Vision RT Research Articles

Assessing Surface Guided Radiation Therapy Benefits for Paediatric Cancer Patients: Dosimetric Implications of Intrafractional Motion - An Institutional Review

<i>Introduction/Background</i>: SGRT, a real-time imaging technique, offers continuous monitoring and motion control during <i>treatment</i>. The investigation aims to assess potential dosimetric alterations in target coverage due to intrafractional motion, considering its impact on patient safety and treatment efficiency. <i>Materials and Methods</i>: A retrospective chart review was conducted to assess intrafractional shifts in 18 paediatric cancer patients. Patient setup employed SGRT using AlignRT (Vision RT Ltd., UK), and the PTV was aligned with CBCT. The study introduced induced shifts of 3 mm, 5 mm, and 7 mm during treatment delivery, assessing their impact on portal dosimetry results for both treatment fields. The gamma index criteria (3%, 3 mm) were employed to evaluate dosimetric accuracy. <i>Results</i>: A total of 18 patients were included, and induced shifts were analyzed for their impact on the planned gamma index values. Significant differences were observed between the Planned Gamma Index and induced shifts of 3 mm, 5 mm, and 7 mm for both treatment fields, highlighting the dosimetric implications of intrafractional motion in paediatric cases. <i>Conclusion</i>: Surface Guided Radiation Therapy (SGRT) is concluded to offer a comprehensive array of benefits for paediatric cases. The dosimetric implications of induced shifts underscore the importance of SGRT in ensuring accurate and safe treatment for paediatric cancer patients.

AN AYURVEDIC MANAGEMENT OF KERATOCONUS – A CASE STUDY

Keratoconus is a bilateral and asymmetric disease that results in progressive thinning and steeping of the cornea, leading to irregular astigmatism and decreased visual acuity. Integrative medicine has no proper treatment, so an attempt is made with ancient medicine. A case of 17year male patient came to Shalakya OPD at Dr B R K R Govt. Ayurvedic College and Hospital with complaints of blurred vision in both eyes, double vision (Rt>lt) and also itching sensation and giddiness while rotating head for eight months and diagnosed with keratoconus at the previous hospital where he has consulted before. In Ayurveda, there is no direct reference to keratoconus; based on the symptoms, it can be considered Vataja Timira, a Drishtigata netra roga. This can be correlated to the advanced condition of keratoconus, considering Vata dosha involvement. The plan of treatment for this is Vatahara and Brumhana Chikitsa. Firstly, Snehapana followed by Shodhana chikitsa like Virechana, Nasya and Samana chikitsa like Saptamruta lauha, Triphala ghruta orally, Kriyakalpa procedure like Akshitarpana, Putapaka, Pindi and Seka. Treatment has been given for three months, and the subject has reported reduced symptoms and showed signs of improvement in vision. Thus, it can be concluded that with the help of Ayurvedic management, the disease did not progress.

A Couch Mounted Smartphone-based Motion Monitoring System for Radiation Therapy

PurposeSurface-guided radiation-therapy (SGRT) systems are being adopted into clinical practice for patient setup and motion monitoring. However, commercial systems remain cost prohibitive to resource-limited clinics around the world. Our aim is to develop and validate a smartphone-based application using LiDAR cameras (such as on recent Apple iOS devices) for facilitating SGRT in low-resource centers. The proposed SGRT application was tested at multiple institutions, and validated using phantoms and volunteers against various commercial systems to demonstrate feasibility. Methods and MaterialsAn iOS application was developed in Xcode and written in Swift using the Augmented-Reality (AR) Kit and implemented on an Apple iPhone 13 Pro with a built-in LiDAR camera. The application contains multiple features: 1) visualization of both the camera and depth video feeds (at a ∼60Hz sample-frequency), 2) region-of-interest (ROI) selection over the patient's anatomy where motion is measured, 3) chart displaying the average motion over time in the ROI, and 4) saving/exporting the motion traces and surface map over the ROI for further analysis. The iOS application was tested to evaluate depth measurement accuracy for: 1) different angled surfaces, 2) different field-of-views over different distances, and 3) similarity to a commercially available SGRT systems (Vision RT AlignRT® and Varian IDENTIFY™) with motion phantoms and healthy volunteers across three institutions. Measurements were analyzed using linear-regressions and Bland-Altman analysis. ResultsCompared to the clinical system measurements (reference), the iOS application showed excellent agreement for depth (r=1.000,p<0.0001; bias=-0.07±0.24cm) and angle (r=1.000,p<0.0001; bias=0.02±0.69°) measurements. For free-breathing traces, the iOS application was significantly correlated to phantom motion (institute 1: r=0.99,p<0.0001; bias=-0.003±0.03cm; institute 2: r=0.98,p<0.0001; bias=-0.001±0.10cm; institute 3: r=0.97,p<0.0001; bias=0.04±0.06cm) and healthy volunteer motion (institute 1: r=0.98,p<0.0001; bias=-0.008±0.06cm; institute 2: r=0.99,p<0.0001; bias=-0.007±0.12cm; institute 3: r=0.99,p<0.0001; bias=-0.001±0.04cm). ConclusionThe proposed approach using a smartphone-based application provides a low-cost platform that could improve access to surface-guided radiation therapy accounting for motion.

