Simultaneous MV‐kV imaging for intrafractional motion management during volumetric‐modulated arc therapy delivery*

Abstract

The purpose of this study was to evaluate the accuracy and clinical feasibility of a motion monitoring method employing simultaneously acquired MV and kV images during volumetric-modulated arc therapy (VMAT). Short-arc digital tomosynthesis (SA-DTS) is used to improve the quality of the MV images that are then combined with orthogonally acquired kV images to assess 3D motion. An anthropomorphic phantom with implanted gold seeds was used to assess accuracy of the method under static, typical prostatic, and respiratory motion scenarios. Automatic registration of kV images and single MV frames or MV SA-DTS reconstructed with arc lengths from 2° to 7° with the appropriate reference fiducial template images was performed using special purpose-built software. Clinical feasibility was evaluated by retrospectively analyzing images acquired over four or five sessions for each of three patients undergoing hypofractionated prostate radiotherapy. The standard deviation of the registration error in phantom using MV SA-DTS was similar to single MV images for the static and prostate motion scenarios (σ = 0.25 mm). Under respiratory motion conditions, the standard deviation of the registration error increased to 0.7mm and 1.7 mm for single MV and MV SA-DTS, respectively. Registration failures were observed with the respiratory scenario only and were due to motion-induced fiducial blurring. For the three patients studied, the mean and standard deviation of the difference between automatic registration using 4° MV SA-DTS and manual registration using single MV images results was 0.07±0.52mm. The MV SA-DTS results in patients were, on average, superior to single-frame MV by nearly 1 mm — significantly more than what was observed in phantom. The best MV SA-DTS results were observed with arc lengths of 3° to 4°. Registration failures in patients using MV SA-DTS were primarily due to blockage of the gold seeds by the MLC. The failure rate varied from 2% to 16%. Combined MV SA-DTS and kV imaging is feasible for intratreatment motion monitoring during VMAT of anatomic sites where limited motion is expected, and improves registration accuracy compared to single MV/kV frames. To create a clinically robust technique, further improvements to ensure visualization of fiducials at the desired control points without degradation of the treatment plan are needed.

Optimizing fiducial visibility on periodically acquired megavoltage and kilovoltage image pairs during prostate volumetric modulated arc therapy

“…Periodic kV imaging triggered at 20 • gantry intervals (every eighteenth of an arc) is scheduled on a TrueBeam Linac (Varian, Palo Alto, CA) and paired with a 4.4 • MV short arc digital tomosynthesis (DTS) simultaneously acquired via iTools-Capture (Varian) to localize the 3D position of implanted gold fiducials in real time. 17 MV-DTS was chosen over a single MV projection to improve the signal to noise ratio of the passively acquired MV images, which is crucial for robust detection and registration of fiducials. Exposure of at least one of the three implanted fiducials consistently throughout the 4.4 • short arc is mandatory for clear fiducial visualization and accurate localization of translations, while full exposure of all three fiducials in the short arc is essential for deriving the rotation information.…”

Technical Note: 3D localization of lung tumors on cone beam CT projections via a convolutional recurrent neural network

“…The length of the series determined length of memory to be analyzed by the CRNN. We configured the length as 10, simulating a 4° short arc, which is often used for 3D localization in the context of digital tomosynthesis . The RNN was constructed with gated recurrent units [GRU, as described in the first block of Fig.…”

Design and validation of a MV/kV imaging‐based markerless tracking system for assessing real‐time lung tumor motion

“…Conventionally, image tracking is performed by registering the MV/kV images to templates created using the planning CT images within a specific respiratory phase to minimize the motion artifact . Since the phantom was well positioned with the guidance of a setup CBCT and in the context of intrafractional tumor tracking, we designed a simpler approach that directly registers the MV/kV images acquired during treatment to the CBCT projections at the same gantry positions.…”

“…To correct for this motion in the image registrations, a motion trace was derived by tracking the transponder motion in the CBCT projections, that is, by registering each projection to a binary‐image template of the transponders at the same gantry angle created from the RCCT at end expiration. Transponder tracking was carried out using an in‐house developed program . Application of the shift correction thus related all motions to the end‐expiration reference position.…”

“…Simultaneously acquired MV/kV projections are complimentary to each other and provide more opportunities to capture the tumor. A combination of the two provides robust real‐time 3D localization results, and has become an indispensable tool for hypofractionated prostate treatment with volumetric modulated arc therapy (VMAT) at our institution . In this paper, we investigate the feasibility of extending the application of MV/kV images to tracking of lung tumors during a VMAT delivery.…”