Megavoltage Imaging Research Articles

New findings on clinical experience on surface-guided radiotherapy for frameless non-coplanar stereotactic radiosurgery treatments.

The aim of this study was to assess the accuracy of a surface-guided radiotherapy (SGRT) system for setup and intra-fraction motion control in frameless non-coplanar stereotactic radiosurgery (fSRS) using actual patient data immobilized with two different types of open-faced masks and employing a novel SGRT systems settings. Forty-four SRS patients were immobilized with two types of open-faced masks. Sixty lesions were treated, involving the analysis of 68 cone-beam scans (CBCT), 157 megavoltage (MV) images, and 521 SGRT monitoring sessions. The average SGRT translations/rotations and 3D vectors (MAG-Trasl and MAG-Rot) were compared with CBCT or antero-posterior MV images for 0° table or non-coplanar beams, respectively. The intrafraction control was evaluated based on the average shifts obtained from each monitoring session. To assess the association between the SGRT system and the CBCT, the two types of masks and the 3D vectors, a generalized estimating equations (GEE) regression analysis was performed. The Wilcoxon singed-rank test for paired samples was performed to detect differences in couch rotation with longitudinal (LNG) and lateral (LAT) translations and/or yaw. The average SGRT corrections were smaller than those detected by CBCT (≤0.5mm and 0.1°), with largest differences in LNG and yaw. The GEE analysis indicated that the average MAG-Trasl, obtained by the SGRT system, was not statistically different (p=0.09) for both mask types, while, the MAG-Rot was different (p=0.01). For non-coplanar beams, the Wilcoxon singed-rank test demonstrated no significantly differences for the corrections (LNG, LAT, and yaw) for any table rotation except for LNG corrections at 65° (p=0.04) and 75° (p=0.03) table angle position; LAT shifts at 65° (p=0.03) and 270° (p<0.001) table angle position, and yaw rotation at 30° (p=0.02) table angle position. The average intrafraction motion was<0.1mm and 0.1° for any table angle. The SGRT system used, along with the novel workflow performed, can achieve the setup and intra-fraction motion control accuracy required to perform non-coplanar fSRS treatments. Both masks ensure the accuracy required for fSRS while providing a suitable surface for monitoring.

Characterization of the on-board megavoltage imager in a magnetic resonance-guided radiotherapy machine for beam output checks

We characterized the on-board megavoltage imager (MVI) of a magnetic resonance-guided radiotherapy machine for beam output checks. Linearity and repeatability of its dose response were investigated. Alignment relative to the beam under clinical circumstances was evaluated for a year using daily measurements. Linearity and short-term repeatability were excellent. Long-term repeatability drifted 0.8 % per year, which can be overcome by monthly cross calibrations. Long-term alignment was stable. Thus, the MVI has suitable characteristics for beam output checks.

Analysis of absorption signal and noise in thin phosphor detectorsfor high-energy transmission radiography

We investigated the detective quantum efficiency (DQE) of thin gadoliniumoxysulfide phosphor-based flat-panel detectors (FPDs) using cascaded-systemsanalysis and Monte Carlo (MC) simulations for applications in megavoltage(MV) x-ray industrial imaging. We decomposed the DQE formula into(dose-independent) upper-limit DQE and (dose-dependent) DQE-reductionfactors. We obtained the absorbed energy distributions (AEDs) forvarious x-ray detector designs and photon energies using MC simulationsand applied the AED analysis to the DQE formula. The investigationsexamined include the x-ray-detector-only DQE and the effect of thecoupling efficiency between the x-ray detector and readout panel,including electronic noise, on the upper-limit DQE. This study confirmsthat the design of the metal build-up layer on the phosphor is effectivefor MV imaging and emphasizes the importance of designing the readoutpanel to maintain the upper-limit DQE. We expect the proposed DQEanalysis to be suitable for designing and evaluating FPDs for high-energynondestructive x-ray testing.

