Intrafractional Imaging Research Articles

634: Audit of DIBH lung SABR patient positioning accuracy via intra-fractional CBCT imaging

634: Audit of DIBH lung SABR patient positioning accuracy via intra-fractional CBCT imaging

Intrafractional motion detection for spine SBRT via X-ray imaging using ExacTrac Dynamic

PurposeDue to its close vicinity to critical structures, especially the spinal cord, standards for safety for spine stereotactic body radiotherapy (SBRT) should be high. This study was conducted, to evaluate intrafractional motion during spine SBRT for patients without individualized immobilization (e.g., vacuum cushions) using high accuracy patient monitoring via orthogonal X-ray imaging. MethodsIntrafractional X-ray data were collected from 29 patients receiving 79 fractions of spine SBRT. No individualized immobilization devices were used during the treatment. Intrafractional motion was monitored using the ExacTrac Dynamic (ETD) System (Brainlab AG, Munich, Germany). Deviations were detected in six degrees of freedom (6 DOF). Tolerances for repositioning were 0.7 mm for translational and 0.5° for rotational deviations. Patients were repositioned when the tolerance levels were exceeded. ResultsOut of the 925 pairs of stereoscopic X-ray images examined, 138 (15 %) showed at least one deviation exceeding the predefined tolerance values. In all 6 DOF together, a total of 191 deviations out of tolerance were recorded. The frequency of deviations exceeding the tolerance levels varied among patients but occurred in all but one patient. Deviations out of tolerance could be seen in all 6 DOF. Maximum translational deviations were 2.6 mm, 2.3 mm and 2.8 mm in the lateral, longitudinal and vertical direction. Maximum rotational deviations were 1.8°, 2.6° and 1.6° for pitch, roll and yaw, respectively. Translational deviations were more frequent than rotational ones, and frequency and magnitude of deviations showed an inverse correlation. ConclusionIntrafractional motion detection and patient repositioning during spine SBRT using X-ray imaging via the ETD System can lead to improved safety during the application of high BED in critical locations. When using intrafractional imaging with low thresholds for re-positioning individualized immobilization devices (e.g. vacuum cushions) may be omitted.

Optimized workflow to minimise intra-fractional motion during stereotactic body radiotherapy of spinal metastases.

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. 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. As the frequency of intra-fractional imaging increased from workflow 1 to 3, the mean intra-fraction 3D translational vector improved from 0.91mm (±0.52mm), to 0.64 (±0.34mm). 85%, 83% and 97% of images were within a tolerance of 1mm/1° for workflows 1, 2 and 3 respectively, based on post treatment CBCT images. The average treatment time for workflow 3 was 13min, as compared to 12min for workflows 1 and 2. The effective dose per treatment for IGRT workflows 1, 2 and 3 measured 0.6mSv, 0.95mSv and 1.8mSv respectively. 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.

Reducing ExacTrac intrafraction imaging uncertainty for prostate stereotactic body radiotherapy using a pre-treatment CBCT.

This study evaluated the intrafractional auto-matching uncertainties of prostate-implanted fiducial markers when using the ExacTrac v6.5 (Brainlab, Feldkirchen, Germany) X-ray stereoscopic system. A customised phantom with 3 implanted gold seeds was initially positioned at the isocentre using a cone beam CT (CBCT) prior to intrafractional imaging. Progressive offsets were applied to the phantom in all six directions (3 translational, 3 rotational) of 0mm, 1mm, 2mm, 0°, 1° and 2°. Subsequently, the ability of the ExacTrac image-matching functions to detect and correct these offsets was tested. For comparison, this procedure was repeated, but without a CBCT for pre-treatment positioning. The auto-matching uncertainties when a CBCT was introduced into the workflow were significantly reduced, and overall, the auto-matching statistics using the implanted marker (seeds) matching function was found to be more precise than the bony anatomy function in-phantom. The total standard deviations for the translational shifts using the implanted marker and bony anatomy functions respectively were 0.1mm and 0.3mm vertically, 0.1mm and 0.3mm longitudinally, and 0.1mm and 0.4mm laterally. The standard deviations for the rotational shifts using the implanted marker and bony anatomy matching functions respectively were 0.2° and 1.2° for the yaw (angle vert), 0.3° and 1.1° for the pitch (angle long), and 0.2° and 1.2° for the roll (angle lat) directions. The reduced uncertainties from introducing a CBCT for initial localisation resulted in decreased probability of inhibits due to false positives during treatment.

