Establishment of correction factors for caesium-137 teletherapy machine at Ocean road cancer institute

To achieve effective clinical outcome in radiation therapy, high accuracy of dose determination and delivery to patient is required. To meet this, entry of correct beam parameters which will result in a match between the prescribed and delivered doses is paramount. The work reported here focused on the establishment of correction factors for Cs-137 for determination and delivery of doses to patients. Since it is unethical to conduct experiment on patients, the standard method was applied to measure doses using tissue equivalent slab phantoms constructed from beeswax. The process of dose delivery was conducted through positioning the dosimeters on the surface of the slab phantom and irradiated with different prescribed doses. The measured doses were recorded on the screen with electrometer connected to the computer. The procedures were repeated through variations of depths by increasing the slab phantoms on the dosimeter. The measured doses were applied to determine the correction factors. With exception of the smaller 4x4 cm2 field size with percentage of deviation of 20.3%, the existing and observed correction factors for Co-60 unit were found to be comparable for 6x6, 8x8 and 10x10 cm2 field sizes. The established correction factors for Cs-137 teletherapy unit were used to determine the treatment time used to deliver different prescribed doses. The measured doses using Cs-137 correction factors were comparable as the percentages of deviations of measured doses from prescribed doses were relatively small for most of the doses. In both Co-60 and Cs-137 units the PDD decreases with increase in depth for all field sizes due to scattering, absorption and attenuation of radiations. From these results it can be seen that, most of the obtained correction factors reproduced good results.