Analysis of tissue‐maximum ratio/scatter‐maximum ratio model relative to the prediction of tissue‐maximum ratio in asymmetrically collimated fields

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Abstract
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.