Intrafraction Motion Research Articles

Implementing online position monitoring for prostate radiotherapy using an in-house position monitoring system: User experience and impact on workforce.

To evaluate the feasibility of prostate intrafraction motion monitoring using the SeedTracker real-time image guidance system in order to improve targeting accuracy in prostate radiotherapy. SeedTracker was used to monitor prostate gold fiducial seeds with kV x-ray imaging during radiotherapy in 30 patients. Feedback from radiation therapists was collected via the use of a user evaluation form. The impact on treatment time was established by using a record and verify system. The effective dose and a risk of exposure-induced cancer death (REID) were estimated for a 60-year-old patient when using the SeedTracker system. A total of 22 radiation therapists completed user evaluation forms. The time taken to prepare a reference data set for one patient varied with three (13.6%) radiation therapists taking less than 2 min, 10 (45.5%) between 2 and 4 min, eight (36.4%) between 4 and 6 min and one (4.5%) between 6 and 8 min. The useability of the SeedTracker system was reported as 'easy' by 21 (95.5%) radiation therapists and 'hard' by 1 (4.5%) radiation therapist. Mean treatment time changed from 6 to 7 min with prostate-only radiotherapy treatment and from 6.9 to 10.2 min with prostate and whole pelvis radiotherapy treatments. The maximum effective dose with the SeedTracker was 1.6276 mSv, and increase in REID was 0.007%. The SeedTracker real-time image guidance system is a feasible tool to use in radiotherapy departments to monitor and correct for prostate intrafraction motion.

WASPE Sleep Deprivation, Paired with an Optical Surface Monitoring System, Can Provide Accurate Radiation Therapy to Pediatric Patients Without the Need for Sedation

This study aimed to explore the use of a protocol for sleep deprivation, combined with a surface optical monitoring system (OSMS)-guided radiation therapy (RT) to eliminate the need for pharmaceutical sedation of pediatric patients with cancer. The study subjects were 28 children aged 0 to 3 years undergoing intensity modulated RT. Parents used the WASPE method to keep the children awake from 6:00 am to 2:00 pm on each day of RT. To compare the potential effects of sleep deprivation versus sedation on treatment, the first 16 patients tested were alternately sleep deprived 1 day and given sedatives the next day for the duration of the RT. In all cases, RT was delivered under the real-time guidance of an OSMS. Twenty-seven of 28 children successfully completed 343 fractions of OSMS-guided RT. The 5-mm threshold for intrafractional motion was exceeded, and treatment was interrupted for patient repositioning during 13 sessions (sleep deprivation: 10 of 253 fractions; sedation: 3 of 90 fractions). Among the first 16 patients, no significant differences in inter- or intrafractional errors were noted on cone beam computed tomography scanning, except for an intrafractional error in the vertical direction for the 8th patient (P=.046). No significant difference in intrafractional errors measured by OSMS was found between the sleep-deprivation and sedation groups, except for 1 in the lateral direction for the 4th patient (P=.042) and 1 in the vertical direction for the 7th patient (P=.05). No differences were found in complete blood counts between the sleep-deprivation and sedation groups. In all, 36 of 52 parents surveyed (69%) preferred sleep deprivation, and only 3 parents (6%) preferred sedatives. The use of a sleep-deprivation protocol with OSMS guidance allows for accurate delivery of RT to children aged 0 to 3 years without the need for sedation and without jeopardizing target coverage. This approach may be useful for children who are resistant to or cannot tolerate sedatives.

Sequential monoscopic image-guided motion compensation in tomotherapy stereotactic body radiotherapy (SBRT) for prostate cancer.

To manage intra-fractional motions, recent developments in tomotherapy enable a unique capability of adjusting MLC/jaw to track the moving target based on the intra-fractional motions detected by sequential monoscopic imaging. In this study, we evaluated the effectiveness of motion compensation with a realistic imaging rate for prostate stereotactic body radiotherapy (SBRT). The obtained results will guide optimizing treatment parameters and image-guided radiation therapy (IGRT) in tomotherapy using this approach. Ten retrospective prostate cases with actual prostate motion curves previously recorded through the Calypso system were used in this study. Based on the recorded peak-to-peak motion, these cases represented either large(>5mm) or median (≤ 5mm) intra-fractional prostate motions. All the cases were re-planned on tomotherapy using 35Gy/5 fractions SBRT regimen and three different jaw settings of 1cm static, 2.5cm static, and 2.5cm dynamic jaw. Two motion compensation methods were evaluated: a complete compensation that adjusted the jaw and MLC every 0.1 s (the same rate as the Calypso motion trace), and a realistic compensation that adjusted the jaw and MLC at an average imaging interval of 6 s from sequential monoscopic images. An in-house 4D dose calculation software was then applied to calculate the dosimetric outcomes from the original motion-free plan, the motion-contaminated plan, and the two abovementioned motion-compensated plans. During the process, various imaging rates were also simulated in one case with unusually large motions to quantify the impact of the KV-imaging rate on the effectiveness of motion compensation. The effectiveness of motion compensation was evaluated based on the PTV coverage and OAR sparing. Without any motion-compensation, the PTV coverage (PTV V100%) of patients with large prostate motions decreased remarkably to 55%-82% when planning with the 1cm jaw but to a less level of 67-94% with the 2.5cm jaw. In contrast, motion compensation improved the PTV coverage(>92%) when combined with the 2.5cm jaw, but less effective, around 75%-94%, with the 1cm jaw. For OAR sparing, the bladder D1cc, bladder D10cc, and rectum D1cc all increased in the motion-contaminated plans. Motion compensation improved OAR sparing to the equivalent level of the original motion-free plans. For patients with median prostate motion, motion-induced degradation in PTV coverage was only observed when planning with the 1cm jaw. After motion compensation, the PTV coverage improved to better than 94% for all three jaw settings. Additionally, the effectiveness of motion compensation depends on the imaging rate. Motion compensation with a typical rate of two KV images per gantry rotation effectively reduces motion-induced dosimetric uncertainties. However, a higher imaging rate is recommended when planning with a 1cm jaw for patients with large motions. Our results demonstrated that the performance of sequential monoscopic imaging-guided motion compensation on tomotherapy depends on the amplitude of intra-fractional prostate motion, the plan parameter settings, especially jaw setting, gantry rotation, and the imaging rate for motion compensation. Creating a patient-specific imaging guidance protocol is essential to balance the effectiveness of motion compensation and achievable imaging rate for intra-fractional motion tracking.

