**1. Introduction**

Radiation therapy would not exist without physics. It begins with the discovery of X-rays. This therapy uses ionizing radiation, which is delivered by a linear accelerator. Linear accelerator is a device that uses high-frequency electromagnetic waves to accelerate charged particles, such as electrons to high energies through a linear tube. The high-energy electron beam itself can be used for treating a superficial target, or it can be made to strike a target to produce x-rays for treating a deep-seated target. Radiation therapy works by damaging the DNA of cancerous cells. Photons cause indirect ionization, which happens as a result of the ionization of water, forming free radicals, which then damage. Charged particles, such as electrons, protons, boron, carbon, and neon ions can cause direct damage to target through high-LET (linear energy transfer) [1]. The main focus of physics in radiation therapy is to increase the level of precision

and accuracy of dose delivery to the target volume. From the 1950s to the late 1980s, the approach to radiation therapy was based on a two-dimensional (2D) approach. In 2D radiation therapy, plans were created manually, and a single beam used to be given from one to four directions [2]. Shielding blocks were used to collimate the beam. Advances in imaging technology like Ultrasound (US), Computed Tomography (CT), Magnetic Resonance Imaging (MRI), etc. significantly changed the practice of radiation therapy from the 2D method to a Three Dimensional Conformal Therapy (3DCT), which conforms to the high radiation dose with uniform intensity to tumor. For precise shaping of treatment field to the target volume, Multi-Leaf Collimator (MLC) system was developed in place of shielding blocks [3]. Advanced form of radiation therapy called Intensity Modulated Radiation Therapy (IMRT) has been developed in the mid-1990s and early 2000s. IMRT can provide conformal dose distribution compared with 3DCRT [3]. Intensity-Modulated Arc Therapy (IMAT) uses the Multi-Leaf Collimator (MLC) dynamically to shape the fields, as well as rotate the gantry in the arc therapy mode. Intensity-Modulated Arc Therapy (IMAT) was further improved with the addition of variable gantry rotation speeds and dose rates and was introduced as volumetric-modulated arc therapy (VMAT) in 2007. Brief descriptions of all these techniques are discussed in the following sections.
