Radiation TherapyAt the beginning of the radiation therapy, before 1950, the kilovoltage x-rays was used for external radiotherapy, which was up to 300 kVp. Later a higher energy radiations were used such as the cobalt-60 radiation and higher energy machines. Eventually the conventional kilovoltage machine started to demise gradually although it is still used today for limited purposes such as superficial skin lesions. A continuous development in manufacturing the high energy machine started the era of the megavoltage beams for external radiotherapy. X- or gamma ray beam are used widely in radiation therapy, this is because the interaction of their beam photons with human body tissues can destroy the tumor cells reproductive capacity. The mechanism of the interaction solely based on the transferred energy from the beam photons to the medium; this initially will cause electrons of the atoms of the absorbing medium to be ejected from atoms. These electrons are moving with high speed and energy, which is sufficient to produce ionization and excitation of the atoms along their path in the tumor tissues. If the deposited energy in the cancerous cell is sufficient, then it will destroy the cell and its component, such as the DNA. Eventually at the end, the cancerous cell will lose its power to reproduce and proliferate and die. However, this sufficient energy is a small portion of the absorbed one; the majority of the absorbed energy is converted to heat which is not sufficient to produce significant biological effect. This situation is desired when the ionizing radiation has to pass through a healthy tissues in order to be delivered to the lesion. The main goal or radiation therapy is to attain the highest cure’s probability and the with least morbidity. This goal can be achieved by maximizing the radiation dose to abnormal cancer cells while minimizing it to the normal cell which are in the path of radiation or adjacent to cancer cells. Fortunately, although the radiation damages both healthy cells and a cancer cells, but healthy cells, usually, have more capability to repair themselves at a faster rate, so they can retain their normal functions than the cancer cells. In general, cancer cells are less efficient than health cells in repairing the damage caused by radiation, and this leads to differential cancer cell killing.Since the beginning, different radiation therapy techniques were introduced, improved and escaladed. These techniques are falling into two categories. The first radiotherapy category delivers the radiation dose to the tumor internally, where the radiation source presents inside the patient body, and this is what is called brachytherapy. The second category is most common in radiotherapy and is called external beam radiotherapy or teletherapy; where the radiation source present outside the patient body, and precisely irradiates the lesion with an accurate prescribed dose. External beam radiotherapy is wide and it contains different radiotherapy techniques. Such as intensity modulated radiation therapy IMRT, volumetric arc therapy VMAT or Rapid Arc, image-guided radiation therapy IGRT, stereotactic radiosurgery SRS and stereotactic body radiation therapy SBRT. This work is dealing with the last two techniques, SRS and SBRT, and they are explained below. However, the other techniques are not since they are out of the scope of this work.Linear Accelerator TreuBeamGeneral InformationA linear accelerator is the most common device to treat cancer with external beam radiation. It is a device that uses high Radio-Frequency (RF) electromagnetic waves to accelerate charged particles i.e. electrons to high energies in a linear path, inside a tube like structure called the accelerator waveguide. The resonance cavity frequency of the medical linacs is about 3 billion Hertz. Then allows these electrons to collide with a heavy metal target. As a result of these collisions, high energy X-Rays (Photons) are produced from the target. These high energy photons will be directed to the patient’s tumor and shaped as they exit the linac to conform to the shape of the tumor. The Gantry is a multi ton piece of metal, it rotates 360 degrees around a point which is called the Isocenter. It contains three components: the electron gun, the accelerator structure and the treatment head. The treatment head contains the treatment beam shaping and monitoring components. Radiaion can be delivered to the tumor from any angle by rotating the gantry and moving the treatment couch.Multi Leaf Collimators (MLC’s)The radiation beams are collimated by adjusting the upper and lower collimator jaws. The jaws are made of high atomic number, Z number, like Tungsten or Lead. The jaws can define a rectangular shaped beam up to 40 cm by 40 cm for X-ray beams. but, as usually the treatment volume is not rectangular, additional shaping is required. Here a lead blocks are attached onto the treatment head under standard collimating system, or the Multi Leaf Collimators (MLC’s) are used.MLC’s are heavy metal field-shaping devices with independent movable leaves used to create a custom like block to spare normal tissue and direct the radian dose to the tumor. Typical MLCs have 40 to 120 leaves, arranged in pairs. By moving and controlling a large number of narrow, closely abutting individual leaves, one can generate almost any desired field shape.In TrueBeam machine from Varian. There are 120 leaves, over a 40×40 cm field. The central 20 cm of the field have 5 mm leaf width, where the outer 20cm of the field have 10mm leaf width. Each leaf can be derived separately as it going to be shown later. Electronic portal imaging devices (EPIDs) Electronic portal imaging devices (EPIDs) are used to measure the x-ray intensity transmitted through a patient from a radiation port during a treatment session. The radiation signal is converted electronically into a two-dimensional (2D) digital radiographic image to verify the correct beam placement in relation to the patient’s anatomy.