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

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.

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).

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.

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.

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.