Measurements of intrafraction motion and interfraction and intrafraction rotation of prostate by three-dimensional analysis of daily portal imaging with radiopaque markers

PURPOSE: To measure the interfraction and intrafraction motion of the prostate during the course of external beam radiotherapy using a video electronic portal imaging device and three-dimensional analysis. METHODS AND MATERIALS: Eighteen patients underwent implantation with two or three gold markers in the prostate before five-angle/11-field conformal radiotherapy. Using CT data as the positional reference, multiple daily sets of portal images, and a three-dimensional reconstruction algorithm, intrafraction translations, as well as interfraction and intrafraction rotations, were analyzed along the three principal axes (left-right [LR], superoinferior [SI], and AP). The overall mean values and standard deviations (SDs), along with random and systematic SDs, were computed for these translations and rotations. RESULTS: For 282 intrafraction translational displacements, the random SD was 0.8 mm (systematic SD, 0.2) in the LR, 1.0 mm (systematic SD, 0.4) in the SI, and 1.4 mm (systematic SD, 0.7) in the AP axes. The analysis of 348 interfraction rotations revealed random SDs of 6.1 degrees (systematic SD, 5.6 degrees ) around the LR axis, 2.8 degrees (systematic SD, 2.4 degrees ) around the SI axis, and 2.0 degrees (systematic SD, 2.2 degrees ) around the AP axis. The intrafraction rotational motion observed during 44 fractions had a random SD of 1.8 degrees (systematic SD, 1.0 degrees ) around the LR, 1.1 degrees (systematic SD, 0.8 degrees ) around the SI, and 0.6 degrees (systematic SD, 0.3 degrees ) around the AP axis. CONCLUSION: The interfraction rotations observed were more important than those reported in previous studies. Intrafraction motion was generally smaller in magnitude than interfraction motion. However, the intrafraction rotations and translations of the prostate should be taken into account when designing planning target volume margins because their magnitudes are not negligible.