Real-time motion monitoring using orthogonal cine MRI during MR-guided adaptive radiation therapy for abdominal tumors on 1.5T MR-Linac.

BACKGROUND: Real-time motion monitoring (RTMM) is necessary for accurate motion management of intrafraction motions during radiation therapy (RT). PURPOSE: Building upon a previous study, this work develops and tests an improved RTMM technique based on real-time orthogonal cine magnetic resonance imaging (MRI) acquired during magnetic resonance-guided adaptive RT (MRgART) for abdominal tumors on MR-Linac. METHODS: A motion monitoring research package (MMRP) was developed and tested for RTMM based on template rigid registration between beam-on real-time orthogonal cine MRI and pre-beam daily reference 3D-MRI (baseline). The MRI data acquired under free-breathing during the routine MRgART on a 1.5T MR-Linac for 18 patients with abdominal malignancies of 8 liver, 4 adrenal glands (renal fossa), and 6 pancreas cases were used to evaluate the MMRP package. For each patient, a 3D mid-position image derived from an in-house daily 4D-MRI was used to define a target mask or a surrogate sub-region encompassing the target. Additionally, an exploratory case reviewed for an MRI dataset of a healthy volunteer acquired under both free-breathing and deep inspiration breath-hold (DIBH) was used to test how effectively the RTMM using the MMRP can address through-plane motion (TPM). For all cases, the 2D T2/T1-weighted cine MRIs were captured with a temporal resolution of 200 ms interleaved between coronal and sagittal orientations. Manually delineated contours on the cine frames were used as the ground-truth motion. Common visible vessels and segments of target boundaries in proximity to the target were used as anatomical landmarks for reproducible delineations on both the 3D and the cine MRI images. Standard deviation of the error (SDE) between the ground-truth and the measured target motion from the MMRP package were analyzed to evaluate the RTMM accuracy. The maximum target motion (MTM) was measured on the 4D-MRI for all cases during free-breathing. RESULTS: The mean (range) centroid motions for the 13 abdominal tumor cases were 7.69 (4.71-11.15), 1.73 (0.81-3.05), and 2.71 (1.45-3.93) mm with an overall accuracy of <2 mm in the superior-inferior (SI), the left-right (LR), and the anterior-posterior (AP) directions, respectively. The mean (range) of the MTM from the 4D-MRI was 7.38 (2-11) mm in the SI direction, smaller than the monitored motion of centroid, demonstrating the importance of the real-time motion capture. For the remaining patient cases, the ground-truth delineation was challenging under free-breathing due to the target deformation and the large TPM in the AP direction, the implant-induced image artifacts, and/or the suboptimal image plane selection. These cases were evaluated based on visual assessment. For the healthy volunteer, the TPM of the target was significant under free-breathing which degraded the RTMM accuracy. RTMM accuracy of <2 mm was achieved under DIBH, indicating DIBH is an effective method to address large TPM. CONCLUSIONS: We have successfully developed and tested the use of a template-based registration method for an accurate RTMM of abdominal targets during MRgART on a 1.5T MR-Linac without using injected contrast agents or radio-opaque implants. DIBH may be used to effectively reduce or eliminate TPM of abdominal targets during RTMM.

Feasibility of real-time motion tracking using cine MRI during MR-guided radiation therapy for abdominal targets.

PURPOSE: Real-time high soft-tissue contrast magnetic resonance imaging (MRI) from the MR-Linac offers the best opportunity for accurate motion tracking during radiation therapy delivery via high-frequency two-dimensional (2D) cine imaging. This work investigates the efficacy of real-time organ motion tracking based on the registration of MRI acquired on MR-Linac. METHODS: Algorithms based on image intensity were developed to determine the three-dimensional (3D) translation of abdominal targets. 2D and 3D abdominal MRIs were acquired for 10 healthy volunteers using a high-field MR-Linac. For each volunteer, 3D respiration-gated T2 and 2D T2/T1-weighted cine in sagittal, coronal, and axial planes with a planar temporal resolution of 0.6 for 60 s was captured. Datasets were also collected on MR-compatible physical and virtual four-dimensional (4D) motion phantoms. Target contours for the liver and pancreas from the 3D T2 were populated to the cine and assumed as the ground-truth motion. We performed image registration using a research software to track the target 3D motion. Standard deviations of the error (SDE) between the ground-truth and tracking were analyzed. RESULTS: Algorithms using a research software were demonstrated to be capable of tracking arbitrary targets in the abdomen at 5 Hz with an overall accuracy of 0.6 mm in phantom studies and 2.1 mm in volunteers. However, this value is subject to patient-specific considerations, namely motion amplitude. Calculation times of < 50 ms provide a pathway of real-time motion tracking integration. A major challenge in using 2D cine MRI to track the target is handling the full 3D motion of the target. CONCLUSIONS: Feasibility to track organ motion using intensity-based registration of MRIs was demonstrated for abdominal targets. Tracking accuracy of about 2 mm was achieved for the motion of the liver and pancreatic head for typical patient motion. Further development is ongoing to improve the tracking algorithm for large and complex motions.

