Neuroendocrine tumours (NETs) are tumours which arise from cells of the neuroendocrine system, which is the mechanism by which the hypothalamus maintains homeostasis. Neuroendocrine cells secrete hormones, a type of signalling molecule, that play a role in many of the body's different processes including growth and development.
Surgical resection remains the only curative approach to NETs. However, due to non-specific representations, over 50% of NETs are unresectable at diagnosis. Patients with metastatic high grade, poorly differentiated tumours have a median overall survival of 5 months, as compared to 33 months for those with metastatic low or intermediate grade, well differentiated disease. The liver is the most common site for metastasis and is directly correlated to a much poorer prognosis if moderately to poorly differentiated disease is seen, with 5-year survival of 50% less than those without liver metastases.
Current treatment for non-resectable NETs include somatostatin analogues, systematic anti-cancer therapy (with etoposide/carboplatin being the most commonly prescribed regime), radionuclide therapy, Meta-iodobenzylguanidine Therapy (MiBGG) and Peptide Related Radiation Therapy (PRRT). The last showing great promise in clinical trials with progression free survival at 20 months of 65.2% in the Netter1 trial. However, this therapy is reliant on tumoural expression and a key density of the somatostatin 2 receptor (SSTR2), a feature that is often lost with increasing grade and aggressiveness of disease. In short, the current proposed treatments for non-resectable NETs are still to be optimised and depend heavily on patient status, SSTR2 expression and in the case of chemotherapy, is based on evidence of treatment regimens for other types of cancer.
Selective Internal Radiation Therapy (SIRT), by hepatic arterial delivery of yttrium-90 (Y-90) labelled microspheres, is a safe and effective locoregional therapy that combines a dual anticancer therapy, combining the effects of hepatic arterial embolization with targeted delivery of high dose radiation. This selectively delivers a tumouricidal dose of beta-radiation to the liver tumour, while maintaining a low radiation dose to surrounding normal tissue. 20% of the blood supply of healthy liver comes from the hepatic artery, while 90% of liver tumours derive \>90% of their blood supply from the same artery and so the hepatic artery is therefore a compelling target to deliver largely tumour specific treatment, sparing healthy liver. SIRT microspheres are also small enough to get trapped in the liver microvasculature but too large to pass through capillary beds and should therefore not reach other places of the body to cause unwanted side effects.
TheraSpheres consist of an yttrium-90 containing glass microsphere. Yttrium-90 is a pure beta emitter with a half-life of approximately 64.1 hours. It has the same toxicity profile as many chemotherapy agents with the common side effects reported as fatigue, anorexia, pain, nausea and vomiting. In a past clinical trial on NETs, SIRT has shown a radiological response rate of 63% and a median survival of 70 months.
