Cube Phantom Research Articles

Development of a phantom for assessing the precision of setup in skin mark-less surface-guided radiotherapy.

Surface-guided radiotherapy (SGRT) is adopted by several institutions; however, reports on the phantoms used to assess the precision of the SGRT setup are limited. The purpose of this study was to develop a phantom to verify the accuracy of the irradiation position during skin mark-less SGRT. An acrylonitrile butadiene styrene (ABS) plastic cube phantom with a diameter of 150mm on each side containing a dummy target of 15mm and two types of body surface-shaped phantoms (breast/face shape) that could be attached to the cube phantom were fabricated. Films can be inserted on four sides of the cubic phantom (left, right, anterior and posterior), and the center of radiation can be calculated by irradiating the dummy target with orthogonal MV beams. Three types of SGRT using a VOXELAN-HEV600M (Electronics Research&Development Corporation, Okayama, Japan) were evaluated using this phantom: (i) SGRTCT-a SGRT set-up based solely on a computed tomography (CT)-reference image. (ii) SGRTCT + CBCT-a method where cone beam computed tomography (CBCT) matching was performed after SGRTCT. (iii) SGRTScan-a resetup technique using a scan reference image obtained after completing the (ii) step. Both the breast and face phantoms were recognized in the SGRT system without problems. SGRTScan ensure precision within 1mm/1° for breast and face verification, respectively. All SGRT methods showed comparable rotational accuracies with no significant disparities. The developed phantom was useful for verifying the accuracy of skin mark-less SGRT position matching. The SGRTScan demonstrated the feasibility of achieving skin-mark less SGRT with high accuracy, with deviations of less than 1mm. Additional research is necessary to evaluate the suitability of the developed phantoms for use in various facilities and systems. This phantom could be used for postal surveys in the future.

Efficient EPID-based quality assurance of beam time delay for respiratory-gated radiotherapy with validation on Catalyst™ and AlignRT™ systems.

To propose a straightforward and time-efficient quality assurance (QA) approach of beam time delay for respiratory-gated radiotherapy and validate the proposed method on typical respiratory gating systems, Catalyst™ and AlignRT™. The QA apparatus was composed of a motion platform and a Winston-Lutz cube phantom (WL3) embedded with metal balls. The apparatus was first scanned in CT-Sim and two types of QA plans specific for beam on and beam off time delay, respectively, were designed. Static reference images and motion testing images of the WL3 cube were acquired with EPID. By comparing the position differences of the embedded metal balls in the motion and reference images, beam time delays were determined. The proposed approach was validated on three linacs with either Catalyst™ or AlignRT™ respiratory gating systems. To investigate the impact of energy and dose rate on beam time delay, a range of QA plans with Eclipse (V15.7) were devised with varying energy and dose rates. For all energies, the beam on time delays in AlignRT™ V6.3.226, AlignRT™ V7.1.1, and Catalyst™ were 92.13 5.79ms, 123.11 6.44ms, and 303.44 4.28ms, respectively. The beam off time delays in AlignRT™ V6.3.226, AlignRT™ V7.1.1, and Catalyst™ were 121.87 1.34ms, 119.33 0.75ms, and 97.69 2.02ms, respectively. Furthermore, the beam on delays decreased slightly as dose rates increased for all gating systems, whereas the beam off delays remained unaffected. The validation results demonstrate the proposed QA approach of beam time delay for respiratory-gated radiotherapy was both reproducible and time-efficient to practice for institutions to customize accordingly.

Focus stacking single-event particle radiography for high spatial resolution images and 3D feature localization

Objective. We demonstrate a novel focus stacking technique to improve spatial resolution of single-event particle radiography (pRad), and exploit its potential for 3D feature detection. Approach. Focus stacking, used typically in optical photography and microscopy, is a technique to combine multiple images with different focal depths into a single super-resolution image. Each pixel in the final image is chosen from the image with the largest gradient at that pixel's position. pRad data can be reconstructed at different depths in the patient based on an estimate of each particle's trajectory (called distance-driven binning; DDB). For a given feature, there is a depth of reconstruction for which the spatial resolution of DDB is maximal. Focus stacking can hence be applied to a series of DDB images reconstructed from a single pRad acquisition for different depths, yielding both a high-resolution projection and information on the features’ radiological depth at the same time. We demonstrate this technique with Geant4 simulated pRads of a water phantom (20 cm thick) with five bone cube inserts at different depths (1 × 1 × 1 cm3) and a lung cancer patient. Main results. For proton radiography of the cube phantom, focus stacking achieved a median resolution improvement of 136% compared to a state-of-the-art maximum likelihood pRad reconstruction algorithm and a median of 28% compared to DDB where the reconstruction depth was the center of each cube. For the lung patient, resolution was visually improved, without loss in accuracy. The focus stacking method also enabled to estimate the depth of the cubes within few millimeters accuracy, except for one shallow cube, where the depth was underestimated by 2.5 cm. Significance. Focus stacking utilizes the inherent 3D information encoded in pRad by the particle's scattering, overcoming current spatial resolution limits. It further opens possibilities for 3D feature localization. Therefore, focus stacking holds great potential for future pRad applications.

