Surface guided radiotherapy for high precision treatments of brain metastases

Abstract: Purpose/Background High accuracy treatment techniques such as stereotactic radiotherapy (SRT) requires precise patient positioning prior to and during treatment. The Catalyst™ is an optical surface scanning (OSS) system that has been utilized for patient positioning and real time monitoring during radiotherapy. The company behind the system recently released a novel algorithm for calculating the isocenter shift, specialized for SRT treatments. The aim of this master thesis was to evaluate if the OSS system with the novel SRT algorithm provides suffcient accuracy for positioning and real time monitoring of SRT treatments. Material and methods A study was performed using a RANDO Alderson phantom (Alderson et al. (1962) [1]), an open-face mask and the OSS system. For positioning at couch angle 0°, the agreement between the isocenter shift calculated by the OSS system and the isocenter shift suggested after imageverification with Cone-Beam Computed Tomography (CBCT) was evaluated. For non-coplanar treatments the accuracy of positioning and monitoring was evaluated by isolating the couch offset and the uncertainties in the OSS systems calculation of the isocenter shift. Furthermore, an evaluation of the dosimetric effect of patient positioning uncertainties in clinical non-coplanar SRT treatment plans was carried out. Results The agreement between the OSS system and the CBCT system for different tumor positions were within 0.5 mm in the longitudinal direction and within 0.3 mm in the vertical and lateral directions. For all rotational directions the agreement were within 0.9°. The OSS system indicated that when rotating the couch used in this study, the position of the phantom relative to the treatment isocenter was shifted up to 1.2 mm. The couch rotation offset were larger for larger couch angles, although within 0.6 mm. The OSS system's uncertainty in the calculation of the isocenter position was within 0.5 mm. Within this study, the worst-case scenario for current work ow entails a risk of a 21.6% decrease of V(95%). However, with the OSS system as a complement for positioning the worst-case scenario would instead be limited to a 11.1% decrease of V(95%). Conclusions The OSS system evaluated within this thesis has the potential to improve patient positioning for SRT treatments. It has been concluded that the OSS system with the novel SRT algorithm show excellent agreement with the CBCT system and has the ability to validate the position of a phantom with 0.5 mm accuracy, at all couch angles. When tracking the surface, the only additional uncertainties are the motion and deformation of the surface. Thus, the OSS system has no problem monitoring the phantom position. However, the system must be further tested on volunteers and patients before clinical implementation, for which there will be some surface motion and deformation. This master thesis is the first step towards commissioning of the OSS system and open-face masks for SRT treatments in the clinic.