Selective Internal Radiation Therapy—Tackling the Tumor, Sparing the Organ

US Oncological Review, 2008;4(1):68-71

Primary liver cancer is the fifth most common cause of cancer in the world, with hepatocellular cancer (HCC) accounting for a large majority of cases. HCC has the highest incidence in East Asia and sub-Saharan Africa and is the fastest-growing cause of cancer-related death in the US.1 The liver is also a common site for metastases from other primary cancers, especially colorectal cancer because of portal venous drainage of the bowel. Approximately 20% of colon cancer patients are found to have stage IV disease at the time of initial diagnosis.2 In 40% of these cases metastases are confined to the liver, and most of these patients have no extra-hepatic metastases, even at the time of death.3
The only curative therapy for liver cancer is surgical removal of the tumor.4,5 Surgery may prolong survival in well-selected patients with liver metastases from colon cancer,3 although only 10–20% of patients with liver metastases are reasonable surgical candidates.4 If surgery is not possible, local ablative therapies such as radiofrequency ablation are possible for some patients with a limited number of small tumors.6 For those with liver tumors who cannot be treated surgically or with ablative therapy, the prognosis is generally poor. Systemic chemotherapy can downsize some metastases to the point of resectability,3 but primary liver tumors do not generally respond to this treatment.1 External-beam radiation is limited by the relatively poor tolerance of the normal liver to standard radiation techniques.7
The vascular supply of liver tumors offers a unique opportunity to selectively deliver therapeutic agents to liver tumors while sparing the rest of the organ. Liver tumors receive >80% of their blood supply via the hepatic artery, whereas the portal vein provides about 75% of the blood supply to the normal liver parenchyma.8 In addition, the microvascular density of liver tumors is three to 200 times greater than the surrounding liver parenchyma.9 These differences have been exploited in the development of treatments that are delivered via the hepatic artery, including transarterial chemotherapy, embolization to induce tumor ischemia, and embolization using drug-eluting microspheres or radiation-emitting microspheres.8
Two types of selective internal radiation therapy (SIRT) radiolabeled microspheres are available, one of which is made of glass (TheraSphere®, MDS Norton Inc., Canada) and the second of which is made of resin (SIRSpheres ®, Sirtex Medical Inc., Australia). Both carry 90yttrium (Y), a beta emitter with a half-life of 64.1 hours and an average energy of 0.94 megaelectron volts (MeV), which corresponds to a maximum range of 1.1cm within tissue with a mean path of 2.5mm. The two formulations differ considerably in terms of particle size and average dose of radioactivity delivered.10 In 2002, the US Food and Drug Administration (FDA) gave pre-market approval of SIR-Spheres as a brachytherapy device for the treatment of metastases from colorectal cancer. TheraSphere is approved as a humanitarian device for use in patients with unresectable HCC.8
Treatment with Sir-Spheres is considered to be the third-line option for patients with colorectal metastatic disease that is confined to the liver or with minimal extra-hepatic involvement. A few reports have suggested better long-term survival in patients with colorectal cancer when SIRT was used in conjunction with other chemotherapy.11 In most cases, patients have exhausted other treatment options and no longer respond to chemotherapy, cannot tolerate or refuse chemotherapy, or choose to receive both SIRT and chemotherapy.12
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