Computed Tomography Attenuation of Three-Dimensional (3D) Printing Materials-Depository to Aid in Constructing 3D-Printed Phantoms.

Three-dimensionally printed phantoms are increasingly used in medical imaging and research due to their cost-effectiveness and customizability, offering valuable alternatives to commercial phantoms. The purpose of this study was to assess the computed tomography (CT) attenuation characteristics of 27 resin materials from Formlabs, a 3D printing equipment and materials manufacturer. Cube phantoms (both solid and hollow constructions) produced with each resin were subjected to CT scanning under varying tube current-time products with attenuation measurements recorded in Hounsfield units (HU). The resins exhibited a wide range of attenuation values (-3.33 to 2666.27 HU), closely mimicking a range of human tissues, from fluids to dense bone structures. The resins also demonstrated consistent attenuation regardless of changes in the tube current. The CT attenuation analysis of FormLabs resins produced an archive of radiological imaging characteristics of photopolymers that can be utilized to construct more accurate tissue mimicking medical phantoms and improve the evaluation of imaging device performance.

3D Image Reconstruction Capability of Peripheral On-chip ECT Sensor

The miniaturisation of electrical tomography sensors such as EIT has become popular recently and these miniaturised sensors were investigated for 2D and 3D imaging of cells in EIT. Nevertheless, research on miniaturisation of ECT sensor also existed which were based on peripheral electrode sensor which had electrodes arranged on the surface around the sensing chamber. However, the study on the miniature peripheral ECT sensor was limited to 2D image reconstruction and detection of presents of samples in the sensing chamber only and lacked the works on the 3D imaging capability. Therefore, this article investigates the 3D image reconstruction capability of peripheral on-chip ECT sensor through simulation approach by simulating a cube phantom at several test positions in the sensing chamber. Eventually, the 3D and 2D images were reconstructed and the 3D imaging capability of the sensor was characterised qualitatively based on visual inspection of the reconstructed image. The simulation results revealed that the 3D imaging capability of peripheral on-chip ECT sensor was limited to the region where electrodes were present only. However, the 2D reconstructed image results showed that the sensor was able to locate the positions of test phantom in the sensing chamber accurately.

Biological modeling of gadolinium-based nanoparticles radio-enhancement for kilovoltage photons: a Monte Carlo study

BackgroundGadolinium-based nanoparticles (GdNPs) are clinically used agents to increase the radiosensitivity of tumor cells. However, studies on the mechanisms and biological modeling of GdNP radio-enhancement are still preliminary. This study aims to investigate the mechanism of radio-enhancement of GdNPs for kilovoltage photons using Monte Carlo (MC) simulations, and to establish local effect model (LEM)-based biological model of GdNP radiosensitization.MethodsThe spectrum and yield of secondary electrons and dose enhancement around a single GdNP and clustered GdNPs were calculated in a water cube phantom by MC track-structure simulations using TOPAS code. We constructed a partial shell-like cell geometry model of pancreatic cancer cell based on transmission electron microscope (TEM) observations. LEM-based biological modeling of GdNP radiosensitization was established based on the MC-calculated nano-scale dose distributions in the cell model to predict the cell surviving fractions after irradiation.ResultsThe yield of secondary electrons for GdNP was 0.16% of the yield for gold nanoparticle (GNP), whereas the average electron energy was 12% higher. The majority of the dose enhancement came from the contribution of Auger electrons. GdNP clusters had a larger range and extent of dose enhancement than single GdNPs, although GdNP clustering reduced radial dose per interacting photon significantly. For the dose range between 0 and 8 Gy, the surviving fraction predicted using LEM-based biological model laid within one standard deviation of the published experimental results, and the deviations between them were all within 25%.ConclusionsThe mechanism of radio-enhancement of GdNPs for kilovoltage photons was investigated using MC simulations. The prediction results of the established LEM-based biological model for GdNP radiosensitization showed good agreement with published experimental results, although the deviation of simulation parameters can lead to large disparity in the results. To our knowledge, this was the first LEM-based biological model for GdNP radiosensitization.

