الفهرس 
Introduction and Literature ReviewOnly 14 pages are availabe for public view
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Abstract
There were an estimated 14.1 million cancer cases around the world and 8.2 million cases died from them (Ferlay et al., 2012).Today, millions of people are living with cancer or have had cancer. Accordingly, radiotherapy is highly used in cancer treatments. It uses high-energy photons or particles, such as x-rays, gamma rays, electron beams and protons, to destroy cancer cells. It is also called radiation therapy, irradiation, or x-ray therapy. It can be delivered alone or used with other treatments, as surgery, hormonal therapy and chemotherapy. In fact, specific drugs are known as radio sensitizers which can actually let the cancer cells more sensitive to radiation and helps the radiation to kill cancer cells well. Cancer cells grow and divide faster than many of the normal cells around them and the radiation works by making small breaks in the DNA molecules within the nucleus in the cells. These DNA breaks stop the cancer cells growing and dividing, causing them to die. The surrounded normal cells can also be affected by radiation, but most recover and go back to working well.
The highest aim for radiation therapy or any cancer therapy is to attain the highest cure with the least toxicity. So, to achieve this, the radiotherapy has been improved to deliver a prescribed dose while minimizing the dose to the surrounding tissues using some advanced techniques such as Intensity- modulated radiotherapy (IMRT) and Volumetric Modulated Arc therapy (VMAT).
Intensity-modulated radiotherapy (IMRT) delivered using step-and- shoot or dynamic sliding window multileaf collimators (MLC) and
Chapter 1 Introduction and Literature review
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Volumetric-modulated arc therapy (VMAT), which combines dynamic MLC delivering along with gantry speed and dose rate modulation, have facilitated complex treatments. These advanced techniques are so complicated and need to be verified before the patient will undergo the treatment, so quality assurance (QA) for IMRT and VMAT is developed to meet this treatment complexity and to verify the desired fluence and hence dose distribution delivered by the linear accelerator.
Recently electronic dosimeters have been introduced by many vendors which able to provide 3D dose information .Octavius 4D (PTW, Freiburg, Germany), is one of such systems that provides measured 3D dose distribution of the IMRT & VMAT plans.
Intensity-Modulated Radiation Therapy (IMRT):
The intensity-modulated fields concept created through inverse planning was first introduced by Brahme in 1988 (Brahme et al., 1982). Given different beams and a required dose distribution, the optimum MLC of the beams could be determined by back projection, leading to non-uniform beam intensities.
Volumetric modulated Arc Therapy (VMAT):
Volumetric modulated arc therapy (VMAT) is a developed radiotherapy treatment modality that delivers radiation while the gantry and emitted radiation are in continuous motion. VMAT allows the dose rate, gantry speed and multi leaf collimator shape to vary during the treatment. However IMRT treatments require setup time in between successive beam angles, VMAT treatments are usually delivered in one or more continuous arcs, therefore reducing the treatment time. Yu., 1995 introduced the intensity modulated arc therapy and led to VMAT systems laterally.