Upright patient positioning for pelvic radiotherapy treatments.

Radiotherapy is typically delivered in supine position. However, upright positioning may affect organ volume, positioning, and movement, compared to supine/prone positioning which might have beneficial impacts. In this study, we report patient positioning data in an upright positioning system designed by Leo Cancer Care®. Sixteen patients with pelvic tumors were included in this study. They had 3 setups in an upright position: an initial setup with acquisition of reference optical images, and 2 repositioning setups. The intra-fraction motion was assessed during two 20-minute chair rotation sessions. The patient comfort in supine and upright position was assessed with a 5-point Likert scale questionnaire. Eight women and 8 men treated on regular linacs between October 2021 and June 2022 were included. Their median age and weight were 62.5years (35 to 81years) and 75.1kg (41 to 107kg). The inter-fraction shift means were -0.5mm (SD=2.5), -0.4mm (SD=1.3) and -0.9mm (SD=2.7) in left-right (LR), antero-posterior (AP), and cranio-caudal (CC) directions, respectively. The intrafraction shifts after 20min were 0.0mm (SD=1.5), 0.2mm (SD=1.1) and 0.0mm (SD=0.3) in LR, CC, and AP directions, respectively. Average global comfort was 4.1 (3 to 5) for the upright position and 3.9 (2 to 5) for the supine position. In conclusion, the first study on pelvic cancer patients positioned in upright position on a chair is promising, and it opens a potential new direction for the treatment of cancer patients. Evaluation of thoracic and head and neck tumors is ongoing, and imaging with vertical CT is expected to start soon.

The dosimetric error due to uncorrected tumor rotation during real-time adaptive prostate stereotactic body radiation therapy.

During prostate stereotactic body radiation therapy (SBRT), prostate tumor translational motion may deteriorate the planned dose distribution. Most of the major advances in motion management to date have focused on correcting this one aspect of the tumor motion, translation. However, large prostate rotation up to 30° has been measured. As the technological innovation evolves toward delivering increasingly precise radiotherapy, it is important to quantify the clinical benefit of translational and rotational motion correction over translational motion correctionalone. The purpose of this work was to quantify the dosimetric impact of intrafractional dynamic rotation of the prostate measured with a six degrees-of-freedom tumor motion monitoringtechnology. The delivered dose was reconstructed including (a) translational and rotational motion and (b) only translational motion of the tumor for 32 prostate cancer patients recruited on a 5-fraction prostate SBRT clinical trial. Patients on the trial received 7.25 Gy in a treatment fraction. A 5 mm clinical target volume (CTV) to planning target volume (PTV) margin was applied in all directions except the posterior direction where a 3 mm expansion was used. Prostate intrafractional translational motion was managed using a gating strategy, and any translation above the gating threshold was corrected by applying an equivalent couch shift. The residual translational motion is denoted as . Prostate intrafractional rotational motion was recorded but not corrected. The dose differences from the planned dose due to + , ΔD( + ) and due to alone, ΔD( ), were then determined for CTV D98, PTV D95, bladder V6Gy, and rectum V6Gy. The residual dose error due to uncorrected rotation, was then quantified: = ΔD( + ) - ΔD( ). Fractional data analysis shows that the dose differences from the plan (both ΔD( + ) and ΔD( )) for CTV D98 was less than 5% in all treatment fractions. ΔD( + ) was larger than 5% in one fraction for PTV D95, in one fraction for bladder V6Gy, and in five fractions for rectum V6Gy. Uncorrected rotation, induced residual dose error, , resulted in less dose to CTV and PTV in 43% and 59% treatment fractions, respectively, and more dose to bladder and rectum in 51% and 53% treatment fractions, respectively. The cumulative dose over five fractions, ∑D( + ) and ∑D( ), was always within 5% of the planned dose for all four structures for everypatient. The dosimetric impact of tumor rotation on a large prostate cancer patient cohort was quantified in this study. These results suggest that the standard 3-5 mm CTV-PTV margin was sufficient to account for the intrafraction prostate rotation observed for this cohort of patients, provided an appropriate gating threshold was applied to correct for translational motion. Residual dose errors due to uncorrected prostate rotation were small in magnitude, which may be corrected using different treatment adaptation strategies to further improve the dosimetricaccuracy.

The delivered dose assessment in pancreas SBRT with the target position determined using an in-house position monitoring system.

This study assessed the delivered dose accuracy in pancreas SBRT by incorporating the real-time target position determined using an in-house position monitoring system. An online image-based position monitoring system, SeedTracker, was developed to monitor radiopaque marker positions using monoscopic x-ray images, available from the Elekta XVI imaging system. This system was applied to patients receiving SBRT for pancreatic cancer on the MASTERPLAN Pilot trial (ACTRN 12617001642370). All patients were implanted pre-treatment with at least three peri-tumoral radiopaque markers for target localisation. During treatment delivery, marker positions were compared to expected positions delineated from the planning CT. The position tolerance of ±3mm from the expected position of the markers was set to trigger a gating event (GE) during treatment. The dosimetric impact of position deviations and actual dose delivered with position corrections was assessed by convolving the plan control point dose matrices with temporal target positions determined during treatment. Eight patients were treated within this study. At least one GE was observed in 38% of the treatment fractions and more than one GE was observed in 10% of the fractions. The position deviations resulted in the mean(range) difference of -0.1(-1.1 - 0.4)Gy in minimum dose to tumour and 1.9(-0.1- 4.6)Gy increase to Dmax to duodenum compared to planned dose. In actual treatment delivery with the patient realignment, the mean difference of tumour min dose and duodenal Dmax was reduced to 0.1(-1.0 - 1.1)Gy and 1.1 (-0.7 - 3.3)Gy respectively compared to the planned dose. The in-house real-time position monitoring system improved the treatment accuracy of pancreatic SBRT in a general-purpose linac and enabled assessment of delivered dose by incorporating the temporal target position during delivery. The intrafraction motion impacts the dose to tumour even if target position is maintained within a 3mm position tolerance.

