Hypofractionated Radiation Therapy Using Fiducial Markers: Assessment of Residual Error and Variation of Prostate Volume during Treatment

Abstract

With the incorporation of fiducial markers as a surrogate for the prostate itself, the systematic and random positioning error can be removed from the treatment. However, geometric uncertainties remain with respect to the prostate’s position. In this study, two topics have been investigated: the residual error in the patient positioning associated to the rigid marker-based registration and the variation of the prostate volume during the treatment. A total of 375 pre-treatment CBCT images from 15 prostate cancer patients treated with a hypofractioned RT (2.7 Gy/session in 25 fractions) are included in the study. Two weeks before the treatment, all patients underwent the implantation of three gold seeds by ultrasound-guided insertion (one at the apex, S1, and two at each side of the prostate base, S2 left, S3 right). The positioning protocol consists of a daily CBCT acquisition followed by a rigid marker-based registration with the planning CT. The position coordinates of the markers and the isocenter are obtained from each study. In each treatment session, the prostate may undergo deformations that can lead to relative displacements between markers different from those of the CT simulation. This fact, together with the subjective alignment of the images, lead to an error associated to the registration. In order to quantify this potential error, the difference between the markers center of mass in the CBCT relative to the treatment isocenter and the center of mass in the CT relative to the planning isocenter was calculated. The residual errors from the combination of systematic and random errors were determined by quadratic addition. Additionally, markers distance variation per treatment session has been analyzed by deducing the separation distance two by two (D12, D13, D23) from markers coordinates. The results obtained for the residual error in the patient positioning are (mm): 0.7 lateral, 1.1 longitudinal, 1.3 vertical. The higher errors detected in the longitudinal and vertical directions could be explained by the prostate rotations on the left-right axis that are not completely compensated by the translational shifts. The distances between markers show a decrease during the treatment course. This fact is associated with a potential reduction in prostate volume. On average, it is observed that in session 12-13 the three distances begin to decrease in magnitude, with D23 showing a smaller reduction at the end of the treatment (about 1 mm). D12 and D13 decrease to 1.6 and 1.8 mm in the last session. The alignment of prostate cancer patients based on CBCT with fiducial markers entails a negligible residual error compared to the margins generally considered. The prostate shows a tendency to reduce its volume during treatment. This fact can have a relevant impact on treatment margins and on treatment re-planning strategies.