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, approximately 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 respective respiratory 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 <21.8 mm-WEL and 9.5 mm-WEL for ITV-RT and gated-RT, respectively for all patients. Gated-RT reduced the beam overshoot volume by approximately a factor of four compared with ITV-RT. Such range fluctuations can affect the efficacy of treatment and result in an excessive dose to a distal critical organ.
Conclusion: Time varying range fluctuation analysis provides information useful for determining appropriate patient-specific treatment parameters in charged particle RT. This analysis can also be useful for optimizing planning and delivery.