Embolization for NET Liver Metastases: RETNET and 896-Patient Data

Embolization for neuroendocrine tumor liver metastases blocks the liver artery branches that feed the tumors. In an Italian review of 896 patients, symptoms improved in about 56% on average, and in the international RETNET randomized trial (13 centers in the USA, Canada, Argentina and Italy) two embolization methods slowed liver progression to a similar degree.

This article summarizes the numbers from both studies, the Japanese guideline and how a Japanese national cancer hospital uses embolization, where Y-90 radioembolization stands in Japan, and what to prepare before treatment.

Why embolization works for NET liver metastases

Neuroendocrine tumors (NETs) arise from hormone-producing cells in organs such as the pancreas, small intestine and rectum. Many grow slowly, but they often spread to the liver; University of Pennsylvania researchers note that about 9 in 10 NET patients develop liver metastases.

When there are many liver lesions, surgery often cannot remove them all. Healthy liver gets most of its blood from the portal vein, while NET metastases feed mainly on the hepatic artery. Blocking selected artery branches therefore cuts the tumor blood supply while the normal liver keeps going on portal blood.

Some NETs release hormones such as serotonin and cause carcinoid syndrome, with flushing and frequent diarrhea. Reducing tumor volume in the liver often eases these symptoms, so embolization is also used for symptom control. The basics of the procedure are explained in our TAE transarterial embolization guide.

What 896 patients showed

A multidisciplinary NET group in Naples, Italy, reviewed 896 patients (979 procedures) treated with hepatic artery embolization and published the results in the Journal of Experimental & Clinical Cancer Research in 2014. Numbers vary widely between the included studies, but the overall picture is as follows.

Del Prete et al., Journal of Experimental & Clinical Cancer Research 2014;33:43.

The authors described hepatic artery embolization as a safe option for unresectable NET liver metastases and recommended it be done at experienced multidisciplinary NET centers. They also noted that metastases outside the liver or an unremoved primary tumor do not by themselves rule out embolization.

RETNET randomized trial

Until recently there was no randomized comparison of embolization methods for NET liver metastases. RETNET (NCT02724540) enrolled patients from 2017 to 2022, and Michael Soulen's team at the University of Pennsylvania presented the results at the 2025 NANETS Symposium.

Of 151 patients, 78 received bland embolization (TAE) with small particles only, and 73 received conventional chemoembolization (cTACE) with chemotherapy and Lipiodol followed by particles. Primary sites were midgut in 54% and pancreas in 36%; more than half had grade 2 tumors.

Soulen et al., NANETS 2025 abstract; Targeted Oncology, October 24, 2025; Interventional News (SIO 2025).

The investigators reported similar results regardless of tumor grade, tumor burden or histology, giving high-level evidence for what clinicians have done for decades. A drug-eluting bead (DEB-TACE) arm was closed after the first safety review, so the trial focused on bland embolization and Lipiodol chemoembolization; NCCN guidance recommends these two methods rather than drug-eluting beads for NETs. See our TACE chemoembolization guide for the general procedure.

Who is a candidate

RETNET enrolled patients with growing liver metastases, hormone symptoms, or tumor occupying more than 25% of the liver. Cancer Research UK says embolization may be offered when NET liver spread causes hormone symptoms and surgery is not possible or other treatments have not controlled the symptoms.

• Preserved liver function (bilirubin and creatinine below 2 mg/dL in the Italian review)
• More than 3 liver lesions or lesions over 3 cm, and lesions with a rich blood supply
• Both functioning (symptomatic) and non-functioning tumors
• Main portal vein blockage, poor liver function or tumor in more than 75% of the liver usually call for another approach first

For small, few lesions, thermal ablation may be considered as well. Endoscopic treatment of small pancreatic NETs is covered in our pancreatic NET radiofrequency ablation article.

How it is done in Japan

The Japan Neuroendocrine Tumor Society (JNETS) guideline synopsis, published in the Journal of Gastroenterology in 2021, lists resection when feasible, and radiofrequency ablation and transarterial chemoembolization, for liver metastases of pancreatic and gastrointestinal NETs. The same guideline does not recommend liver resection for poorly differentiated neuroendocrine carcinoma (NEC), so confirming NET versus NEC on pathology comes first.

The hepatobiliary and pancreatic oncology department of the National Cancer Center Hospital East explains that, for functioning NETs, it uses transarterial embolization and radiofrequency ablation on liver metastases where possible to improve symptoms and course. The same page notes somatostatin analog injections every 4 weeks and that the radioligand Lutathera (177Lu-DOTATATE) is covered by Japanese national insurance.

Details of Y-90 itself are in our Y-90 radioembolization guide. For NET liver metastases in Japan today, TAE and chemoembolization are the main options. An overview of image-guided treatment is on our IVR page.

Procedure and recovery

A thin catheter is passed from an artery in the groin or wrist under X-ray guidance; no surgical incision is needed. Combining patient information from Cancer Research UK and University Hospitals Birmingham:

Hospital stay is one or two nights per Cancer Research UK and about a week per University Hospitals Birmingham, depending on the hospital and extent of treatment.

The most common after-effect is post-embolization syndrome: nausea, abdominal pain and fever, usually settling in 2–3 days with painkillers and anti-sickness medicine. Tiredness can last a couple of weeks, and Birmingham reports most people recover within six weeks. Contact the hospital promptly for fever or feeling unwell at home.

What to prepare for a Japanese hospital

Embolization is performed by the IR team, with gastroenterology, hepatobiliary surgery and medical oncology planning together. Bringing the following speeds up the first review.

• Pathology report: NET grade (G1, G2, G3), Ki-67, and confirmation that it is not NEC
• Contrast CT or MRI of the liver from the last 3 months (original image files and reports)
• Somatostatin receptor imaging (such as gallium PET) results, if done
• Liver and kidney blood tests and a current medication list, especially blood thinners
• Date of the last octreotide or lanreotide injection
• A symptom log (flushing, daily bowel movements) and any contrast allergy

FAQ

How many embolization sessions are needed for NET liver metastases?

In the Italian review, most patients had 2–3 sessions about 4 weeks apart. Treatment can be repeated if lesions grow later.

Is bland embolization or chemoembolization better?

In RETNET, liver progression-free survival and symptom control did not differ significantly between the two. The team chooses based on liver function and tumor features.

How is a carcinoid crisis prevented?

UK hospitals give intravenous octreotide starting 8–12 hours before the procedure and continue it afterwards.

Is Y-90 radioembolization available in Japan?

As of November 2026, a pre-approval study of glass-microsphere Y-90 is running at 5 Japanese sites. In Europe, NET liver metastases are among the CE-marked indications.

Related: Portal vein embolization · Tests before liver procedures

This article summarizes overseas official materials and studies as general information, not medical advice. Treatment decisions should be made with the doctors who examine the patient.