Deep learning-based internal gross target volume definition in 4D CT images of lung cancer patients.

Contouring of internal gross target volume (iGTV) is an essential part of treatment planning in radiotherapy to mitigate the impact of intra-fractional target motion. However, it is usually time-consuming and easily subjected to intra-observer and inter-observer variability. So far, few studies have been explored to directly predict iGTV by deep learning technique, because the iGTV contains not only the gross target volume (GTV) but also the motion information of the GTV. This work was an exploratory study to present a deep learning-based framework to segment iGTV rapidly and accurately in 4D CT images for lung cancers. Five models, including 3D UNet, mmUNet with point-wise add merging approach (mmUNet-add), mmUNet with concatenate fusion strategy (mmUNet-cat), gruUNet with point-wise add fusion approach (gruUNet-add), and gruUNet with concatenate method (gruUNet-cat), were adopted for iGTV segmentation. All the models originated from the 3D UNet network, with multi-channel multi-path and convolutional gated recurrent unit (GRU) added in the mmUNet and gruUNet networks, respectively. Seventy patients with lung cancers were collected and 55 cases were randomly selected as the training set, and 15 cases as the testing set. In addition, the segmentation results of the five models were compared with the ground truths qualitatively and quantitatively. In terms of Dice Similarity Coefficient (DSC), the proposed four networks (mmUNet-add, mmUNet-cat, gruUNet-add, and gruUNet-cat) increased the DSC score of 3D UNet from 0.6945 to 0.7342, 0.7253, 0.7405, and 0.7365, respectively. However, the differences were not statistically significant (p > 0.05). After a simple post-processing to remove the small isolated connected regions, the mean 95th percentile Hausdorff distances (HD_95s) of the 3D UNet, mmUNet-add, mmUNet-cat, gruUNet-add, and gruUNet-cat networks were 19.70, 15.75, 15.84, 15.61, and 15.83 mm, respectively, corresponding to 25.35, 25.96, 25.11, 28.23, and 24.47 mm before the post-processing. With regard to runtime, significant elapsed time growths (about 70s and 230s) were observed both in the mmUNet and gruUNet architectures due to the increasing parameters. But the mmUNet structure showed less growth. Our study demonstrated the ability of the deep learning technique to predict iGTVs directly. With the introduction of multi-channel multi-path and convolutional GRU, the segmentation accuracy was improved under certain conditions with a reduced segmentation efficiency and a further research topic when the 3D UNet network would lead to poor performance is elicited. Less efficiency degradation was observed in the mmUNet structure. Besides, the element-wise add fusing strategy was favorable to increase DSC, whereas HD_95 benefited from the concentrate merging approach. Nevertheless, the segmentation accuracy by deep learning still remains to be improved.

Reduction of intrafraction pancreas motion using an abdominal corset compatible with proton therapy and MRI

Background and purposeMotion mitigation is of crucial importance in particle therapy (PT) of patients with abdominal tumors to ensure high-precision irradiation. Magnetic resonance imaging (MRI) is an excellent modality for target volume delineation and motion estimation of mobile soft-tissue tumors. Thus, the aims of this study were to develop an MRI- and PT-compatible abdominal compression device, to investigate its effect on pancreas motion reduction, and to evaluate patient tolerability and acceptance. Materials and methodsIn a prospective clinical study, 16 patients with abdominal tumors received an individualized polyethylene-based abdominal corset. Pancreas motion was analyzed using time- and phase resolved MRI scans (orthogonal 2D-cine and 4D MRI) with and without compression by the corset. The pancreas was manually segmented in each MRI data set and the population-averaged center-of-mass motion in inferior-superior (IS), anterior-posterior (AP) and left–right (LR) directions was determined. A questionnaire was developed to investigate the level of patient acceptance of the corset, which the patients completed after acquisition of the planning computed tomography (CT) and MRI scans. ResultsThe corset was found to reduce pancreas motion predominantly in IS direction by on average 47 % − 51 % as found in the 2D-cine and 4D MRI data, respectively, while motion in the AP and LR direction was not significantly reduced. Most patients reported no discomfort when wearing the corset. ConclusionAn MRI- and PT-compatible individualized abdominal corset was presented, which substantially reduced breathing-induced pancreas motion and can be safely applied with no additional discomfort for the patients. The corset has been successfully integrated into our in-house clinical workflow for PT of tumors of the upper abdomen.

