Caren van Roekel

17 Radioembolization 1. BASIC PRINCIPLES AND INDICATIONS 1.1. Radioembolization Also known as Selective Internal Radiation Therapy (SIRT), radioembolization is a therapy during which radioactive microspheres are injected into the hepatic artery. A microcatheter is placed in the hepatic arterial vasculature and millions of microspheres are administered. Hepatic malignancies are fed mainly by arterial blood and because of preferential arterial flow, the microspheres lodge in small tumor arterioles. There, they emit high-energy β-radiation to induce cell death. In this way, the hepatic tumors are selectively irradiated and the healthy liver tissue is relatively spared, which is a great advantage over conventional external beam radiation therapy (EBRT) (2). Radioembolization is a fast developing field of expertise and therefore, in this chapter, we will confine ourselves to the basics. The essential steps for radioembolization include [1] visceral angiography to map tumor-feeding vessels, embolize collateral vessels and assess portal vein patency, [2] assessment of pulmonary and gastrointestinal shunts by intra- arterial administration of a scout dose, and [3] determination of the optimal therapeutic activity, e.g. dosimetry (1). 1.2. Types of microspheres Ideal properties of radiolabelled micro-particles for sufficient intra-arterial therapy are (3-5): • The microspheres should be easily labelled and resistant to elution of the radioactive label, macrophage removal or radiolysis. • Uniform microsphere size (i.e. greater than microcapillary diameter of around 8 micron and small enough to lodge as distal as possible) and density comparable to that of blood are necessary to prevent settling and ensure uniform distribution. • For efficacy, the radionuclide label must be a high-energy β-emitter with sufficient range and an intermediate half-life of a few days. • The microspheres should be visible by PET (positron emission), SPECT (γ-emission), CT (sufficient density), or MRI (paramagnetic properties), to assess distribution. 2