الفهرس 
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Abstract
The goal of radiotherapy is to deliver a homogenous dose of radiation to tumor, while delivering a dose as low as possible to healthy surrounding tissues. Conventional 3DCRT delivers a homogenous dose to tumor volume with acceptable low dose to normal structures. However, in some cases of tumor with concave shape such as HNC, limits the ability of conventional radiotherapy to shape the dose to the target volumes and to spare the OAR. Significant advances in imaging technology result in more precise localization of tumor and critical organs in 3D.These developments have been mainly driven by the need to reduce the dose to normal tissues. To that end, newer IMRT have been developed. IMRT is an advanced form of high precision of 3DCRT, which use linear accelerator to deliver precise radiation dose to tumor. IMRT allows to deliver radiation dose to conform more precisely to 3D concave shape tumor by modulating the intensity of radiation beam in multiples segments which minimizing the dose to surrounding healthy tissues. Typically, combination radiation beams intensity modulated fields coming from different beam directions produce precise shape of radiation dose. IMRT techniques for treatment HNC replaced conventional 3DCRT, which resulted in much better dose conformity, sparing of OARs and less radiation toxicity.
IMRT divided in to forward and inverse planning. In the forward planning, the medical physicist selects the planning parameters, the computer then calculates the dose distribution and the plans are optimized by the manual iteration. The inverse planning begins by defining the prescription dose to the target volumes with clinical objectives then the planning system algorithm determine the beams parameters which results dose distribution for the targets and the system undergo thousands of iterations to find the best solution for the treatment plan. Inverse IMRT for HNC is a complex due to the large number of OARs locates near to the PTV, so the correct selection of the beams number and direction in HNC IMRT improve the PTV coverage as well as sparing the OARs and reduce the dose to the surrounding normal tissues. The modulation of IMRT can achieve by three delivery techniques: step and shoot IMRT, dynamic IMRT and IMAT with tomotherapy or VMAT. The step and shoot is most commonly available in cancer treatment centers. In this delivery techniques, the beams divided into different segments and the radiation is turn off between the segments. The MLC shape the first segments then the radiation turn on to delivers into the first segment, the radiation then turn off to allow the MLC move to create the next segment and soon. In the dynamic the radiation delivered as the leaves are moving.
The treatment of HNC by IMRT requires 3D definition of the target volumes and OARs. The IMRT process consist of multiple steps for treatment planning until delivery of radiation. IMRT is more conformity for irregular targets and reduce the dose to the OARs. The main disadvantage is increase the treatment delivery times and MUs. This lead to patient discomfort, reduce the machine output and increase the dose to the surrounding healthy tissues around the PTV which arise from the MLC transmission and scatter radiation from the linear accelerator, these doses proportional to the number of MUs. These scatter radiation can increase the risk of secondary malignances. Reduction of irradiated time can be achieved by using different number of beams or segments or by using high modern delivery techniques such as VMAT. Better if IMRT is restricted to lower MUs with faster irradiated time, so these advanced techniques are not commonly available in most countries. The fundamental factors that determine the quality of a plan are the number of beams and their angles. Generally, a larger number of beams would a greater flexibility to achieve a desired dose distribution. However, the more number of beams used, the more the effort is required for planning, QA and treatment delivery. Practically, it is desirable to reduce the number of beams to as few as possible without compromising the quality of the treatment plans. The use of multiple beams from various angles provides more degrees of freedom and thus improves the dose distribution and reduce the dose to the normal tissues.
The treatment planning optimization system help to determine the distribution of the beam intensities which across the treatment volumes, the optimization explores these possibilities to find the optimum intensity maps that are specified with dose and volumes constraints and objectives for PTV and OARs using system priorities. The different plans can be evaluated and compared to select the optimum intensity modulation. The optimum pattern then converted to complex sequences of beam segments which can deliver by moving the MLC during treatment. Comparisons of different beams with regarding target coverage, HI, CI, OARs sparing, MUs and total irradiated time are procedures to determine the superiority of any particular technique.
The aim of the work was to determine the best beams number and segments in order to improve the plans conformity and homogeneity that generate low MUs and faster irradiated time for different types of HNC. This study includes 30 patients with different HNC. IMRT treatment planning techniques were done with step and shoot delivery technique, 5, 7 and 9 beams IMRT were carried out for each patient. The treatment plans for all patients were calculated and optimized using fast superposition algorithm. All plans were generated using equal spaced odd beam number around the target. In all beams 6 MV were used. Multiple segments were created for each beam. Typically maximum iteration was carried out to achieve optimized plans. The beam weight optimized to generate the plan, then the segment weight optimized for all plans by using sliding window methods. The final optimization maps were converted into a way of step and shoot sequence map which delivered by linear accelerator using MLC. IMRT plans were compared based on several criteria: Isodose distributions, the mean and standard deviation with p-values for PTV95%, CI, HI, OARs, number of segments, MUs and total irradiated time were presented and compared in all patients. Statically analyses were compared for all patients used ANOVA testes.
The total results showed that, there was significant difference between 5, 7 and 9 beams IMRT in term of mean values for PTV95% coverage were 96.76, 97.51 and 98.22 respectively with p = 0.005. The conformation mean values were 1.60, 1.49 and 1.34 with p = 0.007. HI values for the PTV were 0.14 ± 0.05, 0.13 ± 0.05 and 0.12 ± 0.04 with p = 0.001. Right parotid were 21.96, 20.72 and 20.43 with p = 0.003. Left parotid were22.14, 21.04 and 20.70 with p = 0.100. Spinal cord 45.34, 44.51 and 43.23 with p = 0.003. Brain stem were 49.52, 49.77 and 48.74 with p = 0.058. Number of segments were 79.85, 106.55 and 131.80 with p = 0.001. MUs were 23879.8, 24252.6and 22501.8 with p = 0.003 and the total irradiated time were 79.60, 80.84 and 75.0 respectively with p = 0.003. In fact that, the plan quality improved with an increasing the number of intensity modulated beams.