Understanding the Benefit of Magnetic Resonance-guided Adaptive Radiotherapy in Rectal Cancer Patients: a Single-centre Study

AimsNeoadjuvant chemoradiotherapy followed by surgery is the mainstay of treatment for patients with rectal cancer. Standard clinical target volume (CTV) to planning target volume (PTV) margins of 10 mm are used to accommodate inter- and intrafraction motion of target. Treating on magnetic resonance-integrated linear accelerators (MR-linacs) allows for online manual recontouring and adaptation (MRgART) enabling the reduction of PTV margins. The aim of this study was to investigate motion of the primary CTV (CTVA; gross tumour volume and macroscopic nodes with 10 mm expansion to cover microscopic disease) in order to develop a simultaneous integrated boost protocol for use on MR-linacs. Materials and methodsPatients suitable for neoadjuvant chemoradiotherapy were recruited for treatment on MR-linac using a two-phase technique; only the five phase 1 fractions on MR-linac were used for analysis. Intrafraction motion of CTVA was measured between pre-treatment and post-treatment MRI scans. In MRgART, isotropically expanded pre-treatment PTV margins from 1 to 10 mm were rigidly propagated to post-treatment MRI to determine overlap with 95% of CTVA. The PTV margin was considered acceptable if overlap was >95% in 90% of fractions. To understand the benefit of MRgART, the same methodology was repeated using a reference computed tomography planning scan for pre-treatment imaging. ResultsIn total, nine patients were recruited between January 2018 and December 2020 with T3a-T4, N0–N2, M0 disease. Forty-five fractions were analysed in total. The median motion across all planes was 0 mm, demonstrating minimal intrafraction motion. A PTV margin of 3 and 5mm was found to be acceptable in 96 and 98% of fractions, respectively. When comparing to the computed tomography reference scan, the analysis found that PTV margins to 5 and 10 mm only acceptably covered 51 and 76% of fractions, respectively. ConclusionPTV margins can be reduced to 3–5 mm in MRgART for rectal cancer treatment on MR-linac within an simultaneous integrated boost protocol.

Intrafraction Prostate Motion Management during Dose-Escalated Linac-Based SBRT

<h3>Purpose/Objective(s)</h3> This study reports the first clinical experience worldwide using a novel electromagnetic (EM) tracking device for intrafraction prostate motion management during dose-escalated Linac-based stereotactic body radiation therapy (SBRT). <h3>Materials/Methods</h3> Thirteen patients with organ-confined prostate cancer underwent dose-escalated SBRT using flattening filter-free (FFF) volumetric modulated arc therapy (VMAT). The prescribed dose was 40 Gy in 5 fractions or 38 Gy in 4 fractions. The EM tracking device consisted of an integrated Foley catheter with a transmitter. Patients were simulated and treated with a filled bladder and an empty rectum. Setup accuracy was achieved by ConeBeam-CT (CBCT) matching and the prostate motion was tracked during all the procedure. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, target position was re-defined by repeating CBCT. Moreover, the displacements that would have occurred without any intrafraction organ motion management (i.e., no interruptions and repositionings) were simulated and analyzed. <h3>Results</h3> In 31 out of 56 monitored fractions (55%), no intervention was required to correct the target position as a result of an excessive displacement. In 25 (45%) a correction was mandated, but only in 10 (18%), the beam delivery was interrupted. Total treatment time lasted on average 10.2 minutes, 6.7 minutes for setup, and 3.5 minutes for beam delivery. The prostate was found inside the 2 mm threshold from its initial position in 96% of the treatment time, i.e., in 94% of the time during the setup, and in 98% during the delivery (beam on + interruptions). Without any intrafraction motion management, the overall mean treatment time and the mean delivery time would have been 6.9 minutes and 3.2 minutes, respectively. The prostate would have been found outside the tolerance in 8% of the total session time, in 4% of the time during the setup, and in 14% during the beam-on phase. The probability of motion > 2 mm in the lateral, longitudinal, and vertical direction after 5 minutes was 3.6% (2/56), 8.9% (5/56), and 14.3% (8/56), respectively. The predominant motion pattern was posterior with a mean motion ≤ 2 mm occurring within 10 minutes. Moreover, the analysis of the rotation angles from all patients showed a systematic rotation in the pitch axis which is absent in the yaw axis. Concurrently, the range and standard deviation of rotation angles were larger in the pitch axis. <h3>Conclusion</h3> EM real-time tracking was successfully implemented for intrafraction motion management during dose-escalated prostate SBRT. Results showed that most of the observed displacements were < 2 mm in any direction; however, there were a non-insignificant number of fractions with motion exceeding the predefined threshold, which would have otherwise gone undetected without intrafraction motion management.

Reducing CTV to PTV Margins by Assessing Intrafractional Movement of Patients with Cervical Cancer Undergoing Definitive Adaptive Radiation

<h3>Purpose/Objective(s)</h3> The standard treatment for locally advanced gynecologic cancers typically involves pelvic chemoradiation. During radiation treatment planning, the cervix and uterus are delineated which constitute the clinical tumor volume (CTV). The expansion from CTV to planning tumor volume (PTV) ranges from 1.5 – 2.0 cm to account for uterocervical intra- and interfractional motion. With adaptive radiation therapy (ART) interfractional movement is accounted for. Thus, there is potential for CTV to PTV margins to be decreased to account for intrafractional movement and reduce the volume treated, but the correct CTV to PTV margin is unknown. In this pilot study, we compared pre-treatment to post-treatment cone beam CTs (CBCT) for patients undergoing radiation to determine intrafractional movement of the uterocervix. <h3>Materials/Methods</h3> A retrospective cohort of 14 patients with cervical cancer (FIGO stage IB3 – IIIC2) treated with definitive chemoradiation between April 2021 – February 2022 was included. Patients had a median age of 46 years (range 30 – 76 years). 9 patients treated with traditional non-adaptive radiation with 74 fractions had the uterocervix contoured on pre-treatment CBCTs and post-treatment CBCTs by a resident physician or physicist that were approved by the supervising attending physician. Anterior-posterior, lateral, and superior-inferior shifts were calculated based on contours and the average shift in all directions was calculated. Using this information, a CTV to PTV expansion was determined and verified on a validation cohort of 5 patients with 66 fractions treated with online CBCT based ART. <h3>Results</h3> For all 9 patients, the average anterior-posterior, lateral, and superior-inferior shifts were 0.45 ± 1.04 cm, 0.13 ± 0.54 cm, and 1.51 ± 1.92 cm, respectively. With a uniform expansion of 5 mm around the pre-treatment CTV, 98.85 ± 1.23% of the post-treatment CTV was covered. A 5 mm expansion to the pre-treatment CTV was applied to our validation cohort treated with ART, and 98.58 ± 1.99% of the post-treatment CTV was covered. <h3>Conclusion</h3> Based on our analysis of intrafractional movement, a CTV to PTV margin of 5 mm is adequate to encompass 99% of the CTV. We will continue to use this reduced margin to treat GYN patients with ART and determine whether our proposed margin is appropriate or needs to be changed. A significantly reduced margin could potentially decrease the toxicities associated with radiation for GYN patients and lead to improved patient reported toxicity outcomes.

