[en] The quality of phantom materials is crucial for accurate dosimetry in radiotherapy. A wide range of factors such as density, electron density and elemental composition can influence the radiation properties, and hence the absorbed dose, of materials. New materials can be tested by direct measurements which requires considerable time and the availability of relatively large amounts of the material. Alternatively, the dosimetric properties of a proposed phantom material can be compared to those of water or tissue using Monte Carlo calculations. The aim of this study was to evaluate the use of a Monte Carlo technique for the investigation of the water/tissue equivalence of phantom materials. The material used for this investigation was Standard dosimetry Agarose (SDA) gel, which is useful in MRI dosimetry. Depth doses in gel and water were calculated for mono-energetic electron beams of 6, 12 and 20 MeV, and photon beams of 60 keV and 6 MV. For each radiation quality the depth dose distributions are in close agreement. This computational technique takes less time to perform, and uses less material, than a direct measurement. 16 refs., 2 figs