الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
CRCs are the third most commonly diagnosed form of cancer globally, comprising 11% of all cancer diagnoses. They are the third leading cause of cancer death worldwide. cancer of the colon is the fourth most incident cancer in the world, while cancer of the rectum is the eighth most incident.
Radiotherapy has already been considered as the standard neoadjuvant treatment for locally advanced cancer rectum. However, we should consider the relationship between position, method of immobilization and the risk of organs at risk (OARs) including small bowel ,bladder ,skin toxicity and head of femur .The small bowel is the most importan dose-limiting structure.
To decrease dose to small bowel the acute or chronic radiation injury, various methods have been advocated to reduce the toxicity of pelvic radiation treatment, such as prone position with a belly board, three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT).
The prone position with a belly board can decrease the volume and the radiation dose of small bowel . IMRT and VMAT or some other methods were confirmed to decrease the irradiated dose and volume of intestine . Different positions, methods of immobilization and techniques also have effect on set-up errors, reproducibility which are essential for accurate delivery of radiation to target organ while avoiding organs at risk. But the lack of overall quantitative analysis of these methods made it difficult to determine the optimal decision for clinical radiation oncologists, and the thorough description of VSB threshold compared with acute toxicity has not been reported yet.