Background: In this research dosimetric parameters of mercury was investigated as a radiotherapy modulator and compared with cerrobend as a common beam attenuator material in most radiotherapy centers. Also several designs for mercury shielding system were evaluated.

Materials and Methods: In this experimental research transmission factor of different thicknesses of cerrobend and mercury at 6MV and 18MV photon beams was measured in order to investigate mercury as a beam modulator. Relative dosimetry was done with ionization chambers and then mercury modulating system for radiotherapy was designed with different cross sections.

Results: The result show that the transmission values increased with field size increment due to increased scatter contribution, for all cerrobend and mercury thicknesses at two energies. The procedure of mercury transmission factor is in agreement with cerrobend transmission factor. The second designed system with hexagonal cross section is in good agreement with patient contour.

Conclusion: The experimental investigation on mercury dosimetric parameters showed that mercury can be proposed as a modifier in radiotherapy because of its profit properties such as being liquid in room temperature, power of attenuation and …. So mercury can be proposed as a beam modifier (at closed system because of its poison vapor) in radiotherapy. Keywords: beam modifiers, Cerrobend, dosimetry, Mercury