A new simple method using the computer code MCNP is proposed for calculation of photon dose conversion factors in the PMMA slab phantom. Absorbed dose is calculated by multiplying fluence buildup in the PMMA slab phantom by virtual energy deposition in the ICRU tissue substance. This method does not need auxiliary calculations to determine the dose conversion factors, such as calculation of backscatter factors in the ICRU tissue cube and the PMMA slab phantoms. The discrepancies between the results of the direct method presented here and a more conventional indirect method is less than 2%, except for low energy photons (<100 keV) at large depths. As the direct method reduces the number of calculational steps, the results are more reliable than those of the indirect method.
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