Can SGRT be a Substitute for Plan Verification Procedure?

Verification of plan (VP) has been part of our pre-treatment workflow for treatment isocenter verification. Currently, our center uses simulator for VP procedure for all our patients before the treatment. We would like to investigate if SGRT could be a good substitute for VP procedure to simplify our pre-treatment workflow. In Group A (A-c, A-t, A-a), 20 patients of each treatment site (cranial, thorax and abdomen) were selected randomly. Patients did not go through VP procedure. During the first fraction of treatment, the therapists were guided by SGRT system (Vision RT, UK) and aligned the patient to 3mm and 1°using a standard region of interest (ROI). First CBCT was taken as a reference to customize the ROI for better suitability. Next, the patient was re-aligned to 1mm and 1°using the new ROI. Second CBCT was acquired, and 6 degrees of freedoms shifts were recorded. In Group B (B-c, B-t, B-a), 20 patients of each treatment site (cranial, thorax and abdominal) that were assigned for VP over the same period as Group A patients. Group B patients were aligned based on the skin markings drawn during VP procedure. CBCTs were taken at the first fraction of treatment and shifts were recorded. A total of 60 CBCT images were analyzed for each group of patients. The absolute mean and standard deviations were shown in Table 1. The results indicated that Group A is superior, if not comparable, to Group B. Table 1: The absolute mean and standard deviations of first fraction of CBCT positioning errors for Group A and B patients. With appropriate ROI, SGRT is a good or superior substitute for plan verification procedure. Localization verification can be done during day one of treatment which ease the pre-treatment workflow to both patients and clinical team. Analysis of customized ROI will be further studied in the future.

The Effect of Three Different Positioning Approaches for Prone Rectum Radiotherapy on Positioning Accuracy and Treatment Margins

The positioning accuracy of rectal radiotherapy patients was greatly affected by the immobilization gadget, breathing motion, bladder volume etc. Prone is one of the common treatment positions for rectum radiotherapy patients as it can displace the small bowels to reduce side effects. Our team would like to further enhance positioning accuracy by evaluating different positioning approaches. A total of 16 patients from each group were selected retrospectively. All patients fulfilled the bladder (±20% of CT-sim bladder volume) and bowel (empty bowel before the treatment) protocol before entering the treatment unit. Group A patients were positioned using commercial prone pelvic board (Orfit, Belgium) combined with thermoplastic membrane and skin markings; Group B patients were fixed with innovative prone pelvic board and vac-bag which positioned using skin markings. First three fractions and once weekly CBCT images were retrieved for data analysis. Group C patients were fixed the same as Group B and positioned using skin markings during the first fraction. New reference surface for the SGRT system (Vision RT Ltd, UK) was captured after CBCT shifts applied. The subsequent treatment, patient was aligned to ±0.1 cm and ±1.5° according to the new reference surface. CBCT shifts that were taken at fraction 2, 3, 4 and once weekly were recorded in 6 degrees of freedom. A total of 112 sets of data were retrieved from each groups respectively. The results were compared using nonparametric tests based on the normal distribution of the data. PTV margins (M) were calculated using the formula M = 2.5Σ+0.7σ. Positioning errors were shown in Table 1. The median positioning errors was statistically significant in lateral, vertical, roll and yaw for Group A and B. Whereas, Group C positioning errors was superior to Group B in all axes (p<0.05) except roll and yaw directions. The margins of Group A, B and C were 0.68cm, 1.17cm and 0.62cm; 0.56cm, 0.99cm and 0.75cm; 0.39cm, 0.36cm and 0.34cm in lateral, longitudinal and vertical directions. The results shown that Group B margins was smaller than Group A in lateral and longitudinal directions. Group C required the least margins. Positioning prone rectum patients using innovative pelvic board with the aid of surface guidance resulted in higher accuracy and smaller margins especially in longitudinal direction. Accurate positioning and good reproducibility could potentially reduce the margins from 6mm to 4mm for our prone rectum radiotherapy patients.

Review of clinical applications and challenges with surface-guided radiation therapy.

