Intensity-based 2D/3D registration using kilo-voltage (kV) and mega-voltage (MV) on-board imaging is a promising approach for real-time tumor motion tracking. So far, the performance of the kV images as well as kV-MV image pairs for 2D/3D registration using only one gantry angle (in anterior-posterior (AP) direction) has been investigated on patient data. In stereotactic body radiation therapy (SBRT), however, various gantry angles are typically used. This study attempts to answer the question of whether automatic 2D/3D registration is possible using kV images as well as kV-MV image pairs for gantry angles other than the AP direction. We also investigated the effect of additional portal MV images paired with kV images to improve 2D/3D registration in extracting cranio-caudal (CC) and AP displacement at arbitrary gantry angles and different fractions. The kV and MV image sequences as well as 3D volume data from five patients suffering from non-small cell lung cancer undergoing SBRT were used. Diaphragm motion served as the reference signal. The CC and AP displacements resulting from the registration results were compared with the corresponding reference motion signal. Pearson correlation coefficients (R value) was used to calculate the similarity measure between reference signal and the extracted displacements resulting from the registration. Signals we found that using 2D/3D registration tumor motion in 5 degrees of freedom (DOF) with kV images and in 6 degrees of freedom with kV-MV image pairs can be extracted for most gantry angles in all patients. Furthermore, our results have shown that the use of kV-MV image pairs increases the overall chance of tumor visibility and therefore leads to more successful extraction of CC as well as AP displacements for almost all gantry angles in all patients. We observed an improvement in registration of at least 0.29% more gantry angle for all patients when we used kV-MV images compared to kV images alone. In addition, an improvement in the R-value was observed in up to 16 fractions in various patients.

MRI-guided radiotherapy (MRIgRT) is a highly complex treatment modality, allowing adaptation to anatomical changes occurring from one treatment day to the other (inter-fractional), but also to motion occurring during a treatment fraction (intra-fractional). In this vision paper, we describe the different steps of intra-fractional motion management during MRIgRT, from imaging to beam adaptation, and the solutions currently available both clinically and at a research level. Furthermore, considering the latest developments in the literature, a workflow is foreseen in which motion-induced over- and/or under-dosage is compensated in 3D, with minimal impact to the radiotherapy treatment time. Considering the time constraints of real-time adaptation, a particular focus is put on artificial intelligence (AI) solutions as a fast and accurate alternative to conventional algorithms.

OBJECTIVE: The current study investigated the impact of abdominal compression on motion and the delivered dose during non-gated, magnetic resonance image (MRI)-guided radiation ablation of adrenal gland metastases.
METHODS: Thirty-one patients with adrenal gland metastases treated to 45-60 Gy in 3-8 fractions on a 1.5 T MRI-linac were included in the study. The patients were breathing freely (n = 14) or with motion restricted by using an abdominal compression belt (n = 17). The time-resolved position of the target in online 2D cine MR images acquired during treatment was assessed and used to estimate the dose delivered to the GTV and abutting luminal organs at risk (OAR).
RESULTS: The median (range) 3D root-mean-square target position error was significantly higher in patients treated without a compression belt [2.9 (1.9-5.6) mm] compared to patients using the belt [2.1 (1.2-3.5) mm] (P < 0.01). The median (range) GTV V95% was significantly reduced from planned 98.6 (65.9-100) % to delivered 96.5 (64.5-99.9) % due to motion (P < 0.01). Most prominent dose reductions were found in patients showing either large target drift or respiration motion and were mainly treated without abdominal compression. Motion did not lead to an increased number of constraint violations for luminal OAR.
CONCLUSIONS: Acceptable target coverage and dose to OAR was observed in the vast majority of patients despite intra-fractional motion during adaptive MRI-guided radiation ablation. The use of abdominal compression significantly reduced the target position error and prevented the most prominent target coverage degradations and is, therefore, recommended as motion management at MRI-linacs.

