Monte Carlo calculation of shielded colpostat effect on rectum received dose in high dose rate brachytherapy with Cobalt-60 sources

Background: In the current study the effect of shielded colpostat on rectum received dose was calculated in cervical brachytherapy using Monte Carlo (MC) method. Materials and Methods: Two Co-60 sources of GZP6 brachytherapy unit used for intracavitary treatments were simulated using MCNPX Monte Carlo code. Also the two types of colpostats including shielded and unshielded were simulated inside a water phantom. The radial dose function, depth doses and dose distribution around sources were calculated with spatial dose resolution of 2x2x2 mm. The effect of tungsten shields on rectum dose was studied. Moreover, the accuracy of treatment planning system (TPS) was verified comparing to MC results. Results: Our results showed that shielded colpostats reduce the rectum dose up to 7% according to the MC calculations. Also, it was found less than 3% difference between TPS and MC results in percentage depth dose and radial doses for unshielded case. Conclusion: It can be concluded that the application of shielded colpostats provide a relatively mild protection (7% reduction in dose) for the rectum in intracavitary brachytherapy due to the higher energy of Co-60 photons. However, application of shielded colpostats in treatment planning system is desirable for more accurate treatments.