الفهرس 
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Abstract
Gamma and x-rays beams used in radiotherapy are contaminated with secondary electrons. Skin dose has two components depending on secondary electrons produced from photon interactions with air, collimator jaws, the patient surface and any other scattering materials these components are: (1) secondary electrons generated in the patient, and (2) contaminant electrons from the treatment head. It is not possible to change the effect of treatment head materials on skin dose in clinical applications, but skin dose can be changed by using different treatment setup parameters. Therefore, the knowledge of how parameters affect the skin dose is essential for proper treatment.
Many authors have assessed dose in the surface and buildup region for normally incident photons and there are a number of articles in the literature concerning surface dose and buildup at open fields, source to surface distances, oblique angles (gantry angles), various blocks tray, filters, and wedge angles. The aim of the present work is assessment the dose distribution in buildup regions generated by our beams with different treatment setup parameters, applying the Eclipse TPS to calculate the dose distribution correctly in buildup regions, and drawing a model able to predict the skin dose with precision of about +/- 5.0 % for the most treatment conditions available applying both treatment units (cobalt (T780E) andlinear accelerator). A ’Markus’ parallel plate PP chamber was used for these measurements in bothcobalt (T780E) and Varian (6.0 MV -15 MV) linear accelerator. Cobalt beam build-up zones were very poorly described by the conventional beam data acquisition procedures and they are usually flatter than those produced by higher energy machines. All our measurements were done in the central part of the beam and were assessed for dose at entry surface.