In-vivo quality assurance of dynamic tumor tracking (DTT) for liver SABR using EPID images.

To assess dynamic tumor tracking (DTT) target localization uncertainty for in-vivo marker-based stereotactic ablative radiotherapy (SABR) treatments of the liver using electronic-portal-imaging-device (EPID) images. The Planning Target Volume (PTV) margin contribution for DTT is estimated. Phantom and patient EPID images were acquired during non-coplanar 3DCRT-DTT delivered on a Vero4DRT linac. A chain-code algorithm was applied to detect Multileaf Collimator (MLC)-defined radiation field edges. Gold-seed markers were detected using a connected neighbor algorithm. For each EPID image, the absolute differences between the measured center-of-mass (COM) of the markers relative to the aperture-center (Tracking Error, (ET )) was reported in pan, tilt, and 2D-vector directions at the isocenter-plane. An acrylic cube phantom implanted with gold-seed markers was irradiated with non-coplanar 3DCRT-DTT beams and EPID images collected. Patient Study: Eight liver SABR patients were treated with non-coplanar 3DCRT-DTT beams. All patients had three to four implanted gold-markers. In-vivo EPID images were analyzed. Phantom Study: On the 125 EPID images collected, 100% of the markers were identified. The average±SD of ET were 0.24±0.21, 0.47±0.38, and 0.58±0.37mm in pan, tilt and 2D directions, respectively. Patient Study: Of the 1430 EPID patient images acquired, 78% had detectable markers. Over all patients, the average±SD of ET was 0.33±0.41mm in pan, 0.63±0.75mm in tilt and 0.77±0.80mm in 2D directions The random 2D-error, σ, for all patients was 0.79mm and the systematic 2D-error, Σ, was 0.20mm. Using the Van Herk margin formula 1.1mm planning target margin can represent the marker based DTT uncertainty. Marker-based DTT uncertainty can be evaluated in-vivo on a field-by-field basis using EPID images. This information can contribute to PTV margin calculations for DTT.

Current Source Density Imaging Using Regularized Inversion of Acoustoelectric Signals.

Acoustoelectric (AE) imaging can potentially image biological currents at high spatial (~mm) and temporal (~ms) resolution. However, it does not directly map the current field distribution due to signal modulation by the acoustic field and electric lead fields. Here we present a new method for current source density (CSD) imaging. The fundamental AE equation is inverted using truncated singular value decomposition (TSVD) combined with Tikhonov regularization, where the optimal regularization parameter is found based on a modified L-curve criterion with TSVD. After deconvolution of acoustic fields, the current field can be directly reconstructed from lead field projections and the CSD image computed from the divergence of that field. A cube phantom model with a single dipole source was used for both simulation and bench-top phantom studies, where 2D AE signals generated by a 0.6 MHz 1.5D array transducer were recorded by orthogonal leads in a 3D Cartesian coordinate system. In simulations, the CSD reconstruction had significantly improved image quality and current source localization compared to AE images, and performance further improved as the fractional bandwidth (BW) increased. Similar results were obtained in the phantom with a time-varying current injected. Finally, a feasibility study using an in vivo swine heart model showed that optimally reconstructed CSD images better localized the current source than AE images over the cardiac cycle.

The spatial accuracy of ring-mounted halcyon linac versus C-arm TrueBeam linac for single-isocenter/multi-target SBRT treatment.