UNASSIGNED: The aim of this study is to quantify the short-term motion of the gastrointestinal tract (GI-tract) and its impact on dosimetric parameters in stereotactic body radiation therapy (SBRT) for pancreatic cancer.
UNASSIGNED: The analyzed patients were eleven pancreatic cancer patients treated with SBRT or proton beam therapy. To ensure a fair analysis, the simulation SBRT plan was generated on the planning CT in all patients with the dose prescription of 40 Gy in 5 fractions. The GI-tract motion (stomach, duodenum, small and large intestine) was evaluated using three CT images scanned at spontaneous expiration. After fiducial-based rigid image registration, the contours in each CT image were generated and transferred to the planning CT, then the organ motion was evaluated. Planning at risk volumes (PRV) of each GI-tract were generated by adding 5 mm margins, and the volume receiving at least 33 Gy (V33) < 0.5 cm3 was evaluated as the dose constraint.
UNASSIGNED: The median interval between the first and last CT scans was 736 s (interquartile range, IQR:624-986). To compensate for the GI-tract motion based on the planning CT, the necessary median margin was 8.0 mm (IQR: 8.0-10.0) for the duodenum and 14.0 mm (12.0-16.0) for the small intestine. Compared to the planned V33 with the worst case, the median V33 in the PRV of the duodenum significantly increased from 0.20 cm3 (IQR: 0.02-0.26) to 0.33 cm3 (0.10-0.59) at Wilcoxon signed-rank test (p = 0.031).
UNASSIGNED: The short-term motions of the GI-tract lead to high dose differences.

UNASSIGNED: This study evaluated translational and rotational intra-fractional patient movement during spinal stereotactic body radiotherapy (SBRT) using 6D positioning based on 3D cone beam computerized tomography (CBCT) and stereoscopic kilovoltage imaging (ExacTrac). The aim was to determine whether additional intra-fractional image verification reduced intra-fractional motion without significantly prolonging treatment time, whilst maintaining acceptable imaging related dose.
UNASSIGNED: A retrospective analysis of 38 patients with 41 primary tumour volumes treated with SBRT between September 2018 and May 2021 was performed. Three different image-guided radiotherapy (IGRT) workflows were assessed. The translational and rotational positioning errors for the different imaging workflows, 3D translational vectors and estimates of imaging dose delivered for the different imaging workflows were evaluated.
UNASSIGNED: As the frequency of intra-fractional imaging increased from workflow 1 to 3, the mean intra-fraction 3D translational vector improved from 0.91 mm (±0.52 mm), to 0.64 (±0.34 mm). 85 %, 83 % and 97 % of images were within a tolerance of 1 mm/1° for workflows 1, 2 and 3 respectively, based on post treatment CBCT images. The average treatment time for workflow 3 was 13 min, as compared to 12 min for workflows 1 and 2. The effective dose per treatment for IGRT workflows 1, 2 and 3 measured 0.6 mSv, 0.95 mSv and 1.8 mSv respectively.
UNASSIGNED: The study demonstrated that the use of additional intra-fractional stereoscopic kilovoltage image-guidance during spinal SBRT, reduced the number of measurements deemed \"out of tolerance\" and treatment delivery could be optimized within a standard treatment timeslot without applying substantial additional radiation dose.

OBJECTIVE: Stereotactic radiotherapy (SRT) for spine metastases with helical tomotherapy requires a long irradiation time due to the high dose per fraction. Since helical tomotherapy can neither confirm nor correct the position during irradiation, a plan with a long irradiation time cannot be used in actual clinical practice, given the intra-fractional motion error. To address this problem, we devised a method called REPEAT irradiation.
METHODS: REPEtitive pAinTing (REPEAT) irradiation is a method of dividing the irradiation for a given fraction per day into several sessions and performing the irradiation after position correction using mega-voltage computed tomography images for each session. In order to evaluate how REPEAT irradiation changes irradiation time and the dose-volume histogram (DVH), a planning study with helical tomotherapy was conducted using CT images of a patient with lumbar spine metastasis.
RESULTS: In this case, we found that dividing 3 irradiation fractions into 3 sessions per day (i.e., 9 fractions=9 sessions in 3 days) using REPEAT irradiation shortened the irradiation time per session and simultaneously improved dose-volume histogram parameters.
CONCLUSIONS: Although the optimal number of sessions may differ depending on the patient\'s condition, the fixing method, the irradiation site, and the calculation parameters, REPEAT irradiation does not require any special equipment and is a simple practical treatment method.