Assessment of the Offline Setup Correction Protocol and Thermoplastic Mask Use for Head and Neck Cancer Radiotherapy: A Single Institution Experience

This study assessed the efficiency of offline setup correction protocol and the use of a thermoplastic mask for head and neck cancer (HNC) patients treated with three-dimensional conformal radiotherapy at Ocean Road Cancer Institute. A prospective study was conducted from April to August 2021 to verify 62 patients’ treatment setup using an offline setup correction protocol while immobilized with a thermoplastic mask. Megavoltage images were matched with digitally reconstructed radiographs obtained during CT simulation to determine the gross set-up deviations. Box plots were used to show the deviations on three consecutive days of the first week and a successive weekly set-up verification in lateral, longitudinal, and vertical directions. The associations between thermoplastic mask types and weekly deviations were analyzed using repeated test ANOVA. A p-value ˂ 0.05 was considered statistically significant. The observed deviations after the use of correction protocol were lower in all three translational directions. There was no statistical significance between types of thermoplastic mask and setup deviations in lateral (p &lt; 0.65), longitudinal (p = 0.19), and vertical (p = 0.12) directions. The offline correction protocol can be used in settings with limited resources and high workloads of patients. Both types of thermoplastic masks are effective in immobilizing HNC patients.

A deep-learning method using single phantom to enhance megavoltage image quality for patient positioning in chest radiotherapy: a feasibility study

A deep-learning method using single phantom to enhance megavoltage image quality for patient positioning in chest radiotherapy: a feasibility study

An evaluation of isocentre shift magnitude and treatment site on image-guided radiation therapy online decision analysis times.

The ability to capture more anatomical detail in cone-beam computed tomography (CBCT) imaging compared to kilovoltage (kV) and megavoltage (MV) imaging, has seen a documented shift towards CBCT image verification and staff adopting more extensive image analysis processes. The timeframe associated with assessing online CBCT images, termed the online decision analysis time, if drawn out, can affect treatment efficiency and accuracy. This study aimed to determine the current CBCT online decision analysis time at Radiation Oncology Princess Alexandra Ipswich Road (ROPAIR) and investigate the influence of isocentre shift magnitude and treatment site considerations on this timeframe. This retrospective clinical audit collected treatment parameters from 202 CBCT images over 2 treatment days. The online decision analysis time was calculated by subtracting the image acquisition timestamp from the image verification shift application timestamp. The quantitative data were analysed using mean, standard deviation, and range in the following categories: all CBCTs, CBCTs grouped by isocentre shift magnitude and CBCTs grouped by treatment site. Content analysis was performed on staff comments made during image analysis. The average online decision analysis time was 2:37 ± 1:28 min. On average approximately, head and neck, spine and extremity treatment sites measured 1 min, pelvis, breast, and chest measured 2-3 min with abdomen measuring 4 min. Common categories reported in staff comments included anatomical changes, repositioning, and organs at risk size. The results provide baseline online decision analysis times. Further refinement is required to determine if the image match method, treatment site considerations, and rotational discrepancies influence this timeframe.

A step closer to automation: kilovoltage and Megavoltage Planar Imaging Quality Assurance, baseline, tolerance and action levels definition and exploration

PurposeTo establish automated quality assurance (QA) procedures for the kilovoltage (kV) and the Megavoltage (MV) imagers of two linear accelerators (LINACS) using a commercial software. MethodsSNC Machine™ phantoms and software were used and the baseline values, tolerance and action levels for various image quality parameters were defined. Scaling, spatial resolution, contrast, uniformity and noise were considered, explored and evaluated utilizing the appropriate phantoms and the accompanying software. kV and MV planar radiographic images, for 6MV and 10MV beams were obtained for each LINAC. For both kV and MV QA tasks, the baseline values for spatial resolution, contrast, uniformity and noise were defined. ResultsSubsequent measurements performed were highly reproducible and within tolerance and action levels, while noise showed variations. The calculated tolerance and action levels for noise were looser compared to the other image quality metrics. ConclusionsAn automated QA workflow of the kV and MV planar radiographic mode of LINAC imagers’ was established and appears to be time effective.

MR-linac daily semi-automated end-to-end quality control verification.

