapy quantitative SPECT/CT, enabling per-cycle and cumulative organ dosimetry that increasingly guides personalised activity or cycle number.

Radiation protection: after each PRRT cycle the patient is a transient radiation source; standard discharge advice covers time, distance and hygiene precautions for several days, with tighter limits around children and pregnant contacts. Pregnancy is a contraindication and breastfeeding is interrupted. Cumulative administered activity over the standard four-cycle course is ~29.6 GBq.

Special situations

• Paediatric: weight-based activity (e.g. ⁶⁸Ga-DOTATOC ~1.6 MBq/kg); as-low-as-reasonably-achievable (ALARA) dose optimisation.
• Pregnancy/breastfeeding: avoid/defer PET and PRRT; interrupt breastfeeding per policy.
• Renal impairment: central to PRRT risk — verify GFR; amino-acid nephroprotection mandatory; extend intervals or reduce activity per toxicity.
• Marrow reserve: monitor counts; cumulative marrow dose limits re-treatment.
• Post-treatment: ¹⁷⁷Lu post-therapy SPECT/CT confirms delivery; restage with SSTR PET and cross-sectional imaging after the course.
• Incidental findings: SSTR-avid meningiomas and benign uptake are common and should not be over-called.
• Previous therapy: recent SSA can blunt uptake; account for timing.

Recent advances

• NETTER-2 (2024): first-line ¹⁷⁷Lu-DOTATATE plus octreotide LAR in newly diagnosed advanced, well-differentiated grade 2–3 GEP-NETs (Ki-67 ≥10% and ≤55%) prolonged median progression-free survival to 22.8 vs 8.5 months with high-dose octreotide LAR (hazard ratio 0.276), with objective response 43.0% vs 9.3% — extending PRRT earlier and into higher grades [4].
• SSTR antagonists (satoreotide) show higher tumour uptake than agonists (investigational).
• Alpha-PRRT (²²⁵Ac-DOTATATE) and combination/tandem β/α regimens (investigational).
• Dosimetry-guided, personalised PRRT and improved quantitation; artificial intelligence for lesion detection and response — emerging, not yet standard.

High-yield numbers

Reporting pearls

1. Always state SSA type and timing — it explains reduced or altered uptake.
2. Use the Krenning score against liver and spleen; specify the index lesions measured.
3. Call out SSTR-negative/FDG-positive discordance — it changes therapy.
4. Recognise physiological uncinate, adrenal, pituitary, and splenic uptake before diagnosing disease.
5. Void before imaging; do not mistake bladder activity for pelvic disease.
6. SUV is scanner- and time-dependent; compare like with like across studies.
7. Add MRI for equivocal liver/bone disease.
8. Explicitly answer the PRRT-eligibility question when relevant.

Common examination traps

1. SSTR PET is not a response criterion on its own — RECIST on CT/MRI remains standard for response.
2. Spleen, not liver, is often the most avid normal organ.
3. Long-acting SSA blocks receptors — do not scan at peak depot levels.
4. High grade means SSTR down, FDG up — pick the right tracer.
5. ¹⁷⁷Lu is a β⁻ emitter with imageable γ — allowing post-therapy imaging; ⁹⁰Y is a pure β⁻ emitter.
6. Krenning ≥ liver (not just “any uptake”) predicts PRRT benefit.
7. NETTER-1’s primary endpoint was PFS; overall survival was not statistically significant (crossover) [2,3].
8. Amino-acid infusion protects kidneys, not marrow.
9. Uncinate-process uptake is a normal variant, a classic false positive.
10. ⁶⁸Ga is generator-produced; ⁶⁴Cu’s longer half-life enables centralised distribution and delayed imaging.

