IntroductionThe utilization of proton therapy has increased sharply with an increasing number of proton centers opening within the United States. The challenges of accurately delivering particle therapy increase over photon based techniques due to the Bragg peak and sharp energy fall off. Thus as intrafractional motion occurs, the water-equivalent path length (WPL) can vary as organs of different density pass through the particle's path, resulting in under coverage of the planning tumor volume (PTV) and higher doses to organs at risk (1-3).The ability to deliver a planned radiation dose, robustness, of a treatment plan depends on factors which cannot be entirely accounted for by a single free breathing planning CT (4). These factors are eloquently described in the Paganetti paper and include factors independent of Original Article

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IntroductionThe utilization of proton therapy has increased sharply with an increasing number of proton centers opening within the United States. The challenges of accurately delivering particle therapy increase over photon based techniques due to the Bragg peak and sharp energy fall off. Thus as intrafractional motion occurs, the water-equivalent path length (WPL) can vary as organs of different density pass through the particle's path, resulting in under coverage of the planning tumor volume (PTV) and higher doses to organs at risk (1-3).The ability to deliver a planned radiation dose, robustness, of a treatment plan depends on factors which cannot be entirely accounted for by a single free breathing planning CT (4). These factors are eloquently described in the Paganetti paper and include factors independent of Original Article