en Radiation Biology Radiation Biology تحقيق بديع Original Research <p><strong>Background:</strong> In the current study the effect of shielded colpostat on rectum received dose was calculated in cervical brachytherapy using Monte Carlo (MC) method. <strong>Materials and Methods:</strong> Two ⁶⁰Co 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 2&times;2&times;2 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. <strong>Results:</strong> 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. <strong>Conclusion:</strong> 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 ⁶⁰Co photons. However, application of shielded colpostats in treatment planning system is desirable for more accurate treatments.</p> Intracavitary brachytherapy, shielded colpostat, rectum dose, high dose rate brachytherpy, cobalt-60 165 171 http://ijrr.com/browse.php?a_code=A-10-1-556&slc_lang=en&sid=1 A. Mesbahi amesbahi2010@gmail.com 790031947532846009766 790031947532846009766 Yes Medical physics department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran A. Haghzadeh 790031947532846009767 790031947532846009767 No Engineering Faculty, Medical Radiation Engineering Department, Azad University, Science and Research branch, Tehran, Iran A.R. Naseri 790031947532846009768 790031947532846009768 No Radiation Oncology Department, Imam Hospital, Tabriz, Iran A.R. Naseri 790031947532846009769 790031947532846009769 No Engineering Faculty, Medical Radiation Engineering Department, Azad University, Science and Research branch, Tehran, Iran