Stereotactic body radiotherapy (SBRT) treatment of oligometastatic lesions via single-isocenter/multi-target (SIMT) plan is more efficient than using multi-isocenter/multitarget SBRT. This study quantifies the spatial positioning accuracy of 2 commercially available LINAC systems for SIMT treatment pertaining to the potential amplification of error as a function of the target's distance-to-isocenter. We compare the Ring-Gantry Halcyon LINAC equipped with the fast iterative conebeam-CT (iCBCT) for image-guided SIMT treatment, and the SBRT-dedicated C-Arm TrueBeam with standard pretreatment CBCT imaging. For both systems, Sun Nuclear's MultiMet Winston-Lutz Cube phantom with 6 metallic BBs distributed at different planes up to 7 cm away from the isocenter was used. The phantom was aligned and imaged via CBCT, and then couch corrections were applied. To treat all 6 BBs, an Eclipse 10-field 3D-conformal Field-in-Field (2×2 cm2 MLC field to each BB) plan for varying gantry, collimator, and couch (TrueBeam only) positions was developed for both machines with 6MV-FFF beam. The plan was delivered through ARIA once a week. The EPID images were analyzed via Sun Nuclear's software for spatial positioning accuracy. On TrueBeam, the treatment plan was delivered twice: once with 3DoF translational corrections and once with PerfectPitch 6DoF couch corrections. The average 3D spatial positioning accuracy was 0.55 ± 0.30 mm, 0.54 ± 0.24 mm, and 0.56 ± 0.28 mm at isocenter, and 0.59 ± 0.30 mm, 0.69 ± 0.30 mm, and 0.70 ± 0.35 mm at 7 cm distance-to-isocenter for Halcyon, TrueBeam 3DoF, and TrueBeam 6DoF, respectively. This suggests there are no clinically significant deviations of spatial uncertainty between the platforms with the distance-to-isocenter. On both platforms, our weekly independent measurements demonstrated the reproducibility for less than 1.0 mm positional accuracy of off-axis targets up to 7 cm from the isocenter. Due to this, no additional PTV-margin is suggested for lesions within 7 cm of isocenter. This study confirms that Halcyon can deliver similar positional accuracy to SBRT-dedicated TrueBeam to off-axis targets up to 7 cm from isocenter. These results further benchmark the spatial uncertainty of our extensively used SBRT-dedicated TrueBeam LINAC for SIMT SBRT treatments.

Development and feasibility evaluation of an AR-assisted radiotherapy positioning system.

PurposeThe aim of this study is to develop an augmented reality (AR)–assisted radiotherapy positioning system based on HoloLens 2 and to evaluate the feasibility and accuracy of this method in the clinical environment.MethodsThe obtained simulated computed tomography (CT) images of an “ISO cube”, a cube phantom, and an anthropomorphic phantom were reconstructed into three-dimensional models and imported into the HoloLens 2. On the basis of the Vuforia marker attached to the “ISO cube” placed at the isocentric position of the linear accelerator, the correlation between the virtual and real space was established. First, the optimal conditions to minimize the deviation between virtual and real objects were explored under different conditions with a cube phantom. Then, the anthropomorphic phantom–based positioning was tested under the optimal conditions, and the positioning errors were evaluated with cone-beam CT.ResultsUnder the normal light intensity, the registration and tracking angles are 0°, the distance is 40 cm, and the deviation reached a minimum of 1.4 ± 0.3 mm. The program would not run without light. The hologram drift caused by the light change, camera occlusion, and head movement were 0.9 ± 0.7 mm, 1.0 ± 0.6 mm, and 1.5 ± 0.9 mm, respectively. The anthropomorphic phantom–based positioning errors were 3.1 ± 1.9 mm, 2.4 ± 2.5 mm, and 4.6 ± 2.8 mm in the X (lateral), Y (vertical), and Z (longitudinal) axes, respectively, and the angle deviation of Rtn was 0.26 ± 0.14°.ConclusionThe AR-assisted radiotherapy positioning based on HoloLens 2 is a feasible method with certain advantages, such as intuitive visual guidance, radiation-free position verification, and intelligent interaction. Hardware and software upgrades are expected to further improve accuracy and meet clinicalbrendaannmae requirements.

Design of a novel 5-camera surface guidance system with multiple imaging isocenters.