Initial Experience with Real-Time Gated Proton Therapy (RGPT) in the Definitive Treatment of Prostate Cancer

<h3>Purpose/Objective(s)</h3> RGPT has the potential to provide instantaneous feedback for intrafraction target motion to maximize patient safety and inform optimal treatment planning. Our purpose was to report our early experience with RGPT and to summarize intrafraction target motion and implications on treatment delivery, including added time, for a pilot population of prostate cancer patients before expansion to other sites with greater motion. <h3>Materials/Methods</h3> RGPT with a proton beam therapy system was commissioned and implemented at our proton center, representing the first experience in the United States. Patients being treated definitively with proton therapy to the prostate ± seminal vesicles-only and who had successful placement of platinum fiducial markers and hydrogel spacer were assessed on a prospective IRB-approved registry. Orthogonal fluoroscopic imaging was taken at a pulse rate of one image per second during fractionated proton therapy to assess positioning of a pre-selected fiducial marker. Thresholds to guide beam-on and beam-off gating were determined for a matching score component (based on quality of the image tracking and the guiding template) and for a shift tolerance component (based on our typical set-up uncertainty robustness of 3mm). The shift tolerances were defined as 2mm for left-right and anterior-posterior, and 3mm for superior-inferior. Each patient was treated with single-field optimization planning and a two lateral beam setup. <h3>Results</h3> To date, 8 prostate cancer patients have been treated with a total of 152 RGPT-guided fields. The table details the mean, maximum, standard deviation, and estimated upper 90^th percentile of directional shifts for the entire dataset for 1) the total RGPT-tracked treatment time and 2) the beam-on time as gated by RGPT tracking. On average, the initial imaging to set up the RGPT system required 2.5 seconds per field. The average total treatment time during RGPT guidance, including periods of both "on" and "off" gating, was 41.3 seconds per field. RGPT tracking triggered "beam-off" for an average of 7.8 seconds per field, increasing the total treatment delivery time by 20.2% across the 152 fields. <h3>Conclusion</h3> RGPT successfully tracked intrafraction fiducial marker motion during proton therapy for prostate cancer, while having minimal impact on total treatment time. Further aggregated RGPT-derived data may help to inform optimal treatment planning parameters such as robust optimization.

Evaluation of Margin for Intra-Fractional Patient Motion during Single-Isocenter Multi Targets Volumetric Modulated Arc Therapy Stereotactic Radiation Therapy for Brain Metastases Using Actual Target Coordinates

<h3>Purpose/Objective(s)</h3> Single-isocenter multi targets volumetric modulated arc therapy (SIMT-VMAT) has been used extensively in stereotactic radiation therapy (SRT) for brain metastases. However, to the best of our knowledge, there is no report on the position displacement of each target by the intra-fractional patient motion (IFPM) during SIMT-VMAT SRT. In particular, the effect of rotational IFPM is assumed to be greater depending on the distance between the center of gravity of the target and the isocenter (DTI). We hypothesized that the margin for IFPM increased depending on the DTI during SIMT-VMAT SRT. Thus, this study aimed to evaluate the margin for the IFPM during SIMT-VMAT SRT by estimating actual target displacement (ATD), which consisted of translational and rotational displacement (RD) for each target by the IFPM. <h3>Materials/Methods</h3> The study included 25 patients (aged 49–83 years; 13 men and 12 women) with 2-13 brain metastases who had previously received SIMT-VMAT SRT, with four fractions in our institution. The total number of targets for all patients was 101 and the DTI range was 14–85 mm. The clamshell-style immobilization device immobilized all patients. Bite-block was used in 20 patients. Cone beam computed tomography (CBCT) images were acquired in pre- and post-treatment within the same imaging center. The IFPM was calculated from both CBCT images. Both ATD and RD were calculated by the difference between the planned target coordinates and the actual target coordinates, which were considered with the IFPM. The Spearman rank correlation coefficient (r_s) was used for the estimation of the correlation of RD with DTI and ATD with DTI. The Wilcoxon-signed rank test was used for comparing ATD, with and without bite-block. Statistical significance was established at <i>P</i> <0.05. <h3>Results</h3> In SIMT-VMAT SRT with bite-block, the median of ATD and RD was 0.37 mm (95% confidence interval (CI): 0.019–0.80 mm) and 0.15 mm (95% CI: 0.016–0.41 mm), respectively. We observed a weak positive correlation in RD with DTI (r_s=0.39, <i>P</i> <0.05) and no correlation in ATD with DTI (r_s=0.035, <i>P</i> =0.53). On the other hand, in SIMT-VMAT SRT without bite-block, the median of ATD and RD was 0.66 mm (95% CI 0.078–1.5 mm) and 0.43 mm (95% CI 0.065–1.0 mm), respectively. We observed a positive correlation in RD with DTI (r_s=0.52, <i>P</i> <0.05) and a weak positive correlation in ATD with DTI (r_s=0.28, <i>P</i> <0.05). There was a significant difference in ATD with and without bite-block (<i>P</i> <0.05). <h3>Conclusion</h3> These results indicate that the margin for the IFPM in SIMT-VMAT SRT with the bite-block would be equal regardless of the DTI. Our results indicated the bite-block increased the accuracy of patient immobilization. Moreover, it was suggested that the margin for the IFPM would be uniformly below 1 mm from the edge of the target.

