Evaluation of image-matching and prostate movement angle using anatomical characteristics obtained from planning computed tomography

INTRODUCTION: Setup errors and prostate movement may be associated with individual patient anatomy and internal physiological conditions. We evaluated image-matching and angle of prostate movement during prostate radiotherapy using anatomical characteristics. METHODS: We included 103 patients who underwent prostate volumetric-modulated arc therapy. Body mass index (BMI) and the volumes of the lesser pelvic cavity, bladder, rectum, and prostate were measured using planning computed tomography. Patients were divided into small- and large-value groups according to the median value of each anatomical characteristic. The medians and standard deviations of bone setup errors and inter- and intra-fraction prostate movements were calculated in the anterior-posterior (AP), left-right (LR), and superior-inferior (SI) directions using pelvic bone and fiducial markers with the ExacTrac X-ray system. Prostate movement angles were calculated from the approximation equation of correlation between the AP and SI direction for both groups (rectum volume, bladder capacity). RESULTS: The average setup errors in the LR direction exhibited a moderate positively correlation (r = 0.43) with lesser pelvic cavity volume in the small-value group, and a moderate negative correlation (r = -0.45) in the large-value group, as determined by BMI and lesser pelvic cavity volume. Intra-fraction prostate movement angles were tilted 6.7° in the AP direction in the large rectal volume group and 3.1° in the SI direction in the large bladder volume group. CONCLUSIONS: Image-matching and prostate movement angles differed between groups stratified according to patient anatomical characteristics, suggesting the potential utility of anisotropic margins and careful patient setup. IMPLICATIONS FOR PRACTICE: Evaluating patient-specific anatomical characteristics before radiotherapy planning may help estimate susceptibility to setup errors and facilitate the definition of anisotropic margins based on individual prostate movement angle information.