Purpose/objective(s)Surface‐guided radiation therapy (SGRT) can track the patient surface noninvasively to complement radiographic image‐guided radiation therapy with a standard 3‐camera system and a single radiation/image isocenter. Here we report the commissioning of a novel SGRT system that monitors three imaging isocenters locations in a proton half‐gantry room with a unique 5‐camera configuration.Materials/methodsThe proton half‐gantry room has three image isocenters, designated ISO‐0, ISO‐1, and ISO‐2, to cover various anatomical sites via a robotic ceiling‐mounted cone‐beam CT. Although ISO‐0 and ISO‐1 are used to image the cranium, head and neck, and thoracic regions, ISO‐2 is often used to image body and extremity sites and contiguous craniospinal target volumes. The five‐camera system was calibrated to the radiographic isocenter by using a stereotactic radiosurgery cube phantom for each image isocenter.ResultsThe performance of this 5‐camera system was evaluated for 6 degrees of freedom in three categories: (1) absolute setup accuracy relative to the radiographic kV image isocenter based on the DICOM reference; (2) relative shift accuracy based on a reference surface capture; and (3) isocenter tracking accuracy from one isocenter to another based on a reference surface capture. The evaluation revealed maximum deviations of 0.8, 0.2, and 0.6 mm in translation and 0.2°, 0.1°, and 0.1° in rotation for the first, second, and third categories, respectively. Comparing the dosimetry and latency with static and gated irradiation revealed a 0.1% dose difference and positional differences of 0.8 mm in X and 0.9 mm in Y with less than 50 ms temporal accuracy.ConclusionThe unique 5‐camera system configuration provides SGRT at the treatment isocenter (ISO‐0) and also imaging isocenter locations (ISO‐0, ISO‐1, and ISO‐2) to ensure correct patient positioning before and after radiographic imaging, especially during transitions from the offset imaging isocenters to the treatment isocenter.

The accuracy of helium ion CT based particle therapy range prediction: an experimental study comparing different particle and x-ray CT modalities

This work provides a quantitative assessment of helium ion CT (HeCT) for particle therapy treatment planning. For the first time, HeCT based range prediction accuracy in a heterogeneous tissue phantom is presented and compared to single-energy x-ray CT (SECT), dual-energy x-ray CT (DECT) and proton CT (pCT). HeCT and pCT scans were acquired using the US pCT collaboration prototype particle CT scanner at the Heidelberg Ion-Beam Therapy Center. SECT and DECT scans were done with a Siemens Somatom Definition Flash and converted to RSP. A Catphan CTP404 module was used to study the RSP accuracy of HeCT. A custom phantom of 20 cm diameter containing several tissue equivalent plastic cubes was used to assess the spatial resolution of HeCT and compare it to DECT. A clinically realistic heterogeneous tissue phantom was constructed using cranial slices from a pig head placed inside a cylindrical phantom (ø150 mm). A proton beam (84.67 mm range) depth-dose measurement was acquired using a stack of GafchromicTM EBT-XD films in a central dosimetry insert in the phantom. CT scans of the phantom were acquired with each modality, and proton depth-dose estimates were simulated based on the reconstructions. The RSP accuracy of HeCT for the plastic phantom was found to be 0.3 ± 0.1%. The spatial resolution for HeCT of the cube phantom was 5.9 ± 0.4 lp cm−1 for central, and 7.6 ± 0.8 lp cm−1 for peripheral cubes, comparable to DECT spatial resolution (7.7 ± 0.3 lp cm−1 and 7.4 ± 0.2 lp cm−1, respectively). For the pig head, HeCT, SECT, DECT and pCT predicted range accuracy was 0.25%, −1.40%, −0.45% and 0.39%, respectively. In this study, HeCT acquired with a prototype system showed potential for particle therapy treatment planning, offering RSP accuracy, spatial resolution, and range prediction accuracy comparable to that achieved with a commercial DECT scanner. Still, technical improvements of HeCT are needed to enable clinical implementation.

The kVCT System Commissioning of a Novel Medical Linear Accelerator Designed for Biology-Guided Radiotherapy