Stereotactic Magnetic Resonance Guided Adaptive Radiation Therapy (SMART) for Abdominopelvic Oligometastases: A Combined Analysis of Prospective and Retrospective Cohorts

<h3>Purpose/Objective(s)</h3> Stereotactic body radiation therapy can be an effective treatment for oligometastases. However, safe delivery of ablative radiation therapy is commonly limited by the proximity of mobile organs at risk (OARs), as interfraction and intrafraction motion can result in severe complications. The goal of this study is to determine the feasibility, safety, and disease control outcomes of stereotactic magnetic resonance guided adaptive radiation therapy (SMART) in patients with abdominopelvic oligometastases. <h3>Materials/Methods</h3> We identified 101 patients with abdominopelvic oligometastases, including 20 patients enrolled on phase 1 protocols, who were consecutively treated with SMART on a 0.35 T MR Linac at a single institution from October 2019 to September 2021, excluding patients with metastases of the liver, kidneys, or adrenal glands. Local control and overall survival were analyzed using the Kaplan-Meier method. Adverse effects were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0. <h3>Results</h3> Overall, 114 sites of disease were treated. The most common histology was prostate adenocarcinoma (53.5%, n=60), and 57.0% (n=65) of sites were centered in the pelvis. 79.8% of sites (n=91) were treated to 8 Gy × 5, and 43.0% (n=49) were treated with breath hold. Online adaptation was used in 86.6% of delivered fractions to either improve the sparing of OARs (57.2%) or target coverage (29.4%). In addition, there were two cases where the on-board MRI allowed for the detection of bowel which had displaced the target to such a degree as to interfere with safe treatment delivery. These two fractions were postponed to either later the same day or a subsequent day. The median time required for adaptation was 24 minutes, and the median time in the treatment room was 58 minutes. With median follow-up of 11.4 months, the 12-month local control was 93% and higher for prostate adenocarcinoma vs other histologies (100% vs 84%, log-rank <i>P</i>=0.009). The 12-month overall survival was 96% and higher for prostate adenocarcinoma vs other histologies (100% vs 91%, log-rank <i>P</i>=0.046). Three patients developed grade 3 toxicities (colonic hemorrhage at 3.4 months and urinary tract obstructions secondary to ureteral stenoses at 10.1 and 18.4 months). <h3>Conclusion</h3> SMART is a feasible and safe method for delivering ablative radiation therapy to abdominopelvic oligometastases. Early data indicate a high rate of local control and low risk of significant toxicity. Adaptive planning was necessary in the large majority of cases. The advantages of SMART warrant its further investigation as a standard option for the treatment of abdominopelvic oligometastases.

A Prospective Feasibility Study Evaluating Skin Surface-Guided Gating as a Method to Manage Intrafraction Motion for Stereotactic Body Radiotherapy on a Novel Ring Gantry Machine

<h3>Purpose/Objective(s)</h3> We recently deployed CT-guided stereotactic body radiation therapy (SBRT) using an O-ring gantry linear accelerator with an artificial intelligence-enhanced treatment planning system capable of online adaptive radiotherapy. The intrafraction motion management solution for this system gates based on optical monitoring of the patient's skin surface. This system has not yet been clinically validated. Therefore, we prospectively evaluated the feasibility of surface-guided optical gating in a Phase I clinical trial for patients with mobile, upper abdominal or lower thoracic malignancies treated with SBRT on this machine. <h3>Materials/Methods</h3> Ten patients were planned for accrual to this study. Eligible patients were medically fit for SBRT, had at least one abdominal or thoracic disease site suitable for treatment, and were capable of breath hold for at least 17 seconds, the minimum duration of the on-board volumetric cone-beam CT. The trial protocol stipulated a treatment prescription in five fractions. Daily adaptation was at the discretion of the daily treating physician. MD arrival, re-contour, re-plan, delivery, and total fraction times were recorded. <h3>Results</h3> Eight patients enrolled and were eligible for evaluation at the time of this submission. Forty of 41 fractions (98%) were successfully delivered. One patient tolerated only partial delivery of his first fraction because of pain, but the remaining dose was successfully delivered in a sixth fraction. Twenty-seven fractions (53%) were adapted online, motivated by improving target coverage (7%), meeting organ constraints (33%), or both (44%). The median total fraction time was 80 minutes (IQR 66 – 96). <h3>Conclusion</h3> Skin surface-guided optical gating proved feasible in nearly all attempted fractions for respiratory gating of upper abdominal and lower thoracic tumors. This methodology is now being incorporated for SBRT on this platform off-trial.

Surface Imaging-Guided Adaptive Radiotherapy (ART): Imaging Protocol and Verification for Inter- and Intra-Fractional Motion Managements

