**1. Introduction**

#### **1.1 Radiation therapy (RT)**

Radiation therapy (RT), a local therapeutic modality for cancer, uses beams of ionizing radiation to inhibit or control the growth of tumor cells and can be practiced alone or in conjunction with other therapies [1]. It is used in approximately 50–60% of cancer treatments for curative and palliative purposes [1–3].

There are two modalities of RT, namely brachytherapy and teletherapy. Brachytherapy uses a source of ionizing radiation that is in contact with tumor tissue and allows higher doses of radiation to reach the target tissue [4, 5]. Teletherapy, also called external RT, is the most common type of RT and is performed using machines, typically linear accelerators, which allow a source of ionizing radiation to be positioned at a certain distance from the patient and programmed to focus on the tumor [4, 5].
