Stereotactic IMRT for prostate cancer: (1) setup accuracy of a new stereotactic body localization system

Abstract

Abstract:

The purpose of this work is to assess prospectively the setup accuracy that can be achieved with a stereotactic body localizer (SBL) in immobilizing patients for stereotactic intensity modulated radiotherapy (IMRT) of the prostate cancer. By quantifying this important factor, we wish to establish margin criteria for stereotactic treatment planning by also taking into account the target mobility in the SBL. We analyzed data from forty CT studies (with slice thickness of 3 mm) involving ten patients with prostate cancer. Each patient had four sets of CT scans during the course of radiotherapy. For the purpose of this study, all four sets of CT scan were taken with the patients immobilized in a customized body pillow formed by vacuum suction. Unlike other immobilization device, this system consists of not only a customized body pillow, but also a fixation sheet used to suppress patient respiratory motion, a stereotactic body frame to provide stereotaxy in the CT images, and a carbon fiber base board to which both the body cushion and the frame are affixed. We identified four bony landmarks and recorded the coordinates of the landmarks in the stereotactic body frame reference. The coordinates obtained from the subsequent scans were compared with those obtained in the reference (the first) scan. The initial planned isocenter was also marked on the patients? skin with fiducials for each CT study. The distance from each bony landmark to the fiducial-based isocenter was measured and compared among the 4 sets of scan. The deviations in distances were also compared to those measured from the landmarks to the stereotactic frame center, in order to determine the effectiveness of the rigid body frame in positioning patients with prostate cancer. Target inter-fraction motion in this system was also studied for five patients by measuring the deviations in distances from the target geometric center to the bony landmarks. Our results show that the body frame system has the ability to immobilize patient bony landmarks to a reproducible position. Mean patient setup errors of 0.92 mm ± 1.95 mm in lateral direction (LAT), -0.3 ± 1.20 mm in anterior-posterior (AP) direction , and 0.9 mm ± 2.12 mm in superior-inferior (SI) direction were observed for the target in the pelvis region. In terms of three-dimensional vector, the combined setup accuracy was 3.0 mm ±1.29 mm. Comparing to the rigid reference, a skin-mark based reference is less reliable for patient repositioning. It was also found that the mean target mobility relative to the bony landmarks is 2.22 mm in the AP direction, 0.17 mm in the LAT direction, and 0.11 mm in the SI direction, with the SD of 3.45, 1.11, and 2.69 mm, respectively. It is concluded that the body immobilization system has the ability to immobilize prostate patients with satisfactory setup accuracy for fractionated extracranial stereotactic radiotherapy. A rigid frame system serves as a reliable alignment reference in terms of repositioning patients into the planning position, while skin-based reference present larger deviations in repositioning patients.