Evaluation of transperineal ultrasound imaging as a potential solution for target tracking during hypofractionated radiotherapy for prostate cancer.

BACKGROUND: Emerging hypofractionated prostate radiotherapy regimens require solutions for accurate target tracking during beam delivery. The goal of this study is to evaluate the performance of the Clarity ultrasound monitoring system for prostate motion tracking. METHODS: Five prostate patients underwent continuous perineum ultrasound imaging during their daily treatments. Initial absolute 3D positions of fiducials implanted in the prostate were estimated from the KV images. Fiducial positions in MV images acquired during beam delivery were compared with predicted positions based on Clarity 3D tracking. The uncertainty in the comparison results was evaluated in a phantom validation study. RESULTS: Continuous real-time ultrasound motion tracking was recorded in 5 patients and 167 fractions for overall of 39.7 h. Phantom validation of the proposed procedure demonstrated that predicted and observed fiducial positions agree within 1.1 mm. In patients agreement between predicted and actual fiducial positions varied between 1.3 mm and 3.3 mm. On average ultrasound tracking reduced the maximum localization error in patients by 20% on average. With the motion corrected, the duration prostate beyond 1 mm from its initial treatment position can be reduced from 37 to 22% of the total treatment time. CONCLUSION: Real-time ultrasound tracking reduces uncertainty in prostate position due to intra-fractional motion. TRIAL REGISTRATION: IRB Protocol #27372 . Date of registration of trial: 12/17/2013.

Feasibility of real-time lung tumor motion monitoring using intrafractional ultrasound and kV cone beam projection images.

PURPOSE: The ability to monitor intrafractional tumor motion is essential for radiation therapy of thoracic and abdominal tumors. This study aims to develop a method to track lung tumor motion using intrafractional continuous ultrasound (US) and periodic cone-beam projection images (CBPI). METHODS: Time-sequenced b-mode US and CBPI data were extracted from the data acquired with the Clarity® and XVI platforms on an Elekta linac, respectively. The data were synchronized through a video capture card (VCE-PRO, IMPERX Inc.) which was triggered by the XVI system. In this way, a system was configured to allow real-time acquisition of the diaphragm position synchronized with periodic acquisition of the lung tumor position. Feasibility of the system was demonstrated by acquiring synchronized data on an in-house motion platform with embedded spheres of different materials and US images of the diaphragm on 5 volunteers of various body sizes. Finally, ultrasound b-mode images and CBPI were also acquired simultaneously from 3 lung cancer patients. RESULTS: Diaphragm motion monitoring under free breathing (FB) was successful with intracostal US imaging. We observed that diaphragm visualization decreased with the increase in the body size of the volunteer. The US system was able to track the motion as small as 2 mm in the phantom. The intrafractional CBPI acquired during VMAT delivery was successfully synchronized with US acquisition in a phantom study. Collected patient data showed a significant correlation between diaphragm motion, an internal surrogate monitored by US, and the tumor motion in superior-inferior (SI) direction monitored by XVI (P Ë‚ 0.0001). CONCLUSIONS: The feasibility of real-time lung tumor motion tracking in SI direction with continuous ultrasound and periodic CBPI was demonstrated. The real-time estimation of the target position from the two streams for lung cancer patients would enable respiration gating or tracking during SBRT.

Development of 3-dimensional transperineal ultrasound for image guided radiation therapy of the prostate: Early evaluations of feasibility and use for inter- and intrafractional prostate localization.

PURPOSE: Transperineal ultrasound (TPUS) allows for continuous imaging of the prostate gland, but the accuracy of TPUS has not been rigorously studied. We determined the feasibility of prostate imaging with TPUS and subsequently compared prostate localization with TPUS and computed tomography (CT). METHODS AND MATERIALS: We completed 2 sequential evaluations of TPUS. The feasibility study included 15 men with localized prostate cancer and tested if TPUS adequately imaged the prostate. Image qualities of the prostate and adjacent normal structures were measured. The subsequent study included 17 men who at the time of initial radiation treatment planning and in 3 subsequent sessions had CT and TPUS imaging performed and compared. RESULTS: Feasibility of TPUS was confirmed in the first trial. After expected hardware and software modifications were completed, TPUS provided near complete edge definition of the prostate in the final 5 patients in the feasibility trial. The second study allowed for the comparison of 30 image sets. The differences between TPUS and CT in each direction (mean + standard deviation) were found to be 0.06 ± 2.86 mm (anteroposterior), 0.49 ± 3.49 mm (superoinferior), and 0.63 ± 3.27 mm (left-right), with no significant difference between the 2 modalities (all P > .32). The Euclidean distance variance using the 2 techniques was 5.25 ± 1.79 mm, which was significantly different. CONCLUSIONS: TPUS provides good imaging of the prostate gland. We noted excellent correlation in gland localization when TPUS is compared with CT scans when comparing routine 3-dimensional positional data. Euclidean distance variation suggests the potential that summation of small errors may in fact lead to significant differences in actual gland positional certainty. The reported difference is within the range of standard planning target volume expansion however requires additional evaluation.

