Establishment of a calibration curve for an isocentric cobalt unit using Monte Carlo simulation

Measurement of dose distribution in patients during radiotherapy is impossible. The Monte Carlo sim- ulation is an alternative method for dose calculations. In routine radiotherapy, the source-to-surface distance （SSD） method is not practical for an isocentric unit because it requires numerous values of tissue-air ratios and inverse square law. Therefore, this method is time consuming. In this paper, the curves of relative depth doses were obtained for three different SSDs using the MCNP4C Monte Carlo simulation and approximated with a single curve called calibration curve. This curve was compared to the curve obtained by published data, differing in approximately 5% in the worst case. It was also observed that the obtained results were more accurate for distances between -5 and 10 cm from source-to-axis distance.