Adaptive radiation therapy (ART) on the integrated Elekta Unity magnetic resonance (MR)-linac requires routine quality assurance to verify delivery accuracy and system data transfer. In this work, our objective was to develop and validate a novel automated end-to-end test suite that verifies data transfer between multiple software platforms and quantifies the performance of multiple machine subcomponents critical to the ART process. We designed and implemented a software tool to quantify the MR and megavoltage (MV) isocenter coincidence, treatment couch positioning consistency, isocenter shift accuracy for the adapted plan as well as the MLC and jaw position accuracy following the beam aperture adaptation. Our tool employs a reference treatment plan with a simulated isocenter shift generated on an MR image of a readily available phantom with MR and MV visible fiducials. Execution of the test occurs within the standard adapt-to-position (ATP) clinical workflow with MV images collected of the delivered treatment fields. Using descriptive statistics, we quantified uncertainty in couch positioning, isocentre shift as well as the jaw and MLC positions of the adapted fields. We also executed sensitivity measurements to evaluate the detection algorithm's performance. We report the results of 301 daily testing instances. We demonstrated consistent tracking of the MR-to-MV alignment with respect to the established value and to detect small changes on the order of 0.2mm following machine service events. We found couch position consistency relative to the test baseline value was within 95% CI [-0.31, 0.26mm]. For phantom shifts that form the basis for the plan adaptation, we found agreement between MV-image-detected phantom shift and online image registration, within±1.5mm in all directions with a 95% CI difference of [-1.29, 0.79mm]. For beam aperture adaptation accuracy, we found differences between the planned and detected jaw positions had a mean value of 0.27mm and 95% CI of [-0.29, 0.82mm] and -0.17mm and 95% CI of [-0.37, 0.05mm] for the MLC positions. Automated fiducial detected accuracy was within 0.08±0.20mm of manual localization. Introduced jaw and MLC position errors (1-10mm) were detected within 0.55mm (within 1mm for 15/256 instances for the jaws). Phantom shifts (1.3 or 5mm in each cardinal direction) from a reference position were detected within 0.26mm. We have demonstrated the accuracy and sensitivity of a daily end-to-end test suite capable of detecting errors in multiple machine subcomponents including system data transfer. Our test suite evaluates the entire treatment workflow and has captured system communication issues prior to patient treatment. With automated processing and the use of a standard vendor-provided phantom, it is possible to expand to other Unity sites.

Dosimetry implications of VMAT delivery angles and intra-fraction motion for SBRT prostate treatments.

For a cohort of retrospectively treated patients with prostate carcinoma using a Stereotactic body radiation therapy (SBRT) technique and with implanted localization fiducials, this study determined whether there were any correlations between theoretical fiducial visibility using intra-fraction megavoltage imaging and intra-fraction motion induced dosimetric effects. Treatment planning data for 20 retrospective patients that used a SBRT prostate technique were investigated in this study. An in-house script was created that split each of the 360-degree volumetric modulated arc therapy arcs into 12 sectors, with each sector being 30 degrees. The script created a total of 24 sectors for each SBRT plan ranging from 180 to 210 degrees to 180 to 150 degrees. Resulting data was assessed to determine whether there was dosimetric impact from intra-fractional prostate motion and if it correlated with the theoretical fiducial visibility. Forty percent of the 4 highest CTV D98% mean dose differences occurred within the angles of 240 and 270 degrees and 25% occurred between the angles of 90 and 120 degrees. The 4 highest average percentage differences in PTV D98% coverage during angular sectors of 270 to 240 degrees, 90 to 120 degrees, 240 to 270 degrees and 60 to 90 degrees were -1.19%, -1.14%, -1.10%, and 1.01% respectively. It was also the case for the PTV D95%in sectors 90 to 120 degrees, 240 to 270 degrees, 270 to 240 degrees and 270 to 300 degrees, which reduced by -0.97%, -0.93%, -0.92%, and -0.82% respectively. From the 4 highest rectal dose differences in V32Gy and V18Gy, it was found that 50% of the highest rectum V32Gyand V18Gy dose increases occurred between the angles of 90 and 120 degrees and 37.5% occurred between the angles of 240 and 270 degrees. The MU for each sector showed that 240_270, 240_210, 270_240, and 120_90 had the highest average MU with 150.8, 134.6, 129.2, and 124.3 respectively. This study demonstrated that there is a high correlation between dosimetric impact of intra-fractional motion and the theoretical fiducial visibility. As a result, modification to treatment plans to enable fiducial visibility at all angular sectors throughout a treatment may not be required. Further sector analysis tests are required to develop patient specific megavoltage imaging gantry angles for SBRT prostate patients.

