Intrafractional respiratory motion for charged particle lung therapy with immobilization assessed by four-dimensional computed tomography.

The aim of this study was to quantify the magnitude of intrafractional lung tumor motion under free-breathing conditions with an immobilization device using four-dimensional computed tomography (4DCT). 4DCT data sets were acquired for 17 patients with lung tumors receiving carbon ion beam therapy. A single respiratory cycle was subdivided into 10 phases, and intrafractional tumor motion was calculated by identifying the gross tumor volume (GTV) center of mass (COM) in two scenarios; respiratory-ungated and -gated treatments, which were based on a whole respiratory cycle and a 30% duty cycle around peak exhalation, respectively. For the respiratory-ungated case, the mean (± standard deviation) GTV-COM displacements from the peak exhalation position over the 17 patients were 0.6 (± 0.8) / 0.9 (± 1.2) mm, 2.0 (± 1.4) / 0.4 (± 0.7) mm, and 0.2 (± 0.5) / 7.8 (± 6.9) mm in left/right, anterior/posterior and superior/inferior directions, respectively, while these were reduced for the respiratory-gated case to 0.3 (± 0.4) / 0.4 (± 0.6) mm (left/right), 0.8 (± 0.7) / 0.3 (± 0.5) mm (anterior/posterior), and 0.1 (± 0.2) / 2.8 (± 2.9) mm (superior/inferior). Quantitative analysis of tumor motion with immobilization is valuable not only for particle beam therapy but also for photon beam therapy.