Based on 0.35 T magnetic resonance-guided radiotherapy, what are the nonisotropic PTV margins required for conventional prostate radiotherapy?

This study aims to calculate planning target volume (PTV) margins for the prostate and seminal vesicles (SVs) from the use of magnetic resonance-guided radiation therapy (MRgRT). And whether nonisotropic PTV margins are beneficial for these structures. Organ motion is linked to the displacement of the prostate and SVs. From the use of MRgRT, the nearby organs at risk (OAR) can be visualized both inter- and intrafraction. This study looked to determine if there is a correlation between interfractional OAR changes and displacements to the prostate and SVs. Inter- and intrafractional data from 20 consecutive prostate cancer patients treated using extreme hypofractionated 0.35 T MRgRT indicated prostate and SV motion during treatment. Tracking points (TPs) on 2D sagittal cine-MRI enabled assessment of this intrafractional motion. To determine a correlation between rectal changes and target displacements, the rectal diameter (RD) changes were compared against the displacement differences (DDs) at the prostate and SVs. Eighty percent of patients required intrafractional imaging corrections during radiotherapy, including 16/100 fractions due to rectal volume increases and 24/100 fractions due to bladder volume increases. The frequency of ≥3 mm intrafraction displacement was considerably greater in TPs in the SV than in the prostate. A moderate positive correlation (R2 = 0.417) was shown between RD changes and DDs at the level of the prostate and SVs. The PTV margins required for 90% of the patient cohort for prostate and SVs are nonuniform in different directions, and the margin is larger for SVs. Organ motion contributed toward prostate and SV displacements and showed the importance of a robust bladder and rectal-filling protocol.

Evaluation of inter- and intra-fraction 6D motion for stereotactic body radiation therapy of spinal metastases: influence of treatment time

BackgroundThe objective of this study was to analyze the amplitude of translational and rotational movements occurring during stereotactic body radiotherapy (SBRT) of spinal metastases in two different positioning devices. The relevance of intra-fractional imaging and the influence of treatment time were evaluated.MethodsTwenty patients were treated in the supine position either (1) on a body vacuum cushion with arms raised and resting on a clegecel or (2) on an integrated SBRT solution consisting of a SBRT table top, an Orfit™ AIO system, and a vacuum cushion. Alignments between the cone beam computed tomography (CBCT) and the planning computed tomography allowed corrections of inter- and intra-fraction positional shifts using a 6D table. The absolute values of the translational and rotational setup errors obtained for 329 CBCT were recorded. The translational 3D vector, the maximum angle, and the characteristic times of the treatment fractions were calculated.ResultsAn improvement in the mean (SD) inter-fraction 3D vector (mm) from 7.8 (5.9) to 5.9 (3.8) was obtained by changing the fixation devices from (1) to (2) (p < 0.038). The maximum angles were less than 2° for a total of 87% for (1) and 96% for (2). The mean (SD) of the intra-fraction 3D vectors (mm) was lower for the new 1.1 (0.8) positioning fixation (2) compared to the old one (1) 1.7 (1.7) (p = 0.004). The angular corrections applied in the intra-fraction were on average very low (0.4°) and similar between the two systems. A strong correlation was found between the 3D displacement vector and the fraction time for (1) and (2) with regression coefficients of 0.408 (0.262–0.555, 95% CI) and 0.069 (0.010–0.128, 95% CI), respectively. An accuracy of 1 mm would require intra-fraction imaging every 5 min for both systems. If the expected accuracy was 2 mm, then only system (2) could avoid intra-fractional imaging.ConclusionsThis study allowed us to evaluate setup errors of two immobilization devices for spine SBRT. The association of inter- and intra-fraction imaging with 6D repositioning of a patient is inevitable. The correlation between treatment time and corrections to be applied encourages us to move toward imaging modalities which allow a reduction in fraction time.