Beam output checks of a commercial high-field magnetic resonance-guided radiotherapy machine with its on-board megavoltage imager.

Beam output checks of a commercial high-field magnetic resonance-guided radiotherapy machine can be performed with its on-board megavoltage imager (MVI). This is a fast and efficient method, but only recommended for daily checks. The aim of our study was to show its suitability for weekly checks by investigating its long-term agreement with the golden standard: ionization chamber measurements in a water tank. For one year, the output deviations obtained with both methods were compared. The difference was 0.1±0.3 (1SD) percentage point. This indicated an excellent agreement, and translated into a tolerance level of±2%.

Improving image quality and reducing dose with 2.5 MV diamond target volume-of-interest cone beam CT imaging.

Over the past decades, continuous efforts have been made to improve megavoltage (MV) image quality versus dose characteristics, including the implementation of low atomic number (Z) targets in MV beamlines and the development of more efficient detectors. Recently, a diamond target beam within a commercial radiotherapy treatment platform demonstrated improved planar contrast-to-noise-ratio (CNR) per unit dose using a novel 2.5 MV sintered diamond target beam, which enabled image acquisition on the order of mGy. The present work assesses cone beam CT (CBCT) image quality characteristics for the novel 2.5 MV diamond target beam and the effects of volume-of-interest (VOI) collimation on the image quality and imaging dose distribution. A sintered diamond target was incorporated into the target arm of the linear accelerator, replacing the 2.5 MV commercial copper imaging target. CBCT image quality was evaluated against the commercial imaging beam with regard to spatial resolution and CNR versus dose. In addition to full-field acquisitions, we investigated VOI techniques that collimate the imaging beam to preselected anatomy, to determine potential image quality improvements and dose sparing capacity. Using an anthropomorphic phantom, VOI regions were defined to encompass the maxillary and ethmoid sinuses and ranged in dimension from 3 cm to 4.85 cm equivalent radius. The MLC was fit to each VOI structure throughout a full CBCT arc and the corresponding MLC sequences were produced as XML scripts for acquisition. Calibrated radiochromic film was used in phantom to measure cumulative axial dose distributions during each CBCT acquisition. In full-field CBCT, the 2.5 MV diamond target beam demonstrated improved CNR versus dose compared to the commercial imaging beam, by factors of up to 1.7. The calculated modulation transfer function (MTF) displayed an increase of nearly 30% in f50 for the 2.5 MV diamond target beam compared to the commercial beam. Using VOI techniques, CNR increased monotonically as a function of equivalent radius at the bone-tissue interface. At the bone-sinus interface, the CNR for the full-field case was slightly decreased compared to the largest VOI case. Imaging dose in the anteroposterior direction increased with increasing VOI equivalent radius. The novel 2.5 MV sintered diamond target beam presents a simple modification to the commercial imaging beam which provides improved image quality in full-field CBCT and the potential for simultaneous dose sparing and CNR improvement at high-contrast interfaces using VOI acquisition techniques.

Region-of-interest intra-arc MV imaging to facilitate sub-mm positional accuracy with minimal imaging dose during treatment deliveries of small cranial lesions.

To automate the generation of region-of-interest (ROI) apertures for use with megavoltage imaging for online positional corrections during cranial stereotactic radiosurgery. Digitally reconstructed radiographs (DRRs) were created for a 3D-printed skull phantom at 5degree gantry angle increments for a three-arc beam arrangement. At each angle, 3000 random rectangular apertures were generated, and 100 shifts on a grid were applied to the anatomy within the frame. For all shifts, the mutual information (MI) between the shifted and unshifted DRR was calculated to derive an average MI gradient. The top 10% of apertures that minimized registration errors were overlaid and discretely thresholded to generate imaging plans. Imaging was acquired with the skull while implementing simulated patient motion on a linac. Control point-specific couch motions were derived to align the skull to its planned positioning. Apertures with a range of repositioning errors less than 0.1mm possessed a 42% larger average MI gradient when compared with apertures with a range greater than 1mm. Dose calculations with Monte Carlo exhibited an 84% reduction in the dose received by 50% of the skull with the 50% thresholded plan when compared to a constant 22×22cm2 imaging plan. For all different imaging plans (with and without motion), the calculated median 3D-errors with respect to the tracking of a metal-BB fiducial positioned at isocenter in the skull were sub-mm except for the 80% thresholded plan. Sub-mm positional errors are achievable with couch motions derived from control point-specific ROI imaging. Smaller apertures that conform to an anatomical ROI can be utilized to minimize the imaging dose incurred at the expense of larger errors.

