The range sensitivity of particle radiotherapy presents a treatment localization challenge in the presence of internal target motion due to physiological processes such as respiration. In order to minimize the excessive irradiation of normal tissue while ensuring target coverage arising from upstream density fluctuations along the beam trajectory, it is essential to carefully select treatment ports that present minimal path length variation due to internal motion. Direct visualization of the range sensitivity of selected treatment ports allows for optimization of the particle treatment plan in the presence of respiratory motion.