**2. Radionuclides**

The growth in nuclear medicine has been stimulated by introducing several new radionuclides and radiopharmaceuticals. They have been used to treat benign and malignant tumors. Types of radiation that are relevant to RPT are electrons and α-particles. Electron emissions are classified by energy and by the type of decay; Auger electrons, beta (β)-particles are related to RPT [2]. Currently, β particle emitters radionuclides are mostly used for therapy purposes. However, they have a limitation of radiobiological properties. An alpha particle emitting as a new generation radionuclides is being developed, with advantages in high energy and a short path length, which show higher efficacy [19]. Below we discuss the physical differences between a beta particle, Auger electrons, and an alpha particle.

### **2.1 Beta particles**

Beta particles are produced in the beta decay process, wherein an unstable nucleus, a neutron, is converted to a proton, creating an energetic electron (beta particle) [2, 19]. They are the most frequently used for RPT agents and widely available. Many of them also emit photon energy that is easily imaged. Beta particles are negatively charged and have a relatively long path length from 0.0 to 12 mm.

They have low linear energy transfer (LET) of approximately 0.2 keV/μm, and more particles are required for a similar absorbed dose as alpha particles (**Figure 2**). For high energy beta, like 90Y and 188Re, which they energy 2.28 Mev and 2.21 MeV respectively (**Table 1**), they can cause crossfire doses to neighbor cells. So, they are preferable for higher volume solid tumors, poorly perfuse tumors, and less suited for targeting micro-metastases [3, 8, 23]. For the small tumor, low-energy β-rays such as lutetium-177 (177Lu) would be more β-emitting efficient [8].

The most familiar and frequent beta particle used is iodine-131 (131I) for hyperthyroidism and thyroid cancers therapy [20, 24, 25]. Subsequently, samarium-153, lutetium-177, yttrium-90 and have been introduced over the last 40 years (**Table 1**) [2]. Several other β-emitting radionuclides have been investigated or considered. However, those agents have not widely adopted, related to several reasons: limited availability, complex radiochemistry process, or the absence of a commercial products [26]. A variety of reasons for the shift to different radionuclides of the different β-particle emitters used over time. For example, an early evaluation of changing to different radionuclides was based on the tumor to non-tumor-absorbed dose ratio [2]. 90Y has a high-energy β-particle, and it is widely available like 131Iodine. It was used in colloidal form, mainly for rheumatoid treatment [27, 28]. 90Y labeled antibodies initially focused on ovarian cancer, followed by hematological cancers and radiopeptide therapy [2, 29, 30]. 90Y is a popular radionuclide for RPT because of the clinical impact of 90Y-impregnated microspheres used for hepatic metastases therapy [31–33]. Lutetium-177 becomes

#### **Figure 2.**

*Linear energy transfer alpha and beta particles and auger electron on DNA. Alpha particles have high LET (*∼*80 keV/*μ*m) compared with the low LET (*∼*0.2 keV/*μ*m) of beta particles, and auger electron intermediate LET 4–26 keV/*μ*M. Thus, alpha particles result in more double-strand breaks in DNA.*


*Radiopharmaceuticals in Modern Cancer Therapy DOI: http://dx.doi.org/10.5772/intechopen.99334*

#### **Table 1.**

*Characteristics of alpha and beta emitters radionuclides for therapy [6, 8, 20–22].*

popular because it emits photons in the 100–200-keV optimal imaging range and has a β-particle energy between 131I and 90Y, which is appropriate for therapy, particularly for small tumors [2, 8]. All these factors, along with a half-life that is compatible with the pharmacokinetics of both antibodies and peptides. It is a reactor production and widely available, with relatively straightforward conjugation chemistry [2]. Samarium-153 (153Sm) is a β-emitting radionuclide that is used for palliative treatment in breast and prostate cancer with bone metastases, and other primary cancers [34, 35]. Radiopharmaceuticals therapy agent that uses the ethylenediamine-tetra-methylene-phosphonic acid (EDTMP) chelator, binding samarium-153 through six ligands (four phosphate groups and two amines) is FDA approved. 153Sm alternative formulation as 153Sm-DOTMP (1,4,7, 10-tetraazacyclododecanetetramethylenephosphonic acid), which is thought to have a more favorable chelant-to-metal ratio [2].