Reproducibility of chestwall and heart position using surface-guided versus RPM-guided DIBH radiotherapy for left breast cancer.

This study compared the reproducibility of chestwall and heart position using surface-guided versus RPM (real-time position management)-guided deep inspiration breath hold (DIBH) radiotherapy for left sided breast cancer. Forty DIBH patients under either surface-guided radiotherapy (SGRT) or RPM guidance were studied. For patients treated with tangential fields, reproducibility was measured as the displacements in central lung distance (CLD) and heart shadow to field edge distance (HFD) between pretreatment MV (megavoltage) images and planning DRRs (digitally reconstructed radiographs). For patients treated with volumetric modulated arc therapy (VMAT), sternum to isocenter (ISO) distance (StID), spine to rib edge distance (SpRD), and heart shadow to central axis (CAX) distance (HCD) between pretreatment kV images and planning DRRs were measured. These displacements were compared between SGRT and RPM-guided DIBH. In tangential patients, the mean absolute displacements of SGRT versus RPM guidance were 0.19 versus 0.23 cm in CLD, and 0.33 versus 0.62 cm in HFD. With respect to planning DRR, heart appeared closer to the field edge by 0.04 cm with surface imaging versus 0.62 cm with RPM. In VMAT patients, the displacements of surface imaging versus RPM guidance were 0.21 versus 0.15 cm in StID, 0.24 versus 0.19 cm in SpRD, and 0.72 versus 0.41 cm in HCD. Heart appeared 0.41 cm further away from CAX with surface imaging, whereas 0.10 cm closer to field CAX with RPM. None of the differences between surface imaging and RPM guidance was statistically significant. In conclusion, the displacements of chestwall were small and were comparable with SGRT- or RPM-guided DIBH. The position deviations of heart were larger than those of chestwall with SGRT or RPM. Although none of the differences between SGRT and RPM guidance were statistically significant, there was a trend that the position deviations of heart were smaller and more favorable with SGRT than with RPM guidance in tangential patients.

Investigation of image quality of MV and kV CBCT with low-Z beams and high DQE detector.

To investigate cone beam computed tomography (CBCT) image quality using novel combinations of kilovoltage (kV) and megavoltage (MV) beams and detectormaterials. MV and kV CBCT imaging was simulated using the Fastcat hybrid Monte Carlo application. CBCT imaging with various beam energies was investigated: 2.5 and 6 MV photon beams generated with carbon, aluminum, and tungsten targets and a 120 kVp x-ray tube beam based off of a Varian Truebeam on-board imager (OBI). Cadmium tungstate (CWO), gadolinium oxysulfide (GOS), and cesium iodide (CsI) detectors with identical pixel pitch of 0.784 mm were evaluated. Modulation transfer functions (MTF) for all detector/beam combinations were calculated. MV and kV CBCT images for each detector/beam combination of a contrast phantom containing inserts with rib and spongiosa bone, lung, and adipose tissues were simulated with an imaging dose of 7 mGy. Contrast to noise ratio (CNR) of all inserts were compared for all detector/beam combinations. CBCT images of an anthropomorphic head phantom with silver amalgam fillings were also generated. The CWO/120 kVp beam combination resulted in the highest MTF at low frequencies and the CsI detector showed the highest MTF for all other beams and at high frequencies. The CWO/120 kVp beam combination showed the highest CNR for all tissues. The unoptimized CWO/2.5 MV carbon target beam showed the highest CNR of the MV beam/detector combinations with CNR 4% and 17% worse than the optimized Truebeam CsI 120 kVp setup with a bowtie filter and antiscatter grid. Additionally, the CWO 2.5 MV setup showed qualitative reduction of metal artifacts surrounding silver amalgam fillings in an anthropomorphic headphantom. This finding makes a compelling case that further optimization of this CWO carbon target setup could produce CBCT images with similar CNR to current OBI CBCT for equivalent dose with added resilience to metalartifacts.