Intrafractional relationship changes between an external breathing signal and fiducial marker positions in pancreatic cancer patients.

Background and purposeThe purpose of this study of pancreatic cancer patients treated with respiratory‐guided stereotactic body radiotherapy (SBRT) on a standard linac was to investigate (a) the intrafractional relationship change (IRC) between a breathing signal and the tumor position, (b) the impact of IRC on the delivered dose, and (c) potential IRC predictors.Materials and methodsWe retrospectively investigated 10 pancreatic cancer patients with 2–4 implanted fiducial markers in the tumor treated with SBRT. Fluoroscopic images were acquired before and after treatment delivery simultaneously with the abdominal breathing motion. We quantified the IRC as the change in fiducial location for a given breathing amplitude in the left–right (LR), anterior–posterior (AP), and superior–inferior (SI) directions from before to after treatment delivery. The treatment plans were re‐calculated after changing the isocenter coordinates according to the IRCs. Four treatment‐ or patient‐related factors were investigated as potential predictors for IRC using linear models.ResultsThe average (±1 SD) absolute IRCs in the LR, AP, and SI directions were 1.2 ± 1.2 mm, 0.7 ± 0.7 mm, and 1.1 ± 0.8 mm, respectively. The average 3D IRC was 2.0 ± 1.3 mm (range: 0.4–5.3 mm) for a median treatment delivery time of 8.5 min (range: 5.7–19.9 min; n = 31 fractions). The dose coverage of the internal target volume (ITV) decreased by more than 3% points in three of 31 fractions. In those cases, the 3D IRC had been larger than 4.3 mm. The 3D IRC was found to correlate with changes in the minimum breathing amplitude during treatment delivery.ConclusionOn average, 2 mm of treatment delivery accuracy was lost due to IRC. Periodical intrafractional imaging is needed to safely deliver respiratory‐guided SBRT.

Ultrafast single breath-hold cone-beam CT lung cancer imaging with faster linac gantry rotation

PurposeLung tumors treated with hypo-fractionated deep-inspiration breath-hold stereotactic body radiotherapy benefit from fast imaging and treatment. Single breath-hold cone-beam-CT (CBCT) could reduce motion artifacts and improve treatment precision. Thus, gantry speed was accelerated to 18°/s, limiting acquisition time to 10–20 s. Image quality, dosimetry and registration accuracy were compared with standard-CBCT (3°/s). Methods and materialsFor proof-of-concept, image quality was analyzed following customer acceptance tests, CT-dose index measured, and registration accuracy determined with an off-centered ball-bearing-phantom. A lung-tumor patient was simulated with differently shaped tumor-mimicking inlays in a thorax-phantom. Signal-to-noise-ratio, contrast-to-noise-ratio and geometry of the inlays quantified image quality. Dose was measured in representative positions. Registration accuracy was determined with inlays scanned in pre-defined positions. Manual, automatic (clinical software) and objective-automatic (in-house-developed) registration was performed on planning-CT, offsets between results and applied shifts were compared. ResultsImage quality of ultrafast-CBCT was adequate for high-contrast areas, despite contrast-reduction of ∼80% due to undersampling. Dose-output was considerably reduced by 60–83% in presented setup; variations are due to gantry-braking characteristics. Registration accuracy was maintained better than 1 mm, mean displacement errors were 0.0 ± 0.2 mm with objective-automatic registration. Ultrafast-CBCT showed no significant registration differences to standard-CBCT. ConclusionsThis study of first tests with faster gantry rotation of 18°/s showed promising results for ultrafast high-contrast lung tumor CBCT imaging within single breath-hold of 10–20 s. Such fast imaging times, in combination with fast treatment delivery, could pave the way for intra-fractional combined imaging and treatment within one breath-hold phase, and thus mitigate residual motion and increase treatment accuracy and patient comfort. Even generally speaking, faster gantry rotation could set a benchmark with immense clinical impact where time matters most: palliative patient care, general reduction in uncertainty, and increase in patient throughput especially important for emerging markets with high patient numbers.

