In Vivo Validation of Elekta's Clarity Autoscan for Ultrasound-based Intrafraction Motion Estimation of the Prostate During Radiation Therapy

Abstract

PurposeOur purpose was to perform an in vivo validation of ultrasound imaging for intrafraction motion estimation using the Elekta Clarity Autoscan system during prostate radiation therapy. The study was conducted as part of the Clarity-Pro trial (NCT02388308).Methods and MaterialsInitial locations of intraprostatic fiducial markers were identified from cone beam computed tomography scans. Marker positions were translated according to Clarity intrafraction 3-dimensional prostate motion estimates. The updated locations were projected onto the 2-dimensional electronic portal imager plane. These Clarity-based estimates were compared with the actual portal-imaged 2-dimensional marker positions. Images from 16 patients encompassing 80 fractions were analyzed. To investigate the influence of intraprostatic markers and image quality on ultrasound motion estimation, 3 observers rated image quality, and the marker visibility on ultrasound images was assessed.ResultsThe median difference between Clarity-defined intrafraction marker locations and portal-imaged marker locations was 0.6 mm (with 95% limit of agreement at 2.5 mm). Markers were identified on ultrasound in only 3 of a possible 240 instances. No linear relationship between image quality and Clarity motion estimation confidence was identified. The difference between Clarity-based motion estimates and electronic portal–imaged marker location was also independent of image quality. Clarity estimation confidence was degraded in a single fraction owing to poor probe placement.ConclusionsThe accuracy of Clarity intrafraction prostate motion estimation is comparable with that of other motion-monitoring systems in radiation therapy. The effect of fiducial markers in the study was deemed negligible as they were rarely visible on ultrasound images compared with intrinsic anatomic features. Clarity motion estimation confidence was robust to variations in image quality and the number of ultrasound-imaged anatomic features; however, it was degraded as a result of poor probe placement.

Real-time intrafraction motion monitoring in external beam radiotherapy

“…Autoscan’s accuracy was validated in vivo against manual localization of intraprostatic markers in EPID images (Grimwood et al 2018 , Han et al 2018 ) and against RayPilot monitoring (Delcoudert et al 2017 ). Characterisations of prostate motion during treatment describe a gradual drift from the isocentre with substantial inter-patients variations showing maximum recorded shifts >10 mm and a mean SI drift of 0.075 mm min −1 (Ballhausen et al 2015 , Li et al 2017 ).…”

“…Magnetic resonance (MR) imaging was also integrated with treatment machines with two commercial systems (Mutic and Dempsey 2014 , Raaymakers et al 2017 ) where gating is applied on the MRIdian (Green et al 2018 , Tetar et al 2018 ) and multi-leaf collimator (MLC) tracking has been proposed on the Unity (Glitzner et al 2018 ). Add-on systems such as electromagnetic transponders, surface imaging and ultrasound transducers may be interfaced with conventional linacs for automatic gating of the treatment beam (Grimwood et al 2018 , Worm et al 2018 ). In addition, conventional linacs alone may provide 3D motion monitoring capability (Keall et al 2018b ) and mitigation via MLC tracking (Keall et al 2014b , 2018a , Booth et al 2016 ) or couch tracking (Ehrbar et al 2017b ) although the latter has not been used clinically to date.…”

Target tracking accuracy and latency with different 4D ultrasound systems – a robotic phantom study

“…Three 4D US systems with matrix array probes shown in Figure 1 were included in this study: (1) Vivid 7, 3V-D probe (GE Healthcare), (2) Vivid E95, 4Vc-D probe (GE Healthcare) (3) Epiq 7, X6-1 probe (Philips Healthcare). The respective volume sizes were chosen to approximate the typical clinical setting of 58 × 45°in [3] as closely as possible, depending on the available system settings (see Table 1).…”

“…In radiotherapy, the only 4D US guidance system in clinical use [2] utilizes a US probe with mechanical image acquisition and thus limited temporal resolution. While this system has been shown to be suitable for monitoring slowmoving structures such as the prostate [3] or the liver during deep inspiration breath-hold [4], the slow acquisition speed prohibits tracking of fast motion, such as breathing or pulsation, in real-time [5]. Matrix array probes, on the other hand, do not rely on mechanical parts and can therefore achieve much higher volumetric framerates [6].…”

Improving 3D ultrasound prostate localisation in radiotherapy through increased automation of interfraction matching

“…Systems can be used for interfraction and intrafraction motion management [1] , [2] and some radiotherapy departments are using ultrasound as their standard image guidance method for prostate cancer. The Clarity Autoscan system uses a 3D transperineal ultrasound (TPUS) probe and provides continuous imaging of the prostate for intrafraction motion estimation [3] , [4] . The Clarity system uses manual comparison of an image acquired at simulation with one acquired prior to treatment to calculate the couch shift necessary to correct for interfraction motion.…”