Variation in isocentre location of an Elekta Unity MR-linac through full gantry rotation

Objective. The objective of this study was to separately quantify the stability of the megavoltage imager (MVI) and radiation head of an Elekta Unity MRL, throughout full gantry rotation. Approach. A ball-bearing (BB) phantom was attached to the radiation head of the Unity, while a single BB was placed at isocentre. Images were acquired during rotation, using the MVI. These images were processed using an in-house developed MATLAB program to reduce the errors resulted by noise, and the positions of the BBs in the images were analysed to extract MVI and radiation head sag data. Main results. The results returned by this method showed reproducibility, with a mean standard deviation of 7 μm for the position of BBs across all gantry angles. The radiation head was found to sag throughout rotation, with a maximum course of movement of 0.59 mm. The sag pattern was stable over a period greater than a year but showed some dependence on gantry rotation direction. Significance. As MRL is a relatively new system, it is promising to have data supporting the high level of precision on one Elekta Unity machine. Isolating and quantifying the sources of uncertainty in radiation delivery may allow more sophisticated analysis of how the system performance may be improved.

The markerless lung target tracking AAPM Grand Challenge (MATCH) results.

Lung stereotactic ablative body radiotherapy (SABR) is a radiation therapy success story with level 1 evidence demonstrating its efficacy. To provide real-time respiratory motion management for lung SABR, several commercial and preclinical markerless lung target tracking (MLTT) approaches have been developed. However, these approaches have yet to be benchmarked using a common measurement methodology. This knowledge gap motivated the MArkerless lung target Tracking CHallenge (MATCH). The aim was to localize lung targets accurately and precisely in a retrospective in silico study and a prospective experimental study. MATCH was an American Association of Physicists in Medicine sponsored Grand Challenge. Common materials for the in silico and experimental studies were the experiment setup including an anthropomorphic thorax phantom with two targets within the lungs, and a lung SABR planning protocol. The phantom was moved rigidly with patient-measured lung target motion traces, which also acted as ground truth motion. In the retrospective in silico study a volumetric modulated arc therapy treatment was simulated and a dataset consisting of treatment planning data and intra-treatment kilovoltage (kV) and megavoltage (MV) images for four blinded lung motion traces was provided to the participants. The participants used their MLTT approach to localize the moving target based on the dataset. In the experimental study, the participants received the phantom experiment setup and five patient-measured lung motion traces. The participants used their MLTT approach to localize the moving target during an experimental SABR phantom treatment. The challenge was open to any participant, and participants could complete either one or both parts of the challenge. For both the in silico and experimental studies the MLTT results were analyzed and ranked using the prospectively defined metric of the percentage of the tracked target position being within 2mm of the ground truth. A total of 30 institutions registered and 15 result submissions were received, four for the in silico study and 11 for the experimental study. The participating MLTT approaches were: Accuray CyberKnife (2), Accuray Radixact (2), BrainLab Vero, C-RAD, and preclinical MLTT (5) on a conventional linear accelerator (Varian TrueBeam). For the in silico study the percentage of the 3D tracking error within 2mm ranged from 50% to 92%. For the experimental study, the percentage of the 3D tracking error within 2mm ranged from 39% to 96%. A common methodology for measuring the accuracy of MLTT approaches has been developed and used to benchmark preclinical and commercial approaches retrospectively and prospectively. Several MLTT approaches were able to track the target with sub-millimeter accuracy and precision. The study outcome paves the way for broader clinical implementation of MLTT. MATCH is live, with datasets and analysis software being available online at https://www.aapm.org/GrandChallenge/MATCH/ to support future research.

