Eight methods of photon inhomogeneity correction were examined for their photon transport approximations. The methods were categorized according to the different approaches used to model scatter photon dose contribution. They were the ratio of TAR (RTAR) and the modified Batho power law which utilized only the 1-D density information along the primary photon path; the equivalent TAR (ETAR) and the FFT convolution methods which incorporated the 3-D density information of the medium for empirical scatter dose calculation; the differential SAR (DSAR), the delta volume (DV), dose spread array (DSA), and differential pencil beam (DPB) methods which employed explicit 3-D scatter ray-trace calculation. Cobalt-60 measurements in horizontal slab phantoms were used to allow simpler data analysis. RTAR consistently overestimated lung corrections by approximately 10%. The scatter ray-trace approach was not always better as the DSAR calculations were inferior to those using the Batho method. The ray-tracing DV, DPB, and DSA methods agreed with measurements mostly to within 2%, at the expense of long computation time. The nonscatter ray-tracing ETAR and FFT convolution calculations were only slightly inferior in the same geometries. These methods improve on the current 1-D methods and should be seriously considered for fast optimization purposes in practical 3-D treatment planning.
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