To evaluate the use of this new technique, surface-guided radiotherapy (SGRT), for patient setup and motion management in various cancers. Data was collected from 533 patients, who received treatment in our hospital for various malignancies using SGRT from October 2019 to April 2021. We studied patient setup, interfraction position, and patient position during the breath-hold (BH) technique. The main advantage of SGRT is that, it is completely non-invasive and uses visible light to compare the patient's skin surface in the treatment room and planned treatment position. In this analysis, Monaco 5.51.10 (Elekta) treatment planning system, Versa HD Linear Accelerator, and AlignRT 6.2 (Vision RT) SGRT system were used. With SGRT, treatment setup time can be reduced with more precision and techniques like Deep inspiration breathhold (DIBH) can be done with very good compliance. SGRT has shown improved accuracy in patient setup compared to conventional laser setup. The daily kilo voltage imaging frequency can be reduced; it helps in reducing additional radiation exposure due to imaging. SGRT has demonstrated reproducibility with adequate accuracy in BH treatments in DIBH for breast and SBRT.

Phase II Trial of LINAC-Based STereotactic Arrhythmia Radioablation (STAR) for Paroxysmal Atrial Fibrillation in Elderly: Planning and Dosimetric Point of View.

Purpose: Approaching treatment for elderly patients with atrial fibrillation is difficult. A prospective phase II trial evaluating LINAC-based stereotactic arrhythmia radioablation (STAR) safety in this population started in 2021. Dosimetric and planning data were reported. Materials and Methods: A vac-lock bag was used for immobilization in the supine position and a computed tomography (CT, 1 mm) was performed. The clinical target volume (CTV) was defined as the area around the pulmonary veins. An internal target volume (ITV) was added to the CTV to compensate heart and respiratory movement. The planning target volume (PTV) was defined by adding 0-3 mm to the ITV. STAR was performed during free-breathing with a PTV prescription total dose (Dp) of 25 Gy/1 fraction. Flattening filter-free volumetric-modulated arc therapy plans were generated, optimized, and delivered by TrueBeamTM. Image-guided radiotherapy with cone-beam CT and surface-guided radiotherapy with Align-RT (Vision RT) were employed. Results: From May 2021 to March 2022, 10 elderly patients were treated. Mean CTVs, ITVs, and PTVs were 23.6 cc, 44.32 cc, and 62.9 cc, respectively; the mean prescription isodose level and D2% were 76.5% and 31.2 Gy, respectively. The average heart and left anterior descending artery (LAD) Dmean were 3.9 and 6.3 Gy, respectively; the mean Dmax for LAD, spinal cord, left and right bronchus, and esophagus were 11.2, 7.5, 14.3, 12.4, and 13.6 Gy, respectively. The overall treatment time (OTT) was 3 min. Conclusions: The data showed an optimal target coverage, sparing surrounding tissue, in 3 min of OTT. LINAC-based STAR for AF could represent a valid non-invasive alternative for elderly patients who were excluded from catheter ablation.

Investigating the robustness of the AlignRT InBore™ co-calibration process and determining the overall tracking errors

ObjectiveTo determine the overall tracking errors inherent to the co-calibration procedure of AlignRT InBore™’s (Vision RT Ltd., London, UK) ceiling-mounted and ring-mounted cameras. MethodsExtrinsic calibration errors related to the mismatch between ceiling and InBore cameras’ isocentres and treatment isocentre were determined using MV images and the SRS package and compared to traditional plate-based error. Next, using a realistic anthropomorphic female phantom, intrinsic calibration errors were determined while varying source-skin distance (80 to 100 cm), breast board inclination (0° to 12.5°), room lighting conditions (0 to 258 lx), skin colour (dark, white and natural skin colour), and pod occlusion. ResultsMV images of the cube proved plate-based calibration to suffer from large errors especially in the vertical direction (up to 2 mm). Intrinsic calibration errors were considerably lower. Indeed, RTD values of ceiling and InBore cameras showed little variability with isocentre depth (within 1.0 mm/0.4°), surface orientation and breast board inclination (within 0.7 mm/0.3°), changing lighting conditions (within 0.1 mm/0.2°), skin colour/tone (within 0.3 mm/0.3°) and camera pod occlusion (within 0.3 mm/0.2°). ConclusionThe use of MV-images proved critical to maintain co-calibrating errors of ceiling and InBore cameras to Halcyon’s treatment isocentre below 1 mm.

Stability and reproducibility comparisons between deep inspiration breath-hold techniques for left-sided breast cancer patients: A prospective study.