Tattoo-Free Set-ups for Breast Cancer Patients Receiving Regional Nodal Irradiation: Evaluation of Accuracy and Time Efficiency

Purpose/Objective(s)Permanent skin-based tattoos have been considered in treatment planning an essential aspect of ensuring patient positioning accuracy for radiation delivery in breast cancer patients. Despite this benefit, tattoos may also be a reminder of cancer treatment long past the treatment itself, causing unnecessary emotional distress. With the advent of advanced imaging techniques, routine tattoo placement may no longer be necessary. We performed a retrospective study evaluating a tattoo-free method of set-up for breast cancer patients receiving RNI.Materials/MethodsFollowing a transition in our institution's clinical practice toward a tattoo-free set-up in May 2020, we compared a cohort of breast cancer patients, in which tattoos were referenced for daily alignment (“tattoo group”) and a second cohort in which tattoos were placed but not referenced for daily alignment (“no tattoo group”). The number of patients in the two groups were matched by tumor laterality (right versus left) and breath-hold utilization. Patients in both cohorts were positioned using surface-guided imaging with periodic megavoltage (MV) x-ray imaging to confirm patient positioning. The physical shifts obtained from MV imaging, after surface-imaging, and the duration of daily patient treatment were compared between the two cohorts using the Mann-Whitney U-test.ResultsThere were 14 patients in each cohort; 8 left-sided breath-hold patients and 6 right-sided non-breath-hold patients, for a total of 61 recorded duration times and shifts for the tattoo group and 59 recorded duration times and shifts for the no-tattoo group. The average physical 3-dimensional shifts for the no-tattoo group was smaller compared to the tattoo group, mean 4.4 mm versus 5.7 mm (P < 0.01) and patient time in treatment room was also decreased for the no-tattoo group: mean 19.7 min versus 21.3 min (P = 0.03).ConclusionBased on our preliminary results for a cohort of women receiving RNI for breast cancer treatment at our institution, omitting radiation tattoos while utilizing daily surface imaging led to improved patient setup accuracy and decreased patient time on the treatment machine. Utilizing daily surface imaging obviates the requirement for skin-based tattoos in this patient population, improves efficiency of machine utilization, and provides the means for a tattoo-free set-up to become standard practice. Permanent skin-based tattoos have been considered in treatment planning an essential aspect of ensuring patient positioning accuracy for radiation delivery in breast cancer patients. Despite this benefit, tattoos may also be a reminder of cancer treatment long past the treatment itself, causing unnecessary emotional distress. With the advent of advanced imaging techniques, routine tattoo placement may no longer be necessary. We performed a retrospective study evaluating a tattoo-free method of set-up for breast cancer patients receiving RNI. Following a transition in our institution's clinical practice toward a tattoo-free set-up in May 2020, we compared a cohort of breast cancer patients, in which tattoos were referenced for daily alignment (“tattoo group”) and a second cohort in which tattoos were placed but not referenced for daily alignment (“no tattoo group”). The number of patients in the two groups were matched by tumor laterality (right versus left) and breath-hold utilization. Patients in both cohorts were positioned using surface-guided imaging with periodic megavoltage (MV) x-ray imaging to confirm patient positioning. The physical shifts obtained from MV imaging, after surface-imaging, and the duration of daily patient treatment were compared between the two cohorts using the Mann-Whitney U-test. There were 14 patients in each cohort; 8 left-sided breath-hold patients and 6 right-sided non-breath-hold patients, for a total of 61 recorded duration times and shifts for the tattoo group and 59 recorded duration times and shifts for the no-tattoo group. The average physical 3-dimensional shifts for the no-tattoo group was smaller compared to the tattoo group, mean 4.4 mm versus 5.7 mm (P < 0.01) and patient time in treatment room was also decreased for the no-tattoo group: mean 19.7 min versus 21.3 min (P = 0.03). Based on our preliminary results for a cohort of women receiving RNI for breast cancer treatment at our institution, omitting radiation tattoos while utilizing daily surface imaging led to improved patient setup accuracy and decreased patient time on the treatment machine. Utilizing daily surface imaging obviates the requirement for skin-based tattoos in this patient population, improves efficiency of machine utilization, and provides the means for a tattoo-free set-up to become standard practice.

Nontoxic electron collimators.

