Dosimetry for Beta-Emitter Radionuclides by Means of Monte Carlo Simulations

Nowadays, there are interests as well as active investigations devoted to the study and application of radiolabeled molecules able to selectively target and irradiate tumoral cells during nuclear medicine procedures. With this kind of pharmaceuticals, spatial activity distribution with extremely non-uniform characteristics may be assessed in patients. Actually, this feature constitutes precisely themain advantage in view ofmaximizing the discrimination between affected and healthy tissue. The mentioned situation constitutes the main motivation for the present work. In this sense, the chapter is focused on nuclear medicine dosimetry pointing out the main features about how to implement Monte Carlo (MC) approaches to this aim. Nowadays, from a general point of view, therapies with radiopharmaceuticals using beta-emitter radionuclides are growing significantly and very fast. Beta-emitters can be emitters of β− or β+ radiation. Commonly, β+ emitters, like 18F are used for imaging techniques, whereas β− are mainly used with therapeutic purposes, to deliver high dose rate on tumors. Therefore, β− emitters are usually those of more interest for dosimetry. During nuclear medicine procedures, radiopharmaceutical activity distribution may be determined by means of different modalities. Nowadays it is mainly assessed using imaging techniques but otherwise it is also possible to infer it [Stabin (2008)]. This information is then incorporated in the treatment planning system in order to obtain an estimation of the dose distribution. More specifically, patient-specific dose distribution owing to alpha, beta and/or gamma emitters can be calculated starting from activity distribution by means of either direct MC simulation or analytical methods. On the other hand, patient-specific dosimetry requires anatomical information, which shall be further considered as input for establishing mass distribution during MC computations. Patient anatomical information can be suitably extracted from typical non-invasive imaging techniques, like computed tomography (CT) or magnetic resonance imaging (MRI). Many studies have been performed by means of MC applications in Nuclear Medicine up today, both in the imaging field, and regarding dosimetry calculations [F. Botta & Valente (2011), Zubal & Harrel (1992), H. Yoriyaz & dos Santos (2001), M. Ljungberg & Strand (2002)]. I troduction 11