Deep inspiration breath-hold (DIBH) is crucial in reducing the lung and cardiac dose for treatment of left-sided breast cancer. We compared the stability and reproducibility of two DIBH techniques: Active Breathing Coordinator (ABC) and VisionRT (VRT). We examined intra- and inter-fraction positional variation of the left lung. Eight left-sided breast cancer patients were monitored with electronic portal imaging during breath-hold (BH) at every fraction. For each patient, half of the fractions were treated using ABC and the other half with VRT, with an equal amount starting with either ABC or VRT. The lung in each portal image was delineated, and the variation of its area was evaluated. Intrafraction stability was evaluated as the mean coefficient of variation (CV) of the lung area for the supraclavicular (SCV) and left lateral (LLat) field over the course of treatment. Reproducibility was the CV for the first image of each fraction. Daily session time and total imaging monitor units (MU) used in patient positioning were recorded. The mean intrafraction stability across all patients for the LLat field was 1.3±0.7% and 1.5±0.9% for VRT and ABC, respectively. Similarly, this was 1.5±0.7% and 1.6±0.8% for VRT and ABC, respectively, for the SCV field. The mean interfraction reproducibility for the LLat field was 11.0±3.4% and 14.9±6.0% for VRT and ABC, respectively. Similarly, this was 13.0±2.5% and 14.8±9% for VRT and ABC, respectively, for the SCV. No difference was observed in the number of verification images required for either technique. The stability and reproducibility were found to be comparable between ABC and VRT. ABC can have larger interfractional variation with less feedback to the treating therapist compared to VRT as shown in the increase in geometric misses at the matchline.

Evaluation of the accuracy of a six-degree-of-freedom robotic couch using optical surface and cone beam CT images of an SRS QA phantom

Abstract Purpose: To assess the accuracy of the Varian PerfectPitch six-degree-of-freedom (6DOF) robotic couch by using a Varian SRS QA phantom. Methods: The stereotactic radiosurgery (SRS) phantom has five tungsten carbide BBs each with 7·5 mm in diameter arranged with the known geometry. Optical surface images and cone beam CT (CBCT) images of the phantom were taken at different pitch, roll and rotation angles. The pitch, roll, and rotation angles were varied from −3 to 3 degrees by inputs from the linac console. A total of 39 Vision RT images with different rotation angle combinations were collected, and the Vision RT software was used to determine the rotation angles and translational shifts from those images. Eight CBCT images at most allowed rotational angles were analysed by in-house software. The software took the coordinates of the voxel of the maximum CT number inside a 7·5-mm sphere surrounding one BB to be the measured position of this BB. Expected BB positions at different rotation angles were determined by multiplying measured BB positions at zero pitch and roll values by a rotation matrix. Applying the rotation matrix to 5 BB positions yielded 15 equations. A linear least square method was used for regression analysis to approximate the solutions of those equations. Results: Of the eight calculations from CBCT images, the maximum rotation angle differences (degree) were 0·10 for pitch, 0·15 for roll and 0·09 for yaw. The maximum translation differences were 0·3 mm in the left–right direction, 0·5 mm in the anterior–posterior direction and 0·4 mm in the superior–inferior direction. Conclusions: The uncertainties of the 6-DOF couch were examined with the methods of optical surface imaging and CBCT imaging of the SRS QA phantom. The rotational errors were less than 0·2 degree, and the isocentre shifts were less than 0·8 mm.

Reproducibility of Deep-Inspiration Breath Hold treatments on Halcyon™ performed using the first clinical version of AlignRT InBore™: Results of CYBORE study

Background and purposeTo investigate the reproducibility of deep-inspiration breath hold (DIBH) breast cancer treatments on Halcyon™ performed using the first clinical version of AlignRT InBore™ (Vision RT Ltd., London, UK), a Halcyon’s SGRT dedicated solution. Materials and methodsThe ease and feasibility of DIBH treatments was retrospectively investigated for the initial 22 left breast cancer patients treated on Halcyon™ using AlignRT InBore™. Setup time, Cone beam computed tomography (CBCT) imaging and analysis time as well as overall treatment time were recorded. Online and offline review of CBCT images was undertaken to verify the compliance of breast, heart, spine, sternum and diaphragmatic domes positions. ResultsMean duration of patient setup, CBCT analysis and overall treatment time were 4 min, 1.1 min and 14 min respectively. Online review of 520 CBCT acquisitions by therapists showed minimal positioning shifts with AlignRT InBore™ guidance with mean value of vertical, longitudinal, and lateral shifts of 1.7 mm, −1.7 mm, and −0.2 mm respectively. Meanwhile, offline review of 115 CBCTs by the radiation oncologist, showed reproducible breath hold (BH) with average deviation of breast, heart, spine, sternum and diaphragmatic domes respectively within 2.4 mm, 2.9 mm, 3.3 mm, 3.2 mm and 4.5 mm in magnitude. ConclusionAlignRT InBore™ allows for accurate and reproducible DIBH on Halcyon™ with breast and heart organs’ positions within 3 mm in magnitude of expected position and fully compliant with planning margins (5 mm anisotropic CTV-PTV margins).

