الفهرس 
Introduction and aim of the work
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Abstract
The objective of radiotherapy is delivering the necessary dose to the tumor-bearing areas and spares all the normal tissues. Numerous treatment techniques and field arrangements for different tumors and body sites have been developed over the years.
In the present work, we studied the effect of some physical and technical aspects on the accuracy of the calculation dose in (3D-TPS).
•from the CT-number and electron density relationship, treatment planning system used it to correct the heterogeneity of the human body structure according to the electron density of the body relative to the CT-number from the CT machine. For most regions of the body it is appropriate to treat the patient as water equivalent
•As a result of the decrease of the skin dose according to the prescribed dose with decease the source surface distance, in many cases the isocenteric technique is the favorable technique than the source-surface distance technique.
•Because of electron contamination, it is preferable to make the measurements at a depth larger than maximum dose point and then to relate the measured outputs to maximum dose point through the use of Percentage depth dose or tissue maximum ratio.
•The treatment planning system is very accurate for determining the half value layer for the Cerroband blocks used in the blocks and compensator used to protect or compensate the dose for the patients.