PurposeThe goal of this work was to develop and test nontoxic electron collimation technologies for clinical use.MethodsTwo novel technologies were investigated: tungsten‐silicone composite and 3D printed electron cutouts. Transmission, dose uniformity, and profiles were measured for the tungsten‐silicone. Surface dose, relative dose output, and field size were measured for the 3D printed cutouts and compared with the standard cerrobend cutouts in current clinical use. Quality assurance tests including mass measurements, Megavoltage (MV) imaging, and drop testing were developed for the 3D printed cutouts as a guide to safe clinical implementation.ResultsDose profiles of the flexible tungsten‐silicone skin shields had an 80–20 penumbra values of 2–3 mm compared to 7–8 mm for cerrobend. In MV transmission image measurements of the tungsten‐silicone, 80% of the pixels had a transmission value within 2% of the mean. An ∼90% reduction in electron intensity was measured for 6 MeV and a 6.4 mm thickness of tungsten‐silicone and 12.7 mm thickness for 16 MeV. The maximum difference in 3D printed cutout versus cerrobend output, surface dose, and full width at half‐maximum (FWHM) was 1.7%, 1.2%, and 1.5%, respectively, for the 10 cm × 10 cm cutouts.ConclusionsBoth flexible tungsten‐silicone and 3D printed cutouts were found to be feasible for clinical use. The flexible tungsten‐silicone was of adequate density, flexibility, and uniformity to serve as skin shields for electron therapy. The 3D printed cutouts were dosimetrically equivalent to standard cerrobend cutouts and were robust enough for handling in the clinical environment.

Verification of the delivered patient radiation dose for non-coplanar beam therapy.

PurposeThere is an increased interest in using non‐coplanar beams for radiotherapy, including SBRT and SRS. This approach can significantly reduce doses to organs‐at‐risk, however, it requires stringent quality assurance, especially when a dynamic treatment couch is used. In this work, new functionality that allows using non‐coplanar beam arrangements in addition to conventional coplanar beams was added and validated to the previously developed in vivo dose verification system.MethodsThe existing program code was modified to manage the additional treatment couch parameters: angle and positions. Ten non‐coplanar test plans that use a static couch were created in the treatment planning system. Also, two plans that use a dynamic treatment couch were created and delivered using Varian Developer mode, since the treatment planning system does not support a dynamic couch. All non‐coplanar test trajectories were delivered on a simple geometric phantom, using an Edge linear accelerator (Varian Medical Systems) with the megavoltage imager deployed and acquiring megavoltage transmission images that were used to calculate the delivered 3D dose distributions in the phantom with the updated dose calculation algorithm. The reconstructed dose distributions were compared using the 3D chi‐comparison test with 2%/2mm tolerances to the corresponding reference dose distributions obtained from the treatment planning system.ResultsThe chi‐comparison test resulted in at least a 97.0% pass rate over the entire 3D volume for all tested trajectories. For static gantry, static couch non‐coplanar fields, and non‐coplanar arcs using dynamic couch the pass rates observed were at least 98%, while for the static couch, non‐transverse coplanar arc fields, pass rates were at least 97%.ConclusionsA model‐based 3D dose calculation algorithm has been extended and validated for a variety of non‐coplanar beam trajectories of different complexities. This system can potentially be applied for quality assurance of treatment delivery systems that use complex, non‐coplanar beam arrangements.

A Low-Cost Method to Assess the Performance of Surface Guidance Imaging Systems at Non-Zero Couch Angles.

A procedure is presented to assess performance at non-zero couch angles and perform routine quality assurance (QA) on surface-guided radiotherapy (SGRT) imaging systems used for stereotactic radiosurgery (SRS). A low-cost anthropomorphic phantom was used to assess the system under patient-like conditions. The phantom is embedded with a tungsten ball bearing (BB) to facilitate the use of surface imaging (SI) with concurrent megavoltage (MV) imaging to cross-compare and validate SI-reported offsets. Data analysis is done via in-house software that utilized the SGRT system’s log files for automated analysis. This procedure enables users to assess and inter-compare MV-reported offsets with their SGRT system. The analysis provides SGRT system residual error so that users are aware of inherent offsets present in addition to increases in translational offsets due to couch walkout. The procedure was validated with two commercial SGRT systems. The procedure can be used with any surface imaging system and linear accelerator system.