P18 - Comparison of the use of a Vision RT QA cube and a Winston-Lutz test phantom to verify the discrepancy between a linac radiation isocentre and its mechanical isocentre

P18 - Comparison of the use of a Vision RT QA cube and a Winston-Lutz test phantom to verify the discrepancy between a linac radiation isocentre and its mechanical isocentre

Dosimetric Analysis of Intra-Fraction Motion Detected by Surface Guided Radiation Therapy During Linac Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) requires very sharp dose gradients, high conformality and typically uses minimal expansion margins. Immobilization with an open face mask is more comfortable and less invasive than frame-based immobilization but concerns about intra-fraction motion must be addressed. Surface guided radiation therapy (SGRT) is an attractive option for intra-fraction patient monitoring because it is continuous, has sub-millimeter accuracy and uses no ionizing radiation. The purpose of this study was to investigate the dosimetric consequences of uncorrected intra-fraction patient motion detected during frameless Linac-based SRS.All SRS patients (pts) were immobilized in an SRS-specific open face thermoplastic mask with bite plate and were monitored during treatment using SGRT with tolerances of 1 mm translations and 1° rotations; the treatment beam automatically turned off if these tolerances were exceeded. SRS pts received 18 or 21 Gy in 1 fraction or 27 Gy in 3 fractions based on tumor size and location. GTV to PTV margins ranged from 0 to 3 mm. Twelve pts were treated with circular cones and 13 were treated with MLC-based plans with the technique chosen based on PTV size. If the SGRT system detected motion greater than tolerance during treatment, conebeam CT(CBCT) was repeated and the necessary shifts were made prior to continuing treatment. For the 25 pts with intra-fraction 3D vector shifts of at least 1 mm, we offset the isocenter in the treatment planning system using the translational shifts from the repeat CBCT and re-calculated the plans with the planned MU to determine the dosimetric effect the shift would have caused had it not been detected. The GTV and PTV coverage and normal brain V12 for the shifted plans were compared to the planned values. Wilcoxon signed rank tests were used to compare planned and shifted dosimetric indices and median 2 sample tests were used to investigate these differences between cone and MLC plans.From the recalculated plans, the resulting median V12 increased by 0.24 cc (P = 0.006) and the median GTV and PTV coverage decreased 9% (P < 0.001) and 10.2 % (P < 0.001), respectively. PTV coverage decreased more for shifted cone plans than for shifted MLC plans (11.6% vs 4.7%, P = 0.011) but the median V12 differences were found to be significantly larger for MLC plans (-0.34 cc vs -0.01 cc, P = 0.011), due to the sharper dose gradients and higher conformality for the cone plans compared to the MLC plans. Differences in GTV coverage between cone and MLC plans were not statistically significant.SGRT detected clinically meaningful intra-fraction motion during frameless SRS, which could potentially lead to large underdoses and increased normal brain dose if not corrected. Dosimetric differences between planned and shifted plans were statistically significant.R.D. Foster: None. B.J. Moeller: Independent Contractor; Novant Health. Partner; SERO. Stock; Moderna. M. Robinson: None. M. Bright: SEAAPM.J.L. Ruiz: Stock; Obseva USA, Inc. C.J. Hampton: None. J.H. Heinzerling: Research Grant; Vision RT, Inc, AstraZeneca. Honoraria; Vision RT, Inc, AstraZeneca. Consultant; Vision RT, Inc.

The Accuracy of Surface Guided Hypofractionated Radiotherapy for Intracranial Metastasis With Open Face Mask Immobilization

To investigate the applicability of surface imaging for image guidance in the hypofractionated radiotherapy for intracranial metastasis with open face mask immobilization.Nineteen patients were treated with hypofractionated radiotherapy for intracranial metastasis were included. Before the start of treatment, each patient underwent simulation with open face mask immobilization. During treatment, CBCT images were collected for verification each time. Marking based positioning was used for the first fraction and new reference surface images were captured after first CBCT registration (6D of freedom,6DoF) for subsequent treatment. Subsequent treatment was randomly divided into marking guided positioning fraction group (MFG) and surface guided positioning fraction group (SFG). In the SFG, the patient was aligned manually by the therapist until the rotation tolerance was within 1°. The residual translation and rotation errors were corrected using the "Move Couch" function of the optical surface system Align RT (Vision RT, London, UK). The six-dimensional error data of patients with the two positioning methods were compared and expressed as mean ± standard deviation. Meanwhile, the correlation and consistency between the optical surface error data and the gold standard CBCT error data were compared during the MFG. A scientific 2D graphing and statistics software was used for data processing and mapping, and a statistical software was used for mean analysis and normality test. Pearson correlation analysis was used to analyze the correlation, and Bland-Altman plot analysis was used to test the coincidence between the two methods.Total of 103 MFG CBCT images and 87 SFG CBCT images were analyzed. Compared with the marking guided positioning, the 3D magnitude error of the optical surface guided positioning was reduced from 0.35 ± 0.16 cm to 0.14 ± 0.07 cm and the rotation error was reduced from 0.06° ± 1.36°, 0.62° ± 1.18°, 0.21° ± 1.19° to -0.04° ± 0.68°, -0.03° ± 0.73°, 0.04° ± 0.69° on Pitch, Roll and Rotation. The Pearson coefficient R2 was 0.91,0.70,0.78 on Lateral, Longitudinal and Vertical directions, R2 was 0.75,0.85,0.77 on Pitch, Roll and Rtn directions, respectively (all P < 0.01). It showed high correlation between two methods. The 95% agreement limits of agreement were within preset 3 mm tolerance (lateral [-0.19-0.19cm], longitudinal [-0.25cm,0.25cm], vertical[-0.27cm,0.19cm]), and the 95% agreement limits of agreement were within preset 3 ° tolerance (Pitch [-1.76°,1.76°], Roll [-1.54°, 1.60°], Rotation [-2.18°,1.69°]) for two methods.The optical surface error and cone-beam CT error have good correlation and coincidence. The optical surface guided positioning can reduce the setup errors and the results support the application of 3D surface imaging for image guidance in the hypofractionated radiotherapy for intracranial metastasis with open face mask immobilization.

