Investigation of the dose distribution by irregular field-shaping device for photon beam radiation

Abstract

This research work reports the investigation of dose distribution by the irregular field shaping device for photon beam radiation. In this work phantom of size 50 cm x 45 cm, thickness 1.5 cm and the irradiation facilities Co-60 teletherapy unit were used. During dose measurement 0.6 cc ionization chamber and PTW -UNIOOS electrometer were used. At first open field doses for various field sizes such as 5x5cm; lOx 10cm, 15x 15cm, 20x20 cm, 25x25 cm for lateral, vertical and diagonal distances were investigated. The "Half-beam device" which is used for the breast cancer treatment was used as one of the irregular field shaping devices. Three other irregular field shaping devices were made with several rectangular blocks (made of shielding material) inserting into the shielding tray. The doses on points of both side of the central axis were measured by using the irregular field shaping devices. The data were analyzed by making graphs using the points of dose measurement vs. the dose rate of these points and thus showing the effect of the irregular field-shaping device, which are used for the treatment of cancer. Another special block (made of lead alloy) which is routinely used for the pelvis cancer treatment was used with tissue-compensator for the dose measurement of various points, thus to find the effect of the tissue-compensator. For finding the percentage of deviation for doses, the open field data was compared with the blocked field data for the same field size 15x 15cm. The estimated deviations were satisfactory.