<h3>Purpose/Objective(s)</h3> CBCT-guided adaptive radiotherapy (CT-ART) has proven to account for inter-fractional anatomic changes. However, no commercial solution for intra-fractional respiratory motion management existed in CT-ART. Therefore, we investigated intra-fractional motion-managed treatment strategies in CT-ART (or Image-guided RT: IGRT) using optical surface imaging combined with <i>in-situ</i> CBCTs. <h3>Materials/Methods</h3> Eight of ten cancer patients with mobile lower lung or upper abdominal malignancies have completed CT-ART (or IGRT) using Ethos radiotherapy in an IRB-approved clinical trial (phase I). Four of the eight patients underwent additional MR simulations, including cine MRIs, to estimate the tumor/abdominal skin motion correlation. In CT-ART, a pre-configured gating window (±2 mm in vertical position) on surface imaging was used for manually triggering intra-fractional CBCT and treatment beam irradiation in breath-hold (six patients for exhalation and two patients for inhalation). A donut-shaped imaging-surrogate on patient's abdomen was used to localize Region of Interest (ROI) of the abdomen on CBCTs. Two inter-fractional CBCTs at exhalation and inhalation in each fraction were acquired to verify the tumor/imaging-surrogate motions range. Intra-fractional CBCTs were used to quantify the residual motion of the tumor/imaging-surrogate within the pre-configured gating window. Tumor motion was measured by tracking the position of a tumor centroid. <h3>Results</h3> The pre-configured gating window on surface imaging was confirmed based on the correlation of the tumor and abdominal skin motions using cine MRIs. Thirty-six pairs of inter-fractional CBCTs and 100 intra-fractional CBCTs from 36 fractions were analyzed (13 fractions for IGRT and 23 for ART). In the motion range study from the inter-fractional CBCTs, the values of mean ± std were 17.3 ± 12.0 mm in the SI direction for the tumors and 16.6 ± 7.9 mm in the AP direction for the imaging-surrogate, respectively. The residual motion of the imaging-surrogate from intra-fractional CBCTs was 2.5 ± 2.1 mm for SI, 2.3 ± 2.0 mm for RL, and 2.1 ± 2.3 mm for AP directions. The residual motion of the tumors was 2.4 ± 3.1 mm for SI, 2.3 ± 2.3 mm for RL, and 2.2 ± 2.0 mm for AP directions. Trends in the residual motion during the course of treatment fractions and over time through delivery were stable. Inter-fractional residual motions of the tumors were 2.3 ± 0.8 mm for SI, 2.3 ± 0.6 mm for RL, and 2.2 ± 0.7 mm for AP directions. Intra-fractional residual motions of the tumors were 2.4 ± 0.9 mm for SI, 2.4 ± 0.7 mm for RL, and 3.2 ± 1.3 mm for AP. <h3>Conclusion</h3> We demonstrated the inter- and intra-fractional motion-managed treatment strategies in CBCT-guided ART using surface imaging. Since the residual motion of the tumors was comparable to that of the imaging-surrogate within clinical PTV margins (5 mm) but a bit larger than the pre-configured gating window, additional caution is needed with intra-fractional motion management in CT-ART using the optical surface imaging.

Real-Time Motion Monitoring Using Orthogonal Cine MRI during MR-Guided Radiation Therapy for Liver Cancer

<h3>Purpose/Objective(s)</h3> This work investigates the feasibility of real-time motion monitoring (MM) using orthogonal cine MRI during MR guided Adaptative Radiation Therapy (MRgART) for liver tumors that are normally invisible on cine MRI with no contrast agent. <h3>Materials/Methods</h3> An investigational package, Motion Monitoring Research Package (MMRP), was developed to measure real-time target motion between orthogonal cine MRI and the daily baseline 3D MRI. The MMRP package was evaluated on 8 liver cancer patients treated on a 1.5T MR-Linac. For all patients, a 3D mid-position (MidP) image derived from a daily 4D MRI was used to delineate a sub-liver volume (SLV) encompassing the tumor. During the same session, real-time 2D T2/T1-weighted bFFE cine MRI were collected with free breathing during MRgART. The cine images were acquired with a temporal resolution of 200 ms interleaved between coronal and sagittal orientations. MMRP workflow included two steps: (1) creating a 2D template in each orientation using cine data acquired over the first 12 s, followed by a rigid registration between the combined templates and the MidP image over the SLV region; (2) measuring the SLV motion by registering the rest of the 2D cine series with the corresponding template. To evaluate the MM accuracy from MMRP, the ground-truth contours on cine MRI were created manually. Common visible vessels and segments of liver boundaries in proximity to the tumor were used as anatomical landmarks for reproducible delineations on both the 3D and cine images. MM accuracy was measured using the standard deviation of the error (SDE) to compare the center of mass position of the ground-truth and the monitored motion. Maximum of tumor motion was assessed on 4D MRI. <h3>Results</h3> In 6 of the 8 cases, the assessed mean (range) centroid motions were 8.8 (4.6-13.8), 1.5 (0.7-2.8) and 3.2 (1.5-5) mm with mean SDE (range) values of 1.2 (0.7-1.6), 0.94 (0.4-1.7), and 1 (0.4-1.4) mm in superior inferior (SI), left right, and anterior posterior (AP) directions, respectively. The mean (range) of the maximum tumor motion from the 4D MRI was 8 (5-11) mm in SI direction, smaller than the centroid MM, indicating the importance of motion capture using the MM in real-time. For the remaining two cases, quantitative MM was challenging due to target deformation and large through-plane motion in the AP direction. <h3>Conclusion</h3> We have demonstrated the feasibility of accurate real-time MM of a liver tumor using a sub-liver volume as a tumor surrogate, based on real-time orthogonal cine MRI during MRgART. The approach overcomes difficulty due to the invisibility of liver tumor on the T2/T1-w 2D cine without using a contrast agent and the need of implanting radio-opaque clips or the approximation of the whole liver motion as the tumor motion. With further investigation with more patient data, the real-time MM technique may be implemented in MRgART to manage intrafraction liver motion.

Dosimetric Impact of Intrafraction Prostate Motion in Dose-Escalated Linac-Based SBRT