Comparison of the dosimetric accuracy of proton breast treatment plans delivered with SGRT and CBCT setups.

PurposeTo compare the dosimetric accuracy of surface‐guided radiation therapy (SGRT) and cone‐beam computed tomography (CBCT) setups in proton breast treatment plans.MethodsData from 30 patients were retrospectively analyzed in this IRB‐approved study. Patients were prescribed 4256–5040 cGy in 16–28 fractions. CBCT and AlignRT (SGRT; Vision RT Ltd.) were used for treatment setup during the first three fractions, then daily AlignRT and weekly CBCT thereafter. Each patient underwent a quality assurance CT (QA‐CT) scan midway through the treatment course to assess anatomical and dosimetric changes. To emulate the SGRT and CBCT setups during treatment, the planning CT and QA‐CT images were registered in two ways: (1) by registering the volume within the CTs covered by the CBCT field of view; and (2) by contouring and registering the surface surveyed by the AlignRT system. The original plan was copied onto these two datasets and the dose was recalculated. The clinical treatment volume (CTV): V95%; heart: V25Gy, V15Gy, and mean dose; and ipsilateral lung: V20Gy, V10Gy, and V5Gy, were recorded. Multi and univariate analyses of variance were performed to assess the differences in dose metric values between the planning CT and the SGRT and CBCT setups.ResultsThe CTV V95% and lung V20Gy, V10Gy, and V5Gy dose metrics were all significantly (p < 0.01) lower on the QA‐CT in both the CBCT and SGRT setup. The differences were not clinically significant and were, on average, 1.4–1.6% lower for CTV V95% and 1.8%–6.0% lower for the lung dose metrics. When comparing the lung and CTV V95% dose metrics between the CBCT and SGRT setups, no significant difference was observed. This indicates that the SGRT setup provides similar dosimetric accuracy as CBCT.ConclusionThis study supports the daily use of SGRT systems for the accurate dose delivery of proton breast treatment plans.

Validation of a CT-based motion model with in-situ fluoroscopy for lung surface deformation estimation

Many surrogate-based motion models (SMMs), proposed to guide motion management in radiotherapy, are constructed by correlating motion of an external surrogate and internal anatomy during CT-simulation. Changes in this correlation define model break down. We validate a methodology that incorporates fluoroscopic (FL) images acquired during treatment for SMM construction and update. Under a prospective IRB, 4DCT scans, VisionRT (VRT) surfaces, and orthogonal FLs were collected from five lung cancer patients. VRT surfaces and two FL time-series were acquired pre- and post-treatment. A simulated annealing optimization scheme was used to estimate optimal lung deformations by maximizing the mutual information (MI) between digitally reconstructed radiographs (DRRs) of the SMM-estimated 3D images and FLs. Our SMM used partial-least-regression and was trained using the optimal deformations and VRT surfaces from the first breathing-cycle. SMM performance was evaluated using the MI score between reference FLs and the corresponding SMM or phase-assigned 4DCT DRRs. The Hausdorff distance for contoured landmarks was used to evaluate target position estimation error. For four out of five patients, two principal components approximated lung surface deformations with submillimeter accuracy. Analysis of the MI score between more than 4000 pairs of FL and DRR demonstrated that our model led to more similarity between the FL and DRR images compared to 4DCT and DRR images from a model based on an a priori correlation model. Our SMM consistently displayed lower mean and 95th percentile Hausdorff distances. For one patient, 95th percentile Hausdorff distance was reduced by 11 mm. Patient-averaged reductions in mean and 95th percentile Hausdorff distances were 3.6 mm and 7 mm for right-lung, and 3.1 mm and 4 mm for left-lung targets. FL data were used to evaluate model performance and investigate the feasibility of model update. Despite variability in breathing, use of post-treatment FL preserved model fidelity and consistently outperformed 4DCT for position estimation.