Stereotactic Body Radiation Therapy for Thoracic Malignancies: An Analysis of MR-Guided Radiation Therapy Gating Tracking Accuracy

Stereotactic body radiation therapy (SBRT) is effective for thoracic malignancies. Yet, accurate delivery to the target can be challenging due to respiratory motion. OAR sparing is crucial to avoid acute and late toxicities, especially when delivering high SBRT doses per fraction. With MR-Image Guided Radiotherapy (MR-IGRT), lung targets can be tracked in real time on planar intra-fractional MRI and treatment can be interrupted when the target deviates far beyond its planned position. In this study, we sought to evaluate tracking performance of MR-IGRT for lung tumors. Six patients with lung cancer treated with MR-IGRT using respiratory tracking and gating were selected according to observed intra-fraction tumor motion. Cine images, tracked contours, gating signals and gating parameters were collected. For each fraction, up to 200 randomly selected frames from intra-fractional imaging were selected to sample the tumor’s motion extent during beam on and beam off. A radiation oncologist re-contoured the target on all frames. Tracked and manual contours were first compared using Dice similarity coefficient (DSC) and Hausdorff distances (HD). Gating signals were also calculated for manual and tracked contours. From these signals, fractions were determined for: a. beam-off instances where beam should have been-on and b. beam-on frames where beam should have been off. Lastly, target excursions beyond the PTV were identified on beam-on instances. Patients were treated for 3 (n=2) or 5 (n=4) fractions. The target was manually contoured in more than 4600 frames. All patients had cranial-caudal tumor motion greater than 2 cm as measured on 4DCT. Mean GTV volume was 44.4 cc (1.1-143.7). Median treatment duty-cycle (beam-on time %) was 69% (37-100). As summarized in Table 1, DSCs and HDs were systematically larger on beam-off frames than beam-on frames, possibly due to tracking difficulty when the target deviates further from its expected position. Agreement between gating signals generated both contours varied fraction-by-fraction. Compelling metrics for deviation were measured between tracked and manual contours as quantified using conventional algorithms - such as DSC and HD. However, this is a pilot study with limitations and therefore requires further dosimetric analysis and clinical correlation with patient outcome.Abstract TU_24_3553; Table 1DSCs and HDs calculated between manual and tracked contours for beam on/off frames; treatment duty-cycle.PatientBeam OffBeam OnDuty-Cycle (%)DSCHD (mm)DSCHD (mm)MedianMinimumMaximum10.71 ± 0.0718.5 ± 2.40.73 ± 0.0816.8 ± 3.6716610020.66 ± 0.0210.2 ± 2.10.66 ± 0.048.9 ± 2.46055673----0.57 ± 0.085.6 ± 0.610010010040.50 ± 0.1611.4 ± 1.80.55 ± 0.1410.2 ± 1.765537950.81 ± 0.0214.4 ± 1.10.81 ± 0.0412.3 ± 1.847375960.69 ± 0.0712.2 ± 2.40.72 ± 0.058.0 ± 1.39550100Cohort0.67 ± 0.1713.4 ± 5.30.69 ± 0.1610.8 ± 5.56937100 Open table in a new tab

OA126] Intrafractional imaging during volumetric modulated arc therapy (VMAT) prostate treatment in combination with gold anchor fiducials

Purpose Triggered imaging is the use of kV image acquisition during treatment delivery to monitor intrafractional motion of implanted fiducials. An automatically identified fiducial point on the kV image is compared to its expected position predetermined from the treatment planning CT, and the treatment can be paused if the difference exceeds a user-definable tolerance. The purpose of this work was to evaluate triggered imaging in combination with Gold Anchor fiducial markers. Methods Triggered imaging was performed using a spherical homogeneous phantom with images acquired in 0° and 270° on a Truebeam STX linac (Varian Medical Systems, USA). The phantom contained three Gold Anchor fiducials (Naslund Medical, Sweden) and was moved 1–6 mm using HexaMotion (Scandidos, Sweden) during triggered imaging. Since Gold Anchor fiducials are flexible and may end up in various shapes in the patient, the tests were performed with both compressed (around 5 mm) and more elongated fiducials (up to 15 mm). To evaluate how well the system could locate fiducials in patients, triggered imaging was performed during one fraction in five patients undergoing 2-arc VMAT prostate treatment, all with three Gold Anchor fiducials implanted in the prostate. Images were acquired in intervals of 30°, resulting in 24 images per patient. The fiducial tracking accuracy was visually inspected in all images. In both the phantom and patient evaluation a position difference tolerance of 5 mm was used. Results During the phantom tests, the system correctly identified all fiducial positions as being inside or outside their tolerance for movements ≤ 4 or ≥ 6 mm. Negligible prostate motion were observed during the five prostate treatment deliveries. The system correctly identified 316/360 fiducials as being inside tolerance, 36/360 could not be located and 8/360 were falsely identified as another fiducial or bone structure. The main reason for tracking failure was elongated and/or insufficiently separated fiducials. In both the phantom and the patient situation, the system did not always identify the same point on a fiducial. This tracking variation was larger for elongated fiducials. Conclusions Triggered imaging seems to perform satisfactorily when used in combination with well-separated and compressed Gold Anchor fiducials.

