The purpose of this work is to determine appropriate radiation therapy beam margins to account for intrafraction prostate\r\ntranslations for use with real-time electromagnetic position monitoring and correction strategies. Motion was measured\r\ncontinuously in 35 patients over 1157 fractions at 5 institutions. This data was studied using van Herk�s formula of (aS + ?s)\r\nfor situations ranging from no electromagnetic guidance to automated real-time corrections. Without electromagnetic guidance,\r\nmargins of over 10mm are necessary to ensure 95% dosimetric coverage while automated electromagnetic guidance allows the\r\nmargins necessary for intrafraction translations to be reduced to submillimeter levels. Factors such as prostate deformation\r\nand rotation, which are not included in this analysis, will become the dominant concerns as margins are reduced. Continuous\r\nelectromagnetic monitoring and automated correction have the potential to reduce prostate margins to 2-3mm, while ensuring\r\nthat a higher percentage of patients (99% versus 90%) receive a greater percentage (99% versus 95%) of the prescription dose.
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