Neuroendocrine tumours (NETs) are a complex and diverse group of cancers that arise from neuroendocrine cells, which regulate various bodily functions through hormone production. Diagnosing and monitoring NETs can be challenging due to their small size, slow growth, and ability to develop in multiple organs. Unlike many other cancers, NETs often require specialised imaging techniques to confirm their presence, determine their extent, and guide treatment.
Radiological imaging plays a critical role in both initial diagnosis and ongoing management, helping clinicians assess tumour location, grade, spread, and response to therapy. With advancements in imaging technology, clinicians now have access to highly sensitive modalities that can detect even small, early-stage tumours, improving patient outcomes.
There are key imaging techniques used to NET localisation which demonstrates their strengths and limitations how emerging technologies are enhancing diagnostic accuracy.
Neuroendocrine Cancer Australia (NECA), is dedicated to assisting individuals diagnosed with NETs and their loved ones. NECA offers a wealth of resources, educational programs, and advocacy efforts aimed at deepening the understanding of NETs, improving patient care, and encouraging research advancements. Patients can engage with NECA’s comprehensive support and information by calling the NET nurse line.
Importance of imaging in NET diagnosis
Imaging is essential for:
- Identifying the primary tumour – Many NETs are small, slow-growing, and difficult to detect, particularly in the pancreas and intestines.
- Assessing metastasis – NETs can spread to lymph nodes, the liver, bones, and lungs, requiring imaging to evaluate the extent of disease.
- Guiding treatment decisions – Imaging helps determine whether surgery, systemic therapy, or peptide receptor radionuclide therapy (PRRT) is appropriate.
- Monitoring treatment response – Regular scans are used to track tumour progression or shrinkage over time.
Challenges in localising small or slow-growing tumours
NETs often don’t form large masses, making them harder to detect than other cancers. Functional NETs secrete excessive hormones that cause symptoms, but non-functional secrete far less. NETs may remain undetected for years. Some NETs exhibit low metabolic activity, limiting their visibility on conventional imaging like CT or FDG PET scans.
Due to these factors, a combination of imaging techniques is often required for accurate localisation.
Common radiological imaging techniques
Computed Tomography (CT) Scans
CT scans are a widely used imaging tool that provides detailed cross-sectional images of the body. They are particularly effective in:
- Identifying large or metastatic NETs in the liver, lungs, and lymph nodes.
- Assessing tumour vascularity, which helps differentiate NETs from other cancers.
- Evaluating bone involvement when NETs metastasise.
Strengths:
- Widely available and quick to perform.
- Good for detecting liver and lung metastases.
- Contrast-enhanced CT improves visualisation of NET lesions.
Limitations:
- Small, low-grade NETs may not be visible.
- Ionising radiation exposure, making frequent scans a concern for some patients.
Magnetic Resonance Imaging (MRI)
MRI is often used as an alternative to CT, especially when evaluating soft tissue structures. It is particularly beneficial for:
- Liver and pancreatic NETs, as it provides superior contrast resolution compared to CT.
- Patients needing repeat imaging, as MRI does not use ionising radiation.
Best uses for liver and pancreatic NETs
MRI, especially contrast-enhanced MRI, is highly effective at detecting small liver metastases, which can be missed on CT. It is also useful for pancreatic NETs, where fine anatomical details are necessary for surgical planning.
Ultrasound
Ultrasound is a non-invasive imaging technique that can sometimes detect NETs in the liver, pancreas, or intestines. However, it is generally less effective than CT or MRI for small tumours.
Use of Endoscopic Ultrasound (EUS) for pancreatic NETs
EUS is a highly specialised technique that allows for detailed imaging of pancreatic tumours by inserting an ultrasound probe into the gastrointestinal tract. This technique:
- Provides high-resolution images of small pancreatic NETs.
- Allows for fine-needle biopsy, enabling tissue sampling for diagnosis.
Functional imaging and hybrid techniques
Gallium-68 DOTATATE PET/CT
Gallium-68 DOTATATE PET/CT is considered the most effective imaging modality for NET detection. It uses a radiotracer that binds to somatostatin receptors on NET cells, allowing tumours to be visualised with high precision.
Benefits of combining PET with CT:
- High sensitivity and specificity – Can detect even very small tumours.
- Superior imaging of metastases compared to conventional CT or MRI.
- Essential for PRRT eligibility, as it determines if tumours express somatostatin receptors.