Preliminary results on the feasibility of using ultrasound to monitor intrafractional motion during radiation therapy for pancreatic cancer.

PURPOSE: Substantial intrafraction organ motion during radiation therapy (RT) for pancreatic cancer is well recognized as a major limiting factor for accurate delivery of RT. The aim of this work is to determine the feasibility of monitoring the intrafractional motion of the pancreas or surrounding structures using ultrasound for RT delivery. METHODS: Transabdominal ultrasound (TAUS) and 4DCT data were acquired on ten pancreatic cancer patients during radiation therapy process in a prospective study. In addition, TAUS and MRI were collected for five healthy volunteers. The portal vein (PV) and the head of the pancreas (HP) along with other structures were contoured on these images. Volume changes, distance between the HP and PV, and motion difference between the HP and PV were measured to examine whether PV can be used as a motion surrogate for HP. TAUS images were acquired and processed using a research version of the Clarity autoscan ultrasound system (CAUS). Motion monitoring was performed with the ultrasound probe mounted on an arm fixed to the couch. Video segments of the monitoring sessions were captured. RESULTS: On TAUS, PV is better visualized than HP. The measured mean volume deviation for all patients for the HP and PV was 1.4 and 0.6 ml, respectively. The distance between the HP and PV was close to a constant with 0.22 mm mean deviation throughout the ten breathing phases. The mean of the absolute motion difference for all patients was 1.7 ± 0.8 mm in LR, 1.5 ± 0.5 mm in AP, and 2.3 ± 0.7 mm in SI, suggesting that the PV is a good surrogate for HP motion estimation. By using this surrogate, the HP motion tracking using TAUS was demonstrated. CONCLUSIONS: Large intrafractional organ motion due to respiratory and/or bowel motion is a limiting factor in administering curative radiation doses to pancreatic tumors. The authors investigate the use of real-time ultrasound to track pancreas motion. Due to the poor visibility of the pancreas head on an ultrasound image, the portal vein is identified as a surrogate. The authors have demonstrated the feasibility of tracking HP motion through the localization of the PV using TAUS. This will potentially allow real-time tracking of intrafractional motion to justify small PTV-margins and to account for unusual motions, thus, improving normal tissue sparing.

Evaluation of uterine ultrasound imaging in cervical radiotherapy; a comparison of autoscan and conventional probe.

OBJECTIVE: In cervical radiotherapy, it is essential that the uterine position is correctly determined prior to treatment delivery. The aim of this study was to evaluate an autoscan ultrasound (A-US) probe, a motorized transducer creating three-dimensional (3D) images by sweeping, by comparing it with a conventional ultrasound (C-US) probe, where manual scanning is required to acquire 3D images. METHODS: Nine healthy volunteers were scanned by seven operators, using the Clarity(®) system (Elekta, Stockholm, Sweden). In total, 72 scans, 36 scans from the C-US and 36 scans from the A-US probes, were acquired. Two observers delineated the uterine structure, using the software-assisted segmentation in the Clarity workstation. The data of uterine volume, uterine centre of mass (COM) and maximum uterine lengths, in three orthogonal directions, were analyzed. RESULTS: In 53% of the C-US scans, the whole uterus was captured, compared with 89% using the A-US. F-test on 36 scans demonstrated statistically significant differences in interobserver COM standard deviation (SD) when comparing the C-US with the A-US probe for the inferior-superior (p < 0.006), left-right (p < 0.012) and anteroposterior directions (p < 0.001). The median of the interobserver COM distance (Euclidean distance for 36 scans) was reduced from 8.5 (C-US) to 6.0 mm (A-US). An F-test on the 36 scans showed strong significant differences (p < 0.001) in the SD of the Euclidean interobserver distance when comparing the C-US with the A-US scans. The average Dice coefficient when comparing the two observers was 0.67 (C-US) and 0.75 (A-US). The predictive interval demonstrated better interobserver delineation concordance using the A-US probe. CONCLUSION: The A-US probe imaging might be a better choice of image-guided radiotherapy system for correcting for daily uterine positional changes in cervical radiotherapy. ADVANCES IN KNOWLEDGE: Using a novel A-US probe might reduce the uncertainty in interoperator variability during ultrasound scanning.