<h3>Purpose/Objective(s)</h3> The aim of this study was to investigate the impact of observed intrafraction prostate motion on dose metrics and the effect of beam gating and motion correction using a novel electromagnetic (EM) tracking device in dose-escalated Linac-based SBRT. <h3>Materials/Methods</h3> Thirteen patients (56 fractions) with organ-confined prostate cancer received 40 Gy in 5 fractions or 38 Gy in 4 fractions using VMAT technique with flattening filter-free (FFF) arcs. The EM tracking device consisted of an integrated Foley catheter with a transmitter and provided real-time 3D prostate motion data. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, target position was re-defined by repeating a ConeBeam-CT (CBCT). Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed. Both actually delivered treatments (case A) and non-gated treatments (case B) were simulated by incorporating the observed prostate motion for each fraction into the patient original treatment plan with an isocenter shift method. Target and organs at risk (OARs) dosimetric parameters of the two motion-inclusive plans were compared to planned values and protocol dose constraints. <h3>Results</h3> Treatment interruptions were needed in 25 fractions (45%) because of target motion beyond the predefined threshold. The prostate was found within 2 mm from its initial position in 96% of the treatment time, i.e., in 94% of the time during the setup, and in 98% during the delivery. In non-gated treatments, the prostate would have been found outside the tolerance in 8% of the total session time, in 4% of the time during the setup, and in 14% during beam-on. The predominant motion pattern was posterior and its probability increased with time, with a mean motion ≤ 2 mm occurring within 10 minutes. Mean relative dose differences respect to original plans were -0.1% [-1.8 – 1.0] for CTV D99% and -0.2% [-1.6 – 0.7] for PTV D95% in case A. The same values were -1.2% [-8.8 – 0.8] and -1.2% [-5.9 – 0.7] in case B. In both cases, urethra planning organ at risk volume was slightly degraded after taking motion into account, and the rectum and bladder dose metrics showed a favorable underexposition of rectum and an undesirable overdose to bladder. Nevertheless, no protocol dose constraints violations were observed. <h3>Conclusion</h3> Our findings showed that the majority of the observed prostate motion was modest; however, there were a non-insignificant number of fractions with motion exceeding the predetermined threshold. The current CTV to PTV margins, the robustness of original treatment plans, and the fast FFF beams delivery led to minimal degradations of dose metrics for the target and OARs in both cases. Anyway, real-time monitoring with beam gating and motion correction ensured superior results and are recommended in dose-escalated prostate SBRT.

Geometric and Dosimetric Evaluation of Using a Novel Tongue Positioning Device to Reduce Tongue Motions during Radiation Therapy for Head and Neck Cancer

<h3>Purpose/Objective(s)</h3> Inter- and intra-fractional organ motion in the head and neck region can reduce the precision of radiation therapy (RT). Among the organs, a tongue has substantial changes in its position and volume. Therefore, various types of tongue positioning devices have been used during RT for head and neck cancer. Despite of several clinical studies reporting an efficacy of using tongue positioning device, there are no studies investigating the inter-fractional reproducibility of position and shape of tongue while using a tongue positioning device. This study performs geometric and dosimetric evaluation of a novel tongue positioning device during RT for head and neck cancer. <h3>Materials/Methods</h3> Fifty-two patients with head and neck tumors using tongue positioning device were available for analysis. All patients were treated in 28 fractions using VMAT technique with cone-beam CT (CBCT) imaging. CBCT images were taken at 1^st, 7^th, 11^th, 15^th, 19^th, 23^th, and 27^th fractions. In order to evaluate the difference in geometry of the tongue between the CT simulation and the treatment at each fraction, tongues in simulation CT images and CBCT images were contoured in treatment planning system by a trained radiation oncologist. The geometric differences of the tongue between planning CT (pCT) and CBCT were assessed by Dice similarity coefficient (DSC) and averaged Hausdorff distance (AVD). In addition, 2D <i>in vivo</i> measurements using radiochromic films were performed for thirteen patients once a week during RT for 5 weeks. The films were attached to heads of the devices. The planned dose distributions at the surface of the head of device was compared to the measured dose distributions by using gamma analysis with criteria of 3%/3mm. <h3>Results</h3> For entire patients, DSC scored 0.77 to 0.80. The DSC at 1^st fraction (i.e., pCT vs. 1^st CBCT) was 0.80 while the DSC at 27^th fraction (i.e., pCT vs. 27^th CBCT) was 0.77. There was no statistical significance between the DSC at 1^st fraction and the DSC at 27^th fraction (p-value > 0.05). On the other hand, AVD ranged from 1.78 mm to 2.25 mm. The AVD was increased as the fractions were proceeded. There was difference between the AVD at 1^st fraction and the AVD at 23^th fraction and 27^th fraction with a statistical significance (p-value < 0.05). <i>In vivo</i> measurements showed averaged gamma passing rate of 90.54%. There was no significant difference between the measurements at 1^st week and those at other weeks. <h3>Conclusion</h3> Comparison of geometry of the tongue during RT for head and neck cancer was performed with pCT and CBCT. The AVD showed maximum 2.25 mm difference between pCT and CBCT. The AVD difference between the fractions was within 0.47 mm. DSC showed no significant inter-fractional differences. In conclusion, the novel tongue positioning device was found to minimize the inter-fractional variations of position and shape of the tongue.

Surface-guided DIBH radiotherapy for left breast cancer: impact of different thresholds on intrafractional motion monitoring and DIBH stability.

To compare two left breast cancer patient cohorts (tangential vs. locoregional deep-inspiration breath-hold- DIBH treatment) with different predefined beam gating thresholds and to evaluate their impact on motion management and DIBH stability. AnSGRT-based clinical workflow was adopted for the DIBH treatment. Intrafractional monitoring was performed by tracking both the respiratory signal and the real-time displacement between the isocenter on the daily reference surface and on the live surface ("SGRT shift"). Beam gating tolerances were 5 mm/4 mm for the SGRT shifts and 5 mm/3 mm for the gating window amplitude for breast tangential and breast + lymph nodes locoregional treatments, respectively. Atotal of 24patients, 12treated with atangential technique and 12with alocoregional technique, were evaluated for atotal number of 684 fractions. Statistical distributions of SGRT shift and respiratory signal for each treatment fraction, for each patient treatment, and for the two population samples were generated. Lateral cumulative distributions of SGRT shifts for both locoregional and tangential samples were consistent with anull shift, whereas longitudinal and vertical ones were slightly negative (mean values < 1 mm). The distribution of the percentage of beam on time with SGRT shift > 3 mm, > 4 mm, or > 5 mm was extended toward higher values for the tangential sample than for the locoregional sample. The variability in the DIBH respiration signal was significantly greater for the tangential sample. Different beam gating thresholds for surface-guided DIBH treatment of left breast cancer can impact motion management and DIBH stability by reducing the frequency of the maximum SGRT shift and increasing respiration signal stability when tighter thresholds are adopted.