A novel therapeutic machine that combines PET, fan beam kVCT and linear accelerator to support real-time PET-guided treatment, termed biology-guided radiotherapy (BgRT). This study reports the positioning accuracy, image quality, and dose commissioning of the kVCT imaging system of the first clinical BgRT machine at our institute.The helical fan-beam kVCT subsystem consists of an X-ray tube with a voltage of 120kV and a 16-row GOS Ceramic Scintillator detector. The gantry rotation speed is 60 rpm. The tube currents are 45, 67, 133mA for 0.5 pitch; 100, 150, and 300mA for 0.5 pitch; 267 and 400mA for 1.333 pitch. The image slice thickness is 1.25mm with a field of view of 500mm and a resolution of 512 × 512. A ball-cube phantom was used to test the positioning accuracy of the kVCT subsystem with respect to the setup laser and MV beam. Catphan504 phantom was imaged with 0.5 pitch and 150mAs to test the kVCT image quality of the BgRT system, including the uniformity, noise, HU constancy, spatial resolution, geometric distortion, slice thickness, and low contrast visibility and detectability using the PIPSproTM software. The RTI MAS-2 and Piranha detectors were used to verify the accuracy of the mA and voltage. The CTDI dose was measured using a CTDI phantom and a Standard Imaging A101 pencil chamber.The localization error of the ball cube phantom was -0.8, 0.9, and 0.3mm in IEC x, y, and z directions, respectively. For the Catphan504 tests, the vertical and horizontal uniformities were 0.926 and 0.857, respectively. The HU constancy tests of the air, water, polystyrene, acrylic, Delrin, Teflon were all within 40HU to the expected values. The modulation transfer function test results were 0.467, 0.528, and 0.595 lp/mm for F50, F40, and F30, respectively, which showed a comparable spatial resolution to regular CT simulators. The geometric distortion was 0.12mm and slice thickness inaccuracy was 0.13mm. The low contrast detectability was 0.734. The measured mA and voltage were within 5% of the nominal setting of 120 kV and 150 mA. The CTDIw was less than 1.4 cGy for body and 3 cGy for head with a protocol of 120 kVp, 150 mAs, and 1.25 mm slice thickness.This evaluation represents the kVCT characteristics of the novel BgRT system with architecture designed to accommodate CT, PET and a LINAC. The image quality and HU constancy are comparable to our CT simulators and is a useful tool for online adaptive radiotherapy. The commissioning data will serve as a standard for units installed at future clinical sites.

Use of Three-Dimensional Printing in Modelling an Anatomical Structure with a High Computed Tomography Attenuation Value: A Feasibility Study

Introduction: Three-dimensional (3D) printing provides an opportunity to develop anthropomorphic computed tomography (CT) phantoms with anatomical and radiological features mimicking a range of patients’ conditions, thus allowing development of individualised, low dose scanning protocols. However, previous studies of 3D printing in CT phantom development could only create anatomical structures using potassium iodide with attenuation values up to 1200 HU which is insufficient to mimic the radiological features of some high attenuation structures such as cortical bone. This study aimed at investigating the feasibility of using 3D printing in modelling cortical bone with a non-iodinated material. Methods: This study had 2 stages. Stage 1 involved a vat photopolymerisation 3D printer to directly print cube phantoms with different percentage compositions of calcium phosphate (CP) and resin (approach 1), and approach 2 using a material extrusion 3D printer to develop a cube mould for infilling of the CP with hardener as the phantom. The approach able to create the cube phantom with the CT attenuation value close to that of a tibial mid-diaphysis cortex of a real patient, 1475±205 HU was employed to develop a tibial mid-diaphysis phantom. The mean CT numbers of the cube and tibia phantoms were measured and compared with that of the original CT dataset through unpaired t-test. Results: All phantoms were scanned by CT using a lower extremity scanning protocol. The moulding approach was selected to develop the tibia middiaphysis phantom with CT attenuation value, 1434±184 HU which was not statistically significantly different from the one of the original dataset (p = 0.721). Conclusion: This study demonstrates the feasibility to use the material extrusion 3D printer to create a tibial mid-diaphysis mould for infilling of the CP as an anthropomorphic CT phantom and the attenuation value of its cortex matches the real patient’s one.

Development and Longitudinal Analysis of Plan-Based Streamlined Quality Assurance on Multiple Positioning Guidance Systems With Single Phantom Setup.

PurposeThis study was to propose and validate an efficient and streamlined quality assurance (QA) method with a single phantom setup to check performances of patient positioning guidance systems including six-degree-of-freedom (6DoF) couch, X-ray modalities (kV–kV, MV–MV and CBCT), optical surface imaging system (AlignRT), lasers and optical distance indicator (ODI).Methods and MaterialsThe QA method was based on a pseudo-patient treatment plan using the AlignRT cube phantom. The cube was first randomly set up on the couch, and the initial position offsets were acquired by AlignRT and CBCT. The cube was restored to its reference position by 6DoF couch shift, during which the couch motion accuracy and tracking performances of AlignRT and CBCT were derived. After that, the residual offsets were acquired by kV–kV, MV–MV and AlignRT to derive the isocenter discrepancies. Finally, the laser alignment and ODI values were visually inspected. The QA procedure had been internally approved as a standard weekly QA test, and the results over 50 weeks were longitudinally analyzed for clinical validation.ResultsThe 6DoF couch motion errors as well as the tracking errors of AlignRT were sub-millimeter and sub-degree, and no deviation over 1 mm or 1 deg was identified. The ROI mode of isocenter (ISO) in AlignRT exhibited more consistent results than the centroid (CEN). While the isocenter discrepancy between CBCT and kV–kV was negligible, the maximal discrepancies between CBCT and MV–MV were 0.4 mm in LNG and 0.3 deg in PITCH. The isocenter discrepancies between CBCT and AlignRT were <0.5 mm in translation and <0.3 deg in rotation. For AlignRT, the isocenter discrepancies between the DICOM and SGRT references were about 0.6 mm in VRT, 0.5 mm in LNG and 0.2 deg in PITCH. As the therapists became familiar with the workflow, the average time to complete the whole procedure was around 23 min.ConclusionsThe streamlined QA exhibits desirable practicality as an efficient multipurpose performance check on positioning guidance systems. The stability, tracking performance and isocenter congruence of the positioning guidance systems have been fully validated for all clinical image guidance RT application, even SRS/SBRT, which requires the strictest tolerance.

