QUANTITATIVE ASSESSMENT OF RANGE FLUCTUATIONS IN CHARGED PARTICLE LUNG IRRADIATION

Purpose: Water equivalent path length (WEL) variations due to respiration can change the range of a charged particle beam and result in beam overshoot to critical organs or beam undershoot to tumor.We have studied range fluctuations by analyzing four-dimensional computed tomography data and quantitatively assessing potential beam overshoot.Methods and Materials: The maximal intensity volume is calculated by combining the gross tumor volume contours at each respiratory phase in the four-dimensional computed tomography study. The first target volume calculates the maximal intensity volume for the entire respiratory cycle (internal target volume [ITV]-radiotherapy[RT]), and the second target volume is the maximal intensity volume corresponding to gated RT (gated-RT, ~30% phase window around exhalation). A compensator at each respiratory phase is calculated. Two "composite" compensators for ITV-RT and gated-RT are then designed by selecting the minimal compensator depth at the respectiverespiratory phase. These compensators are then applied to the four-dimensional computed tomography data to estimate beam penetration. Analysis metrics include range fluctuation and overshoot volume, both as a function of gantry angle. We compared WEL fluctuations observed in treating the ITV-RT versus gated-RT in 11 lung patients.Results: The WEL fluctuations were