PO-0865: Commissioning and clinical implementation of intra-fractional 4D-CBCT imaging for lung SBRT

PO-0865: Commissioning and clinical implementation of intra-fractional 4D-CBCT imaging for lung SBRT

Investigating the surface dose contribution of intrafractional kV imaging in CyberKnife-based stereotactic radiosurgery

CyberKnife treatment consists of hundreds of noncoplanar beams and numerous intrafractional images that can be taken during a single treatment fraction; thus, doses because of imaging should be considered in this technique. The aim of this study is to investigate the in-field and out-of-field surface doses induced from kV imaging system during stereotactic radiosurgery (SRS) treatment. The imaging-induced surface doses were measured at the center of the imaging field and within ±15-cm distance from the center in both craniocaudal and lateral directions. TLD100H thermoluminescence dosimeters and EBT2 gafchromic films were used to take the measurements at the locations of 0, ±5, ±10, and ±15 cm in the 2 orthogonal directions on abdominal region of a Rando phantom. The surface dose contributions of imaging system for the 4 most commonly used energy options of 90, 100, 110, and 120 kVp with 3 mAs options of 10, 30, and 90 mAs were measured and compared. Imaging dose values have a positive correlation with both parameters of energy and mAs. The energy options of 100, 110, and 120 kVp, in average, induced 60%, 101%, and 141% more doses per mAs than 90 kVp energy in the imaging field center. A threefold increase in mAs values, i.e., from 10 mAs to 30 mAs and from 30 mAs to 90 mAs, caused higher dose in field center with a factor of 2.53 ± 0.08 when the energy value was kept constant. The in-field dose distributions within ±10 cm in both directions showed a flat pattern with a standard deviation lower than 5%, whereas the out-of-field doses at ±15-cm distance from the field center suddenly dropped to almost half of the central doses. Although a single imaging attempt causes a very low dose compared with the therapeutic dose level, one should be aware of the cumulative surface dose increase with higher imaging number. Proper patient setup, fiducial usage, and reduction of both the mAs values and the imaging numbers should be, therefore, considered to keep the cumulative surface dose in a lower level.

Computerized triplet beam orientation optimization for MRI-guided Co-60 radiotherapy.