Proposal for a new operational quantity to assess the effective dose in the event of external exposure to photons

Despite its shortcomings and inconsistencies such as using the kerma approximation, the ICRU tissue, which does not really exist and the radiation quality factor Q defined in water, the personal dose equivalent Hp defined in the 1980s remains until now the operational quantity used to assess the effective dose E for external exposure. To help overcome most of these shortcomings and inconsistencies a new operational quantity Hp(30)water to replace Hp(10) is proposed. Fluence-to Hp(30) water conversion coefficients are calculated for photon energies from 0.02 to 10 MeV using the Monte Carlo transport code EGSnrc. Calculations are performed in a water cube phantom using full transport of secondary charged particles. The comparison of the conversion coefficients of Hp(30) and E shows that Hp(30) water can represent a good envelope of E over the whole energy range considered.

Commissioning of optical surface imaging systems for cranial frameless stereotactic radiosurgery.

PurposeThis study aimed to evaluate and compare different system calibration methods from a large cohort of systems to establish a commissioning procedure for surface‐guided frameless cranial stereotactic radiosurgery (SRS) with intrafractional motion monitoring and gating. Using optical surface imaging (OSI) to guide non‐coplanar SRS treatments, the determination of OSI couch‐angle dependency, baseline drift, and gated‐delivered‐dose equivalency are essential.MethodsEleven trained physicists evaluated 17 OSI systems at nine clinical centers within our institution. Three calibration methods were examined, including 1‐level (2D), 2‐level plate (3D) calibration for both surface image reconstruction and isocenter determination, and cube phantom calibration to assess OSI‐megavoltage (MV) isocenter concordance. After each calibration, a couch‐angle dependency error was measured as the maximum registration error within the couch rotation range. A head phantom was immobilized on the treatment couch and the isocenter was set in the middle of the brain, marked with the room lasers. An on‐site reference image was acquired at couch zero, the facial region of interest (ROI) was defined, and static verification images were captured every 10° for 0°–90° and 360°–270°. The baseline drift was assessed with real‐time monitoring of the motionless phantom over 20 min. The gated‐delivered‐dose equivalency was assessed using the electron portal imaging device and gamma test (1%/1mm) in reference to non‐gated delivery.ResultsThe maximum couch‐angle dependency error occurs in longitudinal and lateral directions and is reduced significantly (P < 0.05) from 1‐level (1.3 ± 0.4 mm) to 2‐level (0.8 ± 0.3 mm) calibration. The MV cube calibration does not further reduce the couch‐angle dependency error (0.8 ± 0.2 mm) on average. The baseline drift error plateaus at 0.3 ± 0.1 mm after 10 min. The gated‐delivered‐dose equivalency has a >98% gamma‐test passing rate.ConclusionA commissioning method is recommended using the 3D plate calibration, which is verified by radiation isocenter and validated with couch‐angle dependency, baseline drift, and gated‐delivered‐dose equivalency tests. This method characterizes OSI uncertainties, ensuring motion‐monitoring accuracy for SRS treatments.

Targeting Accuracy Considerations for Simultaneous Tumor Treating Fields Antimitotic Therapy During Robotic Hypofractionated Radiation Therapy.

