Reproducibility of tumor motion probability distribution function in stereotactic body radiation therapy of lung cancer

Purpose: To evaluate the reproducibility of tumor motion probability distribution function (PDF) in stereotactic body radiation therapy (SBRT) of lung cancer using cine megavoltage (MV) images. Methods and Materials: Cine MV images of 20 patients acquired during three-dimensional conformal (6-11 beams) SBRT treatments were retrospectively analyzed to extract tumor motion trajectories. For each patient, tumor motion PDFs were generated per fraction (PDFn) using three selected "usable" beams. Patients without at least three usable beams were excluded from the study. Fractional PDF reproducibility (Rn) was calculated as the Dice similarity coefficient between PDFnto a "ground-truth" PDF (PDFg), which was generated using the selected beams of all fractions. The mean of Rn, labeled as Rm, was calculated for each patient and correlated to the patient's mean tumor motion rang (Am). Change of Rmduring the course of SBRT treatments was also evaluated. Intra- and intersubject coefficient of variation (CV) of Rmand Amwere determined. Results: Thirteen patients had at least three usable beams and were analyzed. The mean of Rmwas 0.87 (range, 0.84-0.95). The mean of Amwas 3.18 mm (range, 0.46-7.80 mm). Rmwas found to decrease as Amincreases following an equation of Rm= 0.17e-0.9Am+ 0.84. Rmalso decreased slightly throughout the course of treatments. Intersubject CV of Rm(0.05) was comparable to intrasubject CV of Rm(range, 0.02-0.09); intersubject CV of Am(0.73) was significantly greater than intrasubject CV of Am(range, 0.09-0.24). Conclusions: Tumor motion PDF can be determined using cine MV images acquired during the treatments. The reproducibility of lung tumor motion PDF decreased exponentially as the tumor motion range increased and decreased slightly throughout the course of the treatments.