Magnetic resonance imaging (MRI)-guided Co-60 provides daily and intrafractional MRI soft tissue imaging for improved target and critical organ tracking. To increase delivery efficiency, the system uses three Co-60 sources at 120° apart, allowing up to 600 cGy combined dose rate at isocenter. Despite the potential tripling in output, creating a delivery plan that uses all three sources is considerably unintuitive. Here, the authors computerize the triplet orientation optimization using column generation, an approach that was demonstrated effective in integrated beam orientation and fluence optimization for noncoplanar therapies. To achieve a better plan quality without increasing the treatment time, the authors then solve a fluence map optimization (FMO) problem while regularizing the fluence maps to reduce the number of deliverable MLC segments. Three patients-one prostate, one lung, and one head and neck boost plan (H&NBoost)-were evaluated in this study. For each patient, the beamlet doses were calculated using Monte Carlo, under a 0.35 T magnetic field, for 180 equally spaced coplanar beams grouped into 60 triplets. The beamlet size is 1.05 × 0.5 cm determined by the MLC leaf thickness and step size. The triplets were selected using the column generation algorithm. The FMO problem was formulated using an L2-norm dose fidelity term and an L1-norm anisotropic total variation regularization term, which allows controlling the number of MLC segments, and hence the treatment time, with minimal degradation to the dose. The authors' Fluence Regularization and Optimized Selection of Triplets (FROST) plans were compared against the clinical treatment plans (CLNs) produced by an experienced dosimetrist. PTV homogeneity, max dose, mean dose, D95, D98, and D99 were evaluated. OAR max and mean doses, as well as R50, defined as the ratio of the 50% isodose volume over the planning target volume were investigated. The mean PTV D95, D98, and D99 differ by +0.04%, +0.07%, and +0.25% of the prescription dose between planning methods. The mean PTV homogeneity was virtually same with values at 0.8788 (FROST) and 0.8812 (CLN). R50 decreased by 0.67 comparing FROST to CLN. On average, FROST reduced Dmax and Dmean of OARs by 7.30% and 6.08% of the prescription dose, respectively. The manual CLN planning processes required numerous trial and error runs. The FROST plans on the other hand required minimal human intervention. Efficient delivery of MRI-guided Co-60 therapy needs the output of multiple sources yet suffers from unintuitive and laborious manual beam selection processes. Computerized triplet orientation optimization improves both planning efficiency and plan dosimetry. The novel fluence map regularization provides additional controls over the number of MLC segments and treatment time.

TH‐AB‐BRA‐02: Automated Triplet Beam Orientation Optimization for MRI‐Guided Co‐60 Radiotherapy

Purpose:MRI guided Co‐60 provides daily and intrafractional MRI soft tissue imaging for improved target tracking and adaptive radiotherapy. To remedy the low output limitation, the system uses three Co‐60 sources at 120° apart, but using all three sources in planning is considerably unintuitive. We automate the beam orientation optimization using column generation, and then solve a novel fluence map optimization (FMO) problem while regularizing the number of MLC segments.Methods:Three patients—1 prostate (PRT), 1 lung (LNG), and 1 head‐and‐neck boost plan (H&amp;NBoost)—were evaluated. The beamlet dose for 180 equally spaced coplanar beams under 0.35 T magnetic field was calculated using Monte Carlo. The 60 triplets were selected utilizing the column generation algorithm. The FMO problem was formulated using an L2‐norm minimization with anisotropic total variation (TV) regularization term, which allows for control over the number of MLC segments. Our Fluence Regularized and Optimized Selection of Triplets (FROST) plans were compared against the clinical treatment plans (CLN) produced by an experienced dosimetrist.Results:The mean PTV D95, D98, and D99 differ by −0.02%, +0.12%, and +0.44% of the prescription dose between planning methods, showing same PTV dose coverage. The mean PTV homogeneity (D95/D5) was at 0.9360 (FROST) and 0.9356 (CLN). R50 decreased by 0.07 with FROST. On average, FROST reduced Dmax and Dmean of OARs by 6.56% and 5.86% of the prescription dose. The manual CLN planning required iterative trial and error runs which is very time consuming, while FROST required minimal human intervention.Conclusions:MRI guided Co‐60 therapy needs the output of all sources yet suffers from unintuitive and laborious manual beam selection processes. Automated triplet orientation optimization is shown essential to overcome the difficulty and improves the dosimetry. A novel FMO with regularization provides additional controls over the number of MLC segments and treatment time.Varian Medical Systems; NIH grant R01CA188300; NIH grant R43CA183390.

Kilovoltage Imaging of Implanted Fiducials to Monitor Intrafraction Motion With Abdominal Compression During Stereotactic Body Radiation Therapy for Gastrointestinal Tumors

