Intrafractional respiratory motion for charged particle lungtherapy with immobilization assessed by four-dimensionalcomputed 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.