Monitoring of breathing motion in image-guided PBS proton therapy: comparative analysis of optical and electromagnetic technologies

Abstract

BackgroundMotion monitoring is essential when treating non-static tumours with pencil beam scanned protons. 4D medical imaging typically relies on the detected body surface displacement, considered as a surrogate of the patient's anatomical changes, a concept similarly applied by most motion mitigation techniques. In this study, we investigate benefits and pitfalls of optical and electromagnetic tracking, key technologies for non-invasive surface motion monitoring, in the specific environment of image-guided, gantry-based proton therapy.MethodsPolaris SPECTRA optical tracking system and the Aurora V3 electromagnetic tracking system from Northern Digital Inc. (NDI, Waterloo, CA) have been compared both technically, by measuring tracking errors and system latencies under laboratory conditions, and clinically, by assessing their practicalities and sensitivities when used with imaging devices and PBS treatment gantries. Additionally, we investigated the impact of using different surrogate signals, from different systems, on the reconstructed 4D CT images.ResultsEven though in controlled laboratory conditions both technologies allow for the localization of static fiducials with sub-millimetre jitter and low latency (31.6 ± 1 msec worst case), significant dynamic and environmental distortions limit the potential of the electromagnetic approach in a clinical setting. The measurement error in case of close proximity to a CT scanner is up to 10.5 mm and precludes its use for the monitoring of respiratory motion during 4DCT acquisitions. Similarly, the motion of the treatment gantry distorts up to 22 mm the tracking result.ConclusionsDespite the line of sight requirement, the optical solution offers the best potential, being the most robust against environmental factors and providing the highest spatial accuracy. The significant difference in the temporal location of the reconstructed phase points is used to speculate on the need to apply the same monitoring system for imaging and treatment to ensure the consistency of detected phases.

Recent advances in Surface Guided Radiation Therapy

“…dose rate, beam hold times) need to be included into the QA program [ 86 , 95 , 96 ]. Even more complex, is the application of this technology for particle therapy, specifically in pencil beam delivery, where parameters as latency, the impact of room geometries into the system performance and potential prediction algorithms, when using beam-gating or -tracking [ 97 , 98 ] need to be taken into account. Also in ring-like Linacs, a QA program needs to be implemented [ 37 , 99 , 100 ], as well as when the surface is used as a surrogate for 4D-acquisitions [ 101 – 104 ].…”

Experimental validation of a deforming grid 4D dose calculation for PBS proton therapy

“…For all measurements, the delivery log files were saved and used as input to the retrospective 4DDC as described above. Additionally, the position of the measurement device was accurately logged using an optical tracking system (OTS) with a temporal resolution of 60Hz (Fattori et al (2017)). The deliveries were triggered using the gating function of the OTS, ensuring that every delivery started at the same motion phase and allowing for synchronisation of delivery and motion.…”

Current state and future applications of radiological image guidance for particle therapy

“…However, direct translation to particle therapy needs to overcome additional obstacles. Arguably most clinically applicable of all non‐ionizing techniques, optical surface imaging has been shown to improve setup of post‐mastectomy chest wall irradiation patients over radiograph only based techniques and performed more robustly in monitoring respiratory motion than electromagnetic tracking in controlled laboratory conditions . Electromagnetic tracking systems currently suffer from significant environmental distortions which limit the use in a clinical setting of particle therapy.…”

“…Arguably most clinically applicable of all nonionizing techniques, optical surface imaging has been shown to improve setup of post-mastectomy chest wall irradiation patients over radiograph only based techniques 100 and performed more robustly in monitoring respiratory motion than electromagnetic tracking in controlled laboratory conditions. 101 Electromagnetic tracking systems currently suffer from significant environmental distortions which limit the use in a clinical setting of particle therapy. Optical imaging systems may have an important role to play in monitoring patient motion during particle therapy and respiratory motion management than pre-treatment patient positioning when compared to volumetric CBCT/in-room CT image guidance methods.…”