To assess intrafraction respiratory motion using a commercial kilovoltage imaging system for abdominal tumor patients with implanted fiducials and breathing constrained by pneumatic compression during stereotactic body radiation therapy (SBRT). A pneumatic compression belt limited respiratory motion in 19 patients with radiopaque fiducials in or near their tumor during SBRT for abdominal tumors. Kilovoltage images were acquired at 5- to 6-second intervals during treatment using a commercial system. Intrafractional fiducial displacements were measured using in-house software. The dosimetric effect of the observed displacements was calculated for 3 sessions for each patient. Intrafraction displacement patterns varied between patients and between individual treatment sessions. Averaged over 19 patients, 73 sessions, 7.6% of craniocaudal displacements exceeded 0.5cm, and 1.2% exceeded 0.75cm. The calculated single-session dose to 95% of gross tumor volume differed from planned by an average of -1.2% (range, -11.1% to 4.8%)but only for 4 patients was the total 3-session calculated dose to 95% of gross tumor volume more than 3% different from planned. Our pneumatic compression limited intrafractional abdominal target motion, maintained target position established at setup, and was moderately effective in preserving coverage. Commercially available intrafractional imaging is useful for surveillance but can be made more effective and reliable.

Ultrasound Imaging in Radiation Therapy: From Interfractional to Intrafractional Guidance.

External beam radiation therapy (EBRT) is included in the treatment regimen of the majority of cancer patients. With the proliferation of hypofractionated radiotherapy treatment regimens, such as stereotactic body radiation therapy (SBRT), interfractional and intrafractional imaging technologies are becoming increasingly critical to ensure safe and effective treatment delivery. Ultrasound (US)-based image guidance systems offer real-time, markerless, volumetric imaging with excellent soft tissue contrast, overcoming the limitations of traditional X-ray or computed tomography (CT)-based guidance for abdominal and pelvic cancer sites, such as the liver and prostate. Interfractional US guidance systems have been commercially adopted for patient positioning but suffer from systematic positioning errors induced by probe pressure. More recently, several research groups have introduced concepts for intrafractional US guidance systems leveraging robotic probe placement technology and real-time soft tissue tracking software. This paper reviews various commercial and research-level US guidance systems used in radiation therapy, with an emphasis on hardware and software technologies that enable the deployment of US imaging within the radiotherapy environment and workflow. Previously unpublished material on tissue tracking systems and robotic probe manipulators under development by our group is also included.

Setting Action Levels for Abdominal Stereotactic Body Radiation Therapy Monitored With Frequent kV Imaging Surveillance

We hypothesize that intra-fractional motion monitoring with frequent kV imaging surveillance together with an appropriate intervention action level ensures adequate tumor dose to abdominal cancer patients with breathing amplitude restricted by compression who are treated with stereotactic body radiation therapy on a Linac. We retrospectively studied the treatment plans and intra-fractional images of 10 upper-abdominal cancer patients treated on a Linac with hypofractionated IMRT. For all patients, one or more radio-opaque markers were implanted near the tumor. Abdominal compression reduced respiratory marker motion below 6 mm as evaluated by fluoroscopy before each treatment. The PTV was defined by a 5 mm 3D expansion of the GTV. Intra-fractional kV radiographs were automatically acquired at 5-6 second intervals during beam delivery, triggered by the RPM system’s response to a motion phantom mounted above the patient. Planned marker positions are overlaid on each radiograph and displayed on the treatment console; therapists check compression and correct setup if persistent deviation from planned position is seen. kV images were retrospectively analyzed using in-house software to determine marker shifts from their planned positions. To estimate dosimetric effects, the MLC position at each control point is shifted according to the superior-inferior component of the fiducial trajectory, and the resulting dose to GTV and surrounding organs at risk (OAR) is reconstructed. The relationship between the degradation of GTV D95, dose to OAR, mean and maximal fiducial displacement is explored. The allowable mean and maximal marker motion excursion is determined to obey objectives such as GTV D95 degrades less than 5% and no OAR constraints are violated for this patient population. The marker motion excursion ranged from 2 to 6 mm on pre-treatment fluoroscopy, but from 2 to 23 mm on intra-fractional kV projections. The correlation between the two is 0.44. The weak correlation and bigger amplitude argue for intra-fractional motion monitoring and management. The relative difference between the planned GTV D95 and that reconstructed accounting for intra-fractional motion ranges from -8.8% to 4.4%, and averages -1.4%. Without additional intra-fractional intervention, the 5 mm margin around GTV resulted in 10% of fractions violating the objectives. However, the objectives can be met with tools to enforce marker-based action levels that limit mean marker motion to [-2mm 2mm] and maximal excursion to <12 mm. The combination of compression belt, intra-fractional kV imaging surveillance, and advanced intervention methods can ensure patients receive improved dose coverage.