Validating robotic couch isocentricity with 3D surface imaging.

BackgroundA proton therapy system with 190° gantries uses robotic couch rotations to change the treatment beam laterality. Couch rotations are typically validated clinically with post‐rotation radiographic imaging.AimsThis study assesses the specificity and sensitivity of a commercial 3D surface imaging system, AlignRT (Vision RT, London UK) for validating couch rotations.Materials & MethodsIn clinical operation, a reference surface image of the patient is acquired after radiographic setup with couch at 270°, perpendicular to the gantry axis of rotation. The couch is then rotated ±90° to a typical treatment angle, and AlignRT reports a 3D displacement vector. Patient motion, changes in patient surface, non‐coincidence between AlignRT and couch isocenter, and mechanical couch run‐out all contribute to the 3D vector magnitude. To assess AlignRT sensitivity in detecting couch run‐out, volunteers were positioned orthogonal to the proton gantry and reference surface images were captured without x‐ray localization. Subjects were repeatedly rotated ±90⁰ to typical treatment angles and displacement vectors were recorded. Additionally, measurements were performed in which intentional translations of 2, 4, 6, and 8 mm were combined with the intended isocentric rotations. Data sets were collected using a phantom; subjects with a thoracic isocenter and no immobilization; and subjects with a cranial isocenter and thermoplastic immobilization. A total of 300 rotations were measured.ResultsDuring isocentric rotations, the mean AlignRT displacement vectors for the phantom, immobilized, and non‐immobilized volunteers were 0.1 ± 0.1 mm, 0.8 ± 0.1 mm, and 1.1 ± 0.2 mm respectively. 95% of the AlignRT measurements for the immobilized and non‐immobilized subjects were within 1 mm and 2 mm of the actual displacement respectively.DiscussionAfter characterizing the accuracy using phantoms and volunteers, we have shown that a three‐pod surface imaging system can be used to identify gross non‐isocentric patient rotations. Significant positional deviations, either due to improper couch rotation or patient motion, should be followed by radiographic imaging and repositioning.ConculsionAlignRT can be used to verify patient positioning following couch rotations that are applied after the initial x‐ray guided patient setup. Using a three‐pod AlignRt system, positional deviations exceeding 4 mm were flagged with sensitivity and specificity of 90% and 100% respectively.

Abstract P4-12-29: Utility of surface guide radiation therapy (SGRT) in radiation therapy for breast cancer

Abstract Purpose: Radiation therapy has been shown to be a highly effective treatment for breast cancer. Historically, patient setup for daily treatment has required a set of permanent tattoos. However, due to a variety of reasons, it may be hard to align patients to these marks and the marks can also cause distress in many patients. National Comprehensive Cancer Network’s (NCCN) radiation therapy guidelines recommend weekly radiation ports films; however, many centers perform daily image guidance using x-rays for breast treatments prior to radiation delivery, increasing both radiation dose and treatment time. Surface Guided Radiation Therapy (SGRT) is a technique which is being adopted by radiation therapy centers to aid in patient’s setup and treatment delivery accuracy. SGRT is safe and non-invasive, using visible light to compare the patient’s skin surface in the treatment room to the planned treatment position with sub-millimeter accuracy. SGRT can also automatically signal for the treatment delivery system to pause radiation if the patient moves out of the desired position. The aim of this study was to evaluate the AlignRT (Vision RT, Ltd, London, United Kingdom) SGRT system for patient setup accuracy in comparison to a 3-point tattoo setup and daily KV pair imaging. Methods: A total of 20 breast cancer patients (600 radiation fractions) were setup using a 3-point tattoo technique and then positioned using AlignRT. Inter-fraction positions were analyzed between all three techniques, namely tattoo, AlignRT and kV imaging. In addition, the AlignRT system was used to evaluate that patient’s position during breath hold. This deep inspiration breath hold (DIBH) technique is designed to provide cardiac sparing during left breast radiotherapy. Results: Using 3-point tattoo setup, an average magnitude of 5.7±3 mm error from isocenter was measured, compared to 2.4±1 mm using surface guidance. Statistically significant 3d shift vectors were found between patient setup using 3-point tattoo and SGRT techniques, however between SGRT and daily KV imaging 3d shift vectors were not statistically significant. AlignRT based DIBH showed the distance between the heart and chest-wall at the mid tangential beam level, based on weekly port films, to be within 2±1 mm of the planned level. Conclusion: SGRT has been shown to improve the accuracy of patient setup compared to 3-point tattoos and is comparable to daily KV imaging for isocentric setup. SGRT can be used in clinics to reduce the frequency of KV imaging and therefore additional radiation exposure. SGRT can also remove setup tattoo requirements which are placed during simulation which provide a permanent reminder to patients of their cancer and treatment. SGRT has also been demonstrated to ensure adequate and reproducible breath hold for DIBH left sided breast treatments. Citation Format: Snehal Desai. Utility of surface guide radiation therapy (SGRT) in radiation therapy for breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-29.