One-minute review

• Most well-differentiated NETs overexpress SSTR2 — the theranostic target.
• ⁶⁸Ga-DOTATATE/DOTATOC PET/CT is first-line for staging and PRRT selection.
• Activity ~2 MBq/kg (DOTATATE), uptake ~40–90 min; no fasting.
• Hold short-acting SSA ≥12 h; time long-acting around depot dosing.
• Krenning score grades uptake vs liver/spleen; ≥2–3 supports PRRT.
• ¹⁸F-FDG identifies aggressive, dedifferentiated, SSTR-negative disease.
• ¹⁷⁷Lu-DOTATATE PRRT: 7.4 GBq × 4 cycles, 8-weekly, with amino-acid renal protection.
• NETTER-1 established PFS benefit in progressive midgut NETs.
• NETTER-2 supports first-line PRRT in grade 2–3 GEP-NETs.
• Spleen, kidneys, liver, pituitary, adrenals and uncinate process are normal uptake sites.
• Void before imaging; account for SSA and SUV variability.
• SSTR/FDG discordance is a key prognostic and treatment-directing pattern.

Clinical bottom line

Somatostatin-receptor PET/CT is the cornerstone of well-differentiated NET imaging and the gatekeeper for peptide receptor radionuclide therapy, while ¹⁸F-FDG flags aggressive disease that receptor imaging may miss. ¹⁷⁷Lu-DOTATATE PRRT delivers randomised-trial progression-free survival benefit and, following NETTER-2, is moving earlier in the treatment pathway. Correct SSA timing, Krenning-based receptor assessment, and renal protection are the practical determinants of a safe, decision-changing service.

Key references

1. Hope TA, Allen-Auerbach M, Bodei L, et al. SNMMI Procedure Standard/EANM Practice Guideline for SSTR PET: Imaging Neuroendocrine Tumors. J Nucl Med. 2023;64(2):204–210. https://jnm.snmjournals.org/content/64/2/204
2. 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. doi:10.1056/NEJMoa1607427
3. Strosberg JR, Caplin ME, Kunz PL, et al. ¹⁷⁷Lu-Dotatate plus long-acting octreotide versus high-dose long-acting octreotide (NETTER-1): final overall survival and long-term safety. Lancet Oncol. 2021;22(12):1752–1763. doi:10.1016/S1470-2045(21)00572-6
4. Singh S, Halperin D, Myrehaug S, et al. [¹⁷⁷Lu]Lu-DOTA-TATE plus long-acting octreotide versus high-dose long-acting octreotide for newly diagnosed advanced grade 2–3 GEP-NETs (NETTER-2). Lancet. 2024;403(10446):2807–2817. doi:10.1016/S0140-6736(24)00701-3
5. Bodei L, Gnanasegaran G, Giammarile F, et al. Joint EANM, IAEA, and SNMMI practical guidance on somatostatin receptor-targeted radionuclide therapy of neuroendocrine tumours. EANM J. 2026;3:100017. doi:10.1016/j.eanmj.2026.100017
6. Bozkurt MF, Virgolini I, Balogova S, et al. Guideline for PET/CT imaging of neuroendocrine neoplasms with ⁶⁸Ga-DOTA-conjugated somatostatin receptor targeting peptides and ¹⁸F-DOPA. Eur J Nucl Med Mol Imaging. 2017;44(9):1588–1601. doi:10.1007/s00259-017-3728-y
7. Krenning EP, Kwekkeboom DJ, Bakker WH, et al. Somatostatin receptor scintigraphy with [¹¹¹In-DTPA-D-Phe¹]- and [¹²³I-Tyr³]-octreotide: the Rotterdam experience with more than 1000 patients. Eur J Nucl Med. 1993;20(8):716–731.
8. National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Neuroendocrine and Adrenal Tumors. https://www.nccn.org
9. 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. doi:10.1159/000471879
10. Hope TA, Abbott A, Colucci K, et al. NANETS/SNMMI Procedure Standard for Somatostatin Receptor–Based Peptide Receptor Radionuclide Therapy with ¹⁷⁷Lu-DOTATATE. J Nucl Med. 2019;60(7):937–943. doi:10.2967/jnumed.118.230607

Note: doses, timings, thresholds and eligibility values vary by guideline version, equipment, patient age and institutional protocol; verify against your local standards and the most recent society documents before clinical use.