Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations

Current clinical experience in radiation therapy is based upon dose computa tions that report the absorbed dose to water, even though the patient is no t made of water but of many different types of tissue. While Monte Carlo do se calculation algorithms have the potential for higher dose accuracy, they usually transport particles in and compute the absorbed dose to the patien t media such as soft tissue, lung or bone. Therefore, for dose calculation algorithm comparisons, or to report dose to water or tissue contained withi n a bone matrix for example, a method to convert dose to the medium to dose to water is required. This conversion has been developed here by applying Bragg-Gray cavity theory. The dose ratio for 6 and 18 MV photon beams was d etermined by computing the average stopping power ratio for the primary ele ctron spectrum in the transport media. For soft tissue, the difference betw een dose to medium and dose to water is approximately 1.0%, while For corti cal bone the dose difference exceeds 10%. The variation in the dose ratio a s a function of depth and position in the field indicates that for photon b eams a single correction factor can be used for each particular material th roughout the held for a given photon beam energy. The only exception to thi s would be for the clinically non-relevant dose to air. Pre-computed energy spectra for Co-60 to 24 MV are used to compute the dose ratios for these p hoton beams and to determine an effective energy for evaluation of the dose ratio.