OBJECTIVE: Positional uncertainty in spinal stereotactic body radiotherapy (SBRT) may cause fatal error, therefore, we investigated the intra-fractional spinal motion during SBRT and its time dependency.
METHODS: Thirty-one patients who received SBRT using CyberKnife were enrolled in the study. 2D kV X-ray spine images in two directions were taken before and during treatment. Image acquisition intervals during treatment were set at 35-60 sec. Automatic image matchings were performed between the reference digital reconstructed radiography (DRR) and live images, and the spinal position displacements were logged in six translational and rotational directions. If the displacements exceeded 2 mm or 1 degree, the treatment beam delivery was interrupted and the patient position was corrected by moving couch, and the couch adjustments were also logged. Based on the information, the time-dependent accumulated translational and rotational displacements without any couch adjustments were calculated.
RESULTS: Spinal position displacements in all translational and rotational directions were correlated with elapsed treatment time. Especially, Right-Left displacements of >1 mm and >2 mm were observed at 4-6 and 8-10 min after treatment initiation, respectively. Rotational displacements in the Yaw direction >1° were observed at 10-15 min after treatment initiation.
CONCLUSIONS: The translational and rotational displacements systematically increased with elapsed treatment time. It is suggested that the spine position should be checked at least every 4-6 min or the treatment time should be limited within 4-6 minutes to ensure the irradiation accuracy within the millimeter or submillimeter range.

Abdominal organ motions introduce geometric uncertainties to gastrointestinal radiotherapy. This study investigated slow drifting motion induced by changes of internal anatomic organ arrangements using a 3D radial MRI sequence with a scan length of 20 min. Breathing motion and cyclic GI motion were first removed through multi-temporal resolution image reconstruction. Slow drifting motion analysis was performed using an image time series consisting of 72 image volumes with a temporal sampling rate of 17 s. B-spline deformable registration was performed to align image volumes of the time series to a reference volume. The resulting deformation fields were used for motion velocity evaluation and patient-specific motion model construction through principal component analysis (PCA). Geometric uncertainties introduced by slow drifting motion were assessed by Hausdorff distances between unions of organs at risk (OARs) at different motion states and reference OAR contours as well as probabilistic distributions of OARs predicted using the PCA model. Thirteen examinations from 11 patients were included in this study. The averaged motion velocities ranged from 0.8 to 1.9 mm min-1, 0.7 to 1.6 mm min-1, 0.6 to 2.0 mm min-1and 0.7 to 1.4 mm min-1for the small bowel, colon, duodenum and stomach respectively; the averaged Hausdorff distances were 5.6 mm, 5.3 mm, 5.1 mm and 4.6 mm. On average, a margin larger than 4.5 mm was needed to cover a space with OAR occupancy probability higher than 55%. Temporal variations of geometric uncertainties were evaluated by comparing across four 5 min sub-scans extracted from the full scan. Standard deviations of Hausdorff distances across sub-scans were less than 1 mm for most examinations, indicating stability of relative margin estimates from separate time windows. These results suggested slow drifting motion of GI organs is significant and geometric uncertainties introduced by such motion should be accounted for during radiotherapy planning and delivery.

The aim of the study was to investigate the clinical feasibility of whole-brain radiation therapy without a thermoplastic mask. Positioning and intra-fractional motion monitoring were performed using optical surface scanning. The motion threshold was 3 mm/3 degrees. The group mean vector deviation was 1.1 mm. The roll was larger compared to pitch and rotation. Two patients out of 30 were not able to lie still. All other patients completed their treatment successfully without a mask. With a probability of success of 93%, we concluded that irradiation without a mask is a clinically feasible method.

The POP-ART RT study aims to determine to what extent and how intra-fractional real-time respiratory motion management (RRMM) and plan adaptation for inter-fractional anatomical changes (ART), are used in clinical practice and to understand barriers to implementation. Here we report on part I: RRMM.
A questionnaire was distributed worldwide to assess current clinical practice, wishes for expansion or new implementation and barriers to implementation. RRMM was defined as inspiration/expiration gating in free-breathing or breath-hold, or tracking where the target and the beam are continuously realigned.
The questionnaire was completed by 200 centres from 41 countries. RRMM was used by 68% of respondents (\'users\') for a median (range) of 2 (1-6) tumour sites. Eighty-one percent of users applied inspiration breath-hold in at least one tumour site (breast: 96%). External marker was used to guide RRMM by 61% of users. KV/MV imaging was frequently used for liver and pancreas (with fiducials) and for lung (with or without fiducials). Tracking was mainly performed on robotic linacs with hybrid internal-external monitoring. For breast and lung, approximately 75% of respondents used or wished to implement RRMM, which was lower for liver (44%) and pancreas (27%). Seventy-one percent of respondents wished to implement RRMM for a new tumour site. Main barriers were human/financial resources and capacity on the machine.
Sixty-eight percent of respondents used RRMM and 71% wished to implement RRMM for a new tumour site. The main barriers to implementation were human/financial resources and capacity on treatment machines.