Analysis of tissue-maximum ratio/scatter-maximum ratio model relative to the prediction of tissue-maximum ratio in...

The tissue-maximum ratio/scatter-maximum ratio model has been examined as an empirical model capable of predicting the variation in tissue-maximum ratio observed in fields shifted off-axis by independent collimators. This model has been assessed for its sensitivity to the necessary extrapolation of percentage depth dose data and phantom-scatter factors to zero field size and to large field sizes. This model was found to yield good results for both 6- and 24-MV x rays and to be relatively insensitive to the assumptions made regarding unmeasurable beam parameters.