الفهرس 
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Abstract
The goal of radiation therapy is to administer a therapeutic dose of radiation to a target while limiting the side effects caused by delivering the dose to surrounding tissues and vital organs. The ongoing pursuit to achieve an optimal dose distribution has prompted the radiation therapy profession to develop new techniques that incorporate advances in technology. In radiation therapy today, modern techniques that include three-dimensional conformal radiation therapy (3D-CRT) and intensity-modulated radiation therapy (IMRT) are routinely used in the treatment of cancers.There are many advantages for IMRT over 3D-CRT technique. IMRT is capable of sculpting dose distributions to a complex target volume involving concave and convex portions. The technology of IMRT also allows for rapid dose falloff sparing surrounding critical structures. Improved protection of the surrounding tissues using IMRT has decreased the side effects in comparison to 3D-CRT methods in virtually every type of cancer treated.The improved dose distribution achieved using IMRT permits dose escalation to the target volume.Another advantage of IMRT is that it allows a simultaneous integrated boost, which allows the delivery of a higher dose per fraction to areas considered at high risk of disease while prescribing a lower dose per fraction to lower risk regions.
This study included 45 cancer patients (15 prostate cancer patients, 15 nasopharyngeal carcinoma and 15 breast cancer patients) performed with radiotherapy. A total of 90 plans were generated by performing 3D-CRT plan and IMRT plan for each patient.
Our results showed that:
Prostate Cancer:
The mean doses received by PTV D95% and PTV D5% were significantly higher in IMRT plan than in 3D-CRT plan. So IMRT plan had a good PTV coverage than 3D-CRT plan. There was improvement in conformity index and homogeneity index in IMRT plan than in conformal plan. The monitor unit was significantly higher in IMRT plan than in 3D-CRT plan.
Regarding dosimetric parameters for organs at risk, the comparison of IMRT plan with 3D-CRT plan in the same percentage of volume showed that, the doses to the rectum, bladder, right and left femoral head irradiated with IMRT-plan were lower than that for 3D-CRT plan.
Head and neck cancer (Nasopharyngeal carcinoma):
The mean doses received by 95 % of PTV70 (D 95%)and PTV54(D95%) were significantly increased in IMRT plan than in 3D-CRT plan. Both homogeneity index and conformity index significantly improved in IMRT plan than in conformal plan. Furthermore, the monitor unit was significantly increased in IMRT plan than in 3D-CRT plan.
With respect to dosimetric parameters for organs at risk, the comparison of IMRT plan with 3D-CRT-plan showed that, the IMRT plan was superior to 3D-CRT plan in reducing the doses to brain stem, cord, RT parotid, LT parotid and RT and LT optic nerve compared to the 3D-CRT plan. Regarding LT and RT lenses, insignificant
difference was found between IMRT plan and 3D-CRT plan but both results were in tolerance.
Breast cancer
The mean doses received by PTV D95% and PTV D5% were significantly higherin IMRT plan than in 3D-CRT plan. There was improvement in conformity index (CI) and homogeneity index (HI) in IMRT plan than in conformal plan. The monitor unit was significantly higher in IMRT plan than in 3D-CRT plan.
The comparison of IMRT plan with 3D-CRT plan showed that, the mean doses delivered to both lungs, heart, contralateral breast and ipsilateral lung irradiated with IMRT-plan werelower than that for 3D-CRT plan.