First demonstration of intrafractional tumor‐tracked irradiation using 2D phantom MR images on a prototype linac‐MR

To demonstrate intrafractional MR tumor tracking using a prototype linac-MR by delivering radiation to a moving target undergoing simulated tumor motions. A prototype linac-MR at the Cross Cancer Institute was used for intrafractional MR imaging and simultaneous beam delivery. A Varian 52-leaf MK-II multileaf collimator (MLC) was used for beam collimation. The authors used an inhouse built MR compatible motion phantom to simulate tumor motions during tracking with two different motion patterns (sine and modified cosine). Gafchromic film was inserted in the phantom to measure radiation exposure, and this film measurement was converted to dose (cGy) for further analysis. The authors demonstrated intrafractional tracking in various scenarios: [Scenario 0 (S0)] no phantom motion + no beam margin, (S1) no phantom motion + maximum beam margin, (S2) phantom motion + no beam margin, (S3) S2 + MLC tracking, and (S4) S3 + motion prediction. S0 emulates a perfect tumor tracking scenario, and its result was used as a "gold-standard" to evaluate tracking accuracy from other scenarios. The authors compared (1) time difference in phantom and MLC motion curves in S3 and S4, and (2) dose profiles (50% beam width, 80%-20% penumbra width) from scenarios S1-S4 to S0. In S4, no observable time difference exists between the phantom and MLC motion curves, indicating that MLC tracks phantom motion accurately. Comparing S4 to S0, 50% beam width reveals minimal differences of < 0.5 mm, while the increase in 80%-20% penumbra width is limited to 0.4 and 1.7 mm in the sine and modified cosine patterns, respectively. The authors report the first demonstration of intrafractional tumor tracking using 2D MR images. During 2 min of tracking, the authors delivered highly conformal dose to a moving target that simulates tumor motions. Compared to static target irradiation, the 50% beam width remains essentially the same (within 0.5 mm), with an increase in 80%-20% penumbra width of less than 1.7 mm in moving target irradiation. These results illustrate potential dosimetric advantages of intrafractional MR tumor tracking in treating mobile tumors as shown for the phantom case.

OC-0358: MRI guided adaptive cervical cancer brachytherapy: Impact of repeated intrafractional imaging

OC-0358: MRI guided adaptive cervical cancer brachytherapy: Impact of repeated intrafractional imaging

MV-kV Intrafractional Imaging During Volume Modulated Arc Therapy (VMAT): Initial Clinical Results

registration. We now produce these masks through image segmentation making the registration completely automatic. We investigate how the accuracy of the segmented masks affects the registration. Materials/Methods: The program was developed using the Insight Toolkit. Prostate patient CTs were segmented via a number of algorithms. By comparing generated to manual contours, Neighborhood Connected was chosen as the preferred segmentation algorithm. Due to the difficulty in segmenting the bladder from prostate, a new mask combining the bladder and prostate volume, was also produced. Three registration methods will therefore be compared: using manual rectum and bladder masks (previous standard), segmented rectum and bladder masks, and segmented rectum and combined bladder prostate masks. Agreement of manual contours of the prostate (which is not masked individually), in the registered images was used to infer registration accuracy. Results: The DC between the segmented and manual masks was up to 96 and 93% for the bladder and rectum respectively. This allowed the segmented mask guided deformable registration algorithm to achieve the same agreement in prostate contours as when manual masks are used, (both average 90% DC). When the combined bladder prostate-segmented mask is used the DC of the prostate contour improves to 95%, (the rectum and bladder contour DC remains the same). Conclusions: A now fully automated hybrid deformable registration method was developed for the prostate region capable of recovering large initial organ variation. Good agreement of segmented masks to the organ they represent is required to achieve acceptable registration results. Using a combined bladder prostate mask is the preferred method as it simplifies the segmentation process, while allowing for improved prostate registration. This automatic registration is used to produce contours and delivered dose for adaptive therapy and cumulative dose for outcome analysis. Author Disclosure: A.R. Godley: None. M. Itomlenskis: None.