The potential of an optical surface tracking system in non‐coplanar single isocenter treatments of multiple brain metastases

Abstract

To evaluate the accuracy of a commercial optical surface tracking (OST) system and to demonstrate how it can be implemented to monitor patient positioning during non-coplanar single isocenter stereotactic treatments of brain metastases. A 3-camera OST system was used (Catalyst HD™, C-RAD) on a TruebeamSTx with a 6DoF couch. The setup accuracy and agreement between the OST system, and CBCT and kV-MV imaging at couch angles 0°and 270°, respectively, were examined. Film measurements at 3 depths in the Rando-Alderson phantom were performed using a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Setup of the phantom was performed with CBCT at couch 0°and subsequently monitored by OST at other couch angles. Setup data for 7 volunteers were collected to evaluate the accuracy and reproducibility of the OST system at couch angles 0°, 45°, 90°, 315°, and 270°. These results were also correlated to the couch rotation offsets obtained by a Winston-Lutz (WL) test. The Rando-Alderson phantom, as well as volunteers, were fixated using open face masks (Orfit). For repeated tests with the Rando-Alderson phantom, deviations between rotational and translational isocenter corrections for CBCT and OST systems are always within 0.2°(pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0°and 270°, respectively. Dose deviations between the film and TPS doses in the center of the 4 lesions were −1.2%, −0.1%, −0.0%, and −1.9%. Local gamma evaluation criteria of 2%/2 mm and 3%/1 mm yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for the 4 lesions. Regarding the volunteers, the mean translational and rotational isocenter shift values were (0.24 ± 0.09) mm and (0.15 ± 0.07) degrees. Largest isocenter shifts were found for couch angles 45˚and 90˚, confirmed by WL couch rotation offsets. Patient monitoring during non-coplanar VMAT treatments of brain metastases is feasible with submillimeter accuracy.

A retrospective analysis of setup and intrafraction positional variation in stereotactic radiotherapy treatments

“…2) show that using μ ± 2σ to calculate the 95% CI results in a range of (−1.61, 1. 20 Schmidhalter et al also attributed some positioning inaccuracies to patient weight loss and the subsequent increased movement within the mask, although in our study this is not an issue…”

“…In situations where mask material is removed in order to facilitate other means of image guidance (such as optical systems), patient positioning results have been shown to be comparable to that of ExacTrac 20 . Schmidhalter et al also attributed some positioning inaccuracies to patient weight loss and the subsequent increased movement within the mask, although in our study this is not an issue as we are not treating more than 5 fractions 11 .…”

Recent advances in Surface Guided Radiation Therapy

“…For treatments with non-coplanar couch angles, accurate knowledge about the coincidence between radiation and couch rotation isocenter becomes important, as it is not possible to correct for couch walkout during treatment delivery. Regular Winston-Lutz checks provide accurate information about the radiation isocenter for different gantry, collimator and couch angles and can also indirectly provide information about the accuracy of the system [19,54,55].…”

“…Applications of hypofractionated RT (e.g. SRS, SBRT) require measurements of the static accuracy with 6DOF [92], differentiation between single or multiple isocenter plans [93], including couch rotations, walkout effects, and the impact of miscalibration [19,94].…”

“…Another application developed in recent years is the use of SGRT in whole-brain radiation therapy (WBRT) or stereotactic radiosurgery (SRS). Here, an SGRT system has the ability to monitor the surface of the patient within an open-face immobilization mask in planar and non-coplanar treatments [19]. SGRT has also been applied in special techniques such as accelerated partial breast irradiation (APBI), stereotactic body radiation therapy (SBRT), and with limited use in pediatrics [20].…”