Development and prospective in-patient proof-of-concept validationof a surface photogrammetry+CT-based volumetric motion model for lung radiotherapy.

We develop and validate a motion model that uses real-time surface photogrammetry acquired concurrently with four-dimensional computed tomography (4DCT) to estimate respiration-induced changes within the entire irradiated volume, over arbitrarily many respiratory cycles. A research, couch-mounted, VisionRT (VRT) system was used to acquire optical surface data (15Hz, ROI=15×20cm2 ) from the thoraco-abdominal surface of a consented lung SBRT patient, concurrently with their standard-of-care 4DCT. The end-exhalation phase from the 4DCT was regarded as reference and for each remaining phase, deformation vector fields (DVFs) with respect to the reference phase were computed. To reduce dimensionality, the first two principal components (PCs) of the matrix of nine DVFs were calculated. In parallel, ten phase-averaged VRT surfaces were created. Surface DVFs and corresponding PCs were computed. A principal least squares regression was used to relate the PCs of surface DVF to those of volume DVFs, establishing a relationship between time-varying surface and the underlying time-varying volume. Proof-of-concept validation was performed during each treatment fraction by concurrently acquiring 30s time series of real-time surface data and "ground truth" kV fluoroscopic data (FL). A ray-tracing algorithm was used to create a digitally reconstructed fluorograph (DRF), and motion trajectories of high-contrast, soft-tissue, anatomical features in the DRF were compared with those from kV FL. For five of the six fluoroscopic acquisition sessions, the model out-performed 4DCT in predicting contour Dice coefficient with respect to fluoroscopy-derived contours. Similarly, the model exhibited a marked improvement over 4DCT for patch positions on the diaphragm. Model patch position errors varied from 5 to -15mm while 4DCT errors ranged between 5 and -22.4mm. For one fluoroscopic acquisition, a marked change in the a priori internal-external correlation resulted in model errors comparable to those of 4DCT. We described the development and a proof-of-concept validation for a volumetric motion model that uses surface photogrammetry to correlate the time-varying thoraco-abdominal surface to the time-varying internal thoraco-abdominal volume. These early results indicate that the proposed approach can result in a marked improvement over 4DCT. While limited by the duration of the fluoroscopic acquisitions as well as the resolution of the acquired images, the DRF-based proof-of-concept technique developed here is model-agnostic, and therefore, has the potential to be used as an in-patient validation tool for other volumetric motion models.

Evaluation of the Residual Rotation Error of a 6 DOF Robotic Couch Using Optical Surface and CBCT.

Rotation setup errors for SRS treatments are typically less than two degrees which usually causes less dosimetry discrepancy for a typical SRS treatment as the target size is likely less than 4 cm. However this might not be true for the single isocenter and multiple metastatic targets’ technique where, the distances between the isocenter of the treatment and those tumor centers can be very large. A 6DOF couch can be used to correct this setup error by using optical surface and/or Conebeam CT (CBCT) images. The accuracy and consistence between optical surface and CBCT images were studied. The CBCT system and optical surface images of an SRS phantom were taken at different pitch, roll, and rotation angles varied from -3 to 3 degrees by intentionally changing values on the service console. A total 39 combinations of pitch, roll, and rotations using the 6DOF couch were generated. The optical surface images were taken and Vision RT software was used to determine the rotation angles and isocenter shifts. Eight CBCT images corresponding to +- 3 degree changes in pitch, roll, and rotation were taken. The center positions of the 5 tungsten carbide BBs contained in the SRS QA phantom were determined using in-house software. A least square fit was used to determine translation/rotation parameters. Among the 39 calculations from optical images, the maximum rotation angle differences were 0.2, 0.1, and -0.2 degrees for pitch, roll, and couch rotation, respectively. The maximum isocenter shifts were -0.80 mm, 0.70 mm, and -0.10 mm in lateral, longitudinal, and vertical axes, respectively. The maximum residual rotation angles from analysis of CBCT images were 0.20 degree for pitch, roll, and rotation. By using 6DOF couch, the rotation errors can be reduced dramatically compared to initial rotation errors. To overcome this rotation errors without 6DOF couch, one has to introduce extra margin for target which is far away from the treatment iso-center. Six DOF can avoid extra CTV-PTV margin. The residual rotational errors studied for the couch were less than 0.2 degree. This accuracy for the 6DOF system is sufficient for SRS applications even for single isocenter multiple tumor technique and extra CTV-PTV margin might be not necessary for some cases.