Background Determining the scattered radiation dose absorbed by specific body organs and tissues in radiotherapy is very important. In this study we choose thyroid as one of the radiosensitive tissues in the human body to determine absorbed dose in breast cancer patients undergoing radiotherapy. Beside direct dosimetry, the other way to determine the absorbed dose to identified organ of human body is MCNP simulation. Materials and methods In this study, the absorbed dose by thyroid was obtained by two manners: 1. MCNP simulation, 2. Direct measurement by TLD. We modeled each patient's body using three dimensional Monte Carlo Nuclear Particles Code (MCNP-4C). In our simulation, the dose distribution from Co-60 γ-rays, with its photon energy spectra, was used and the received dose to thyroid was calculated by the code. In order to validate the calculated data, we measured the dose distribution at the same selected point during a real radiotherapy by TLDs on the same patients without any build-up-cap. For each patient, we compared the measured dose with the relevant calculated data. Results The comparison showed that the measured data was about 22% higher than the calculated data. The measurement showed that 10% of all received dose was absorbed by the thyroid, While the result of calculation showed that only 9% of that was absorbed by thyroid which is 1% less with respect to measured values. Conclusion The scattered dose to the thyroid was not much. However, clinicians and radiotherapists should be aware of the magnitude of the risk in order to expose the thyroid as little as possible. MCNP version 4C, used in this study, was found to be a valid practical way of performing accurate calculations of 3-D dose distributions from particle interactions in a complex target such as the human body. [ABSTRACT FROM AUTHOR]