Purpose: Tumor treating fields (TTFields) is a novel antimitotic treatment that was first proven effective for glioblastoma multiforme, now with trials for several extracranial indications underway. Several studies focused on concurrent TTFields therapy with radiation in the same time period, but were not given simultaneously. This study evaluates the targeting accuracy of simultaneous radiation therapy while TTFields arrays are in place and powered on, ensuring that radiation does not interfere with TTFields and TTFields does not interfere with radiation. This is one of several options to enable TTFields to begin several weeks sooner, and opens potential for synergistic effects of combined therapy. Methods: TTFields arrays were attached to a warm saline water bath and salt was added until the TTFields generator reached the maximal 2000 mA peak-to-peak current. A ball cube phantom containing 2 orthogonal films surrounded by fiducials was placed in the water phantom, CT scanned, and a radiation treatment plan with 58 isocentric beams was created using a 3 cm circular collimator. Fiducial tracking was used to deliver radiation, the films were scanned, and end-to-end targeting error was measured with vendor-supplied software. In addition, radiation effects on electric fields generated by the TTFields system were assessed by examining logfiles generated from the field generator. Results: With TTFields arrays in place and powered on, the robotic radiosurgery system achieved a final targeting result of 0.47 mm, which was well within the submillimeter specification. No discernible effects on TTFields current output beyond 0.3% were observed in the logfiles when the radiation beam pulsed on and off. Conclusion: A robotic radiosurgery system was used to verify that radiation targeting was not adversely affected when the TTFields arrays were in place and the TTFields delivery device was powered on. In addition, this study verified that radiation delivered simultaneously with TTFields did not interfere with the generation of the electric fields.

Stability assessment of radiation isocenter with the gimbaled linac system

Purpose: We report the results of our year-long radiation isocenter accuracy verification for daily quality assurance (QA) implementation on a Vero4DRT system. Methods: The radiation isocenter was calculated using a cube phantom with a steel ball of diameter 10 mm fixed to the center of the phantom. A single photon beam was set with a field size of 100 × 100 mm 2 . Coincidence of the centroid of the steel ball at kiloVolt X-ray imaging isocenter and megaVolt beam radiation isocenter at each gantry and ring angle was tested. This procedure was performed for gantry angles of 0°, 90°, 180°, and 270°, and ring angles of 0°, 20°, and 340°. The centroid of the steel ball and the center of the radiation field were calculated to analyze the radiation isocenter error. This analysis was automatically calculated using the Daily Check tool in the Vero4DRT system. This QA was implemented between 24 August 2015 and 23 August 2016. Results: The average and standard deviation for pan and tilt directions were 0.12 ± 0.10 mm and -0.20 ± 0.13 mm, respectively. The maximum radiation isocenter accuracy error was 0.50 mm in both directions. Conclusion: The radiation isocenter alignment for the one year duration of the experiment was performed with high accuracy.

Commissioning of applicator-guided stereotactic body radiation therapy boost with high-dose-rate brachytherapy for advanced cervical cancer using radiochromic film dosimetry

PurposeTo describe an EBT3 GAFCHROMIC film-based dosimetry method to be used in commissioning of a combined HDR brachytherapy (HDRB) and stereotactic body radiation therapy (SBRT) boost for treatment of advanced cervical cancer involving extensive residual disease after external beam treatment. Methods and MaterialsA cube phantom was designed to firmly fit an intrauterine tandem applicator and EBT3 radiochromic film pieces. A high-risk clinical target volume (CTVHR, Total) was contoured with an extended arm at one side. The HDRB treatment was planned to cover the proximal CTVHR, Total with 7 Gy and the distal volume, referred to as CTVHR, Distal, was planned by SBRT for dose augmentation. After HDRB treatment delivery, SBRT treatment was delivered within 1 hour by image guidance using the applicator geometry. Intentional 1D and 2D misalignments were introduced to evaluate the effect on target volumes. In addition, effect of film reirradiation at different time gaps and dose levels was evaluated. ResultsFilm dosimetric accuracy, with up to 2 hours gap between irradiations, was shown to be unaffected. A 2%/2 mm gamma analysis between measured and planned doses showed agreement of >99%. Misalignments of more than 2 mm between applicator and SBRT isocenter resulted in suboptimal dose-volume histogram affecting mostly D98% and D90% of CTVHR, Distal. ConclusionsVisualizing how target dose-volume metrics are affected by minor misalignments between SBRT and HDRB dose gradients, in light of achievable phantom-based experimental quality assurance level, encourages the clinical applicability of this technique. Radiochromic film was shown to be a valuable tool to commission procedures combining two different treatment planning systems and modalities with varying dose rates and energy ranges.