RESUMEN
In vivo dosimetry is a method for estimation of overall error in the delivered dose to the patients at the end of radiotherapy process. In this research, two methods for target dose calculation were evaluated on midline and central axis of photon beams in in vivo dosimetry of thorax fields. Entrance and exit doses for anterior and lateral fields of thorax were measured in thorax phantom using diode dosimeter. Also, the doses of some points on midline and central axis were measured in thorax phantom using ionization chamber. The dose at these points was calculated using entrance and exit doses by geometric and arithmetic mean methods. The calculated doses were compared with measured doses. In all cases, arithmetic mean method showed errors from%8.8 to 19% for points on midline and central axis in comparison to measurements. The range of errors for geometric method was from%1.5 to%8 depending on distance from midline. The results showed that doses of points on midline and central axis can be calculated with acceptable accuracy from entrance and exit doses using geometric mean in thorax fields