FDG PET/CT
Unlike Gallium-68 DOTATATE PET, which targets well-differentiated, slow-growing NETs, FDG PET/CT is used to detect aggressive, high-grade neuroendocrine carcinomas (NECs).
- High-grade NECs often do not express somatostatin receptors but have high glucose metabolism, making them visible on FDG PET.
- It is useful for assessing tumour aggressiveness and selecting appropriate treatment strategies.
Comparing FDG PET with Gallium-68 PET:
- Gallium-68 DOTATATE PET is preferred for low-to-intermediate grade NETs.
- FDG PET/CT is used for high-grade, poorly differentiated NECs.
Octreotide Scans (SRS)
Before Gallium-68 PET was widely available, Octreotide scans (Somatostatin Receptor Scintigraphy – SRS) were the standard for NET imaging. This method:
- Uses radiolabelled octreotide to detect somatostatin receptor-positive tumours.
- Can still be useful when Gallium-68 PET is unavailable.
Limitations compared to modern imaging:
- Lower sensitivity – Can miss small or less active tumours.
- Longer imaging time – Requires multiple scans over 24-48 hours.
Selecting the right imaging approach
Choosing the right imaging technique can have a vast impact on treatment.
Factors influencing choice of imaging modality
- Tumour grade and differentiation – Determines whether Gallium-68 DOTATATE PET or FDG PET is needed.
- Tumour location – MRI is often preferred for liver and pancreas, while CT is better for lungs and bones.
- Availability and cost – Gallium-68 PET is the gold standard, but accessibility varies.
Imaging for monitoring and follow-up
How often imaging is recommended depends on the tumour’s grade, stability, and treatment history and your clinicians recommendations. For patients with stable, low-grade neuroendocrine tumours (NETs), scans are typically recommended every 6 to 12 months to monitor for any changes.
In cases involving progressing or high-risk tumours, imaging may be required more frequently, sometimes every 3 to 6 months, to track disease activity. After treatment—such as surgery or peptide receptor radionuclide therapy (PRRT)—regular follow-up imaging is essential to assess the response and watch for signs of recurrence.
Assessing treatment response and disease progression is a key reason for ongoing imaging. If scans show shrinking tumours, it suggests that the treatment is effective. When tumours remain stable, continued monitoring is often recommended to ensure they don’t begin to grow. The appearance of new metastases or changes in tumour activity may signal the need to adjust the treatment approach, such as switching therapies or exploring clinical trials.
Advances and future directions
There are several new technologies available for patients looking to combine imaging techniques.
- Dual-tracer PET imaging – Combining Gallium-68 DOTATATE PET and FDG PET for more comprehensive assessments.
- Artificial intelligence (AI) – Improving image analysis and early detection of NETs.
Research continues to focus on enhancing imaging resolution and tracer specificity, allowing for even earlier detection and better treatment planning.
By utilising the right imaging modalities at the right time, clinicians can improve early diagnosis, treatment planning, and long-term monitoring of NETs.
Further information and support for people diagnosed with NETs is available by calling the NECAÂ NET nurse line.
SOURCES
- Neuroendocrine Tumor Diagnosis and Management
https://ajronline.org/doi/10.2214/AJR.18.19881 - Imaging in Neuroendocrine Tumors: An Update for the Clinician
https://pmc.ncbi.nlm.nih.gov/articles/PMC4526141/ - CT/MRI of Neuroendocrine Tumours
https://pmc.ncbi.nlm.nih.gov/articles/PMC1805060/ - Imaging of Neuroendocrine Tumours (CT/MR/US)
https://www.sciencedirect.com/science/article/abs/pii/S1521690X07000048 - Imaging of Neuroendocrine Neoplasms: Monitoring Treatment Response
https://www.ajronline.org/doi/full/10.2214/AJR.21.27159 - Imaging of Neuroendocrine Tumors of the Pancreas
https://www.sciencedirect.com/science/article/pii/S2211568416302339 - Pancreatic Neuroendocrine Tumours: Spectrum of Imaging Findings
https://gs.amegroups.org/article/view/52017/html - Neuroendocrine Neoplasms of the Gastrointestinal Tract
https://pubs.rsna.org/doi/abs/10.1148/rg.276075001 - A Comprehensive Review on Neuroendocrine Neoplasms
https://pmc.ncbi.nlm.nih.gov/articles/PMC10420169/
