Rando Head Research Articles

Evaluation of a head rest prototype for rotational corrections in three degrees of freedom.

Cranial stereotactic irradiations require accurate reproduction of the planning CT patient position at the time of treatment, including removal of rotational offsets. A device prototype was evaluated for potential clinical use to correct rotational positional offsets in image-guided radiotherapy workflow. Analysis was carried out with a prototype device "RPS head" by gKteso GmbH, rotatable up to 4° in three dimensions by hand wheels. A software tool accounts for the nonrectangular rotation axes and also indicates translational motions to be performed with the standard couch to correct the initial offset and translational shifts introduced by the rotational motion. The accuracy of angular corrections and positioning of an Alderson RANDO head phantom using the prototype device was evaluated for nine treatment plans for cranial targets. Corrections were obtained from cone beam computed tomography (CBCT) imaging. The phantom position was adjusted and the final position was then verified by another CBCT. The long-term stability of the prototype device was evaluated. Attenuation by the device along its three main axes was assessed. A planning study was performed to evaluate if regions of high-density material can be avoided during plan generation. The device enabled the accurate correction of rotational offsets in a clinical setup with a mean residual angular difference of (0.0±0.1)° and a maximum deviation of 0.2°. Translational offsets were less than 1mm. The device was stable over a period of 20 min, not changing the head support plate position by more than (0.7±0.6) mm. The device contains high-density material in the adjustment mechanism and slightly higher density in the support structures. These can be avoided during planning generation maintaining comparable plan quality. The head positioning device can be used to correct rotational offsets in a clinical setting.

Reduction of Radiation Dose to Eye Lens in Cerebral 3D Rotational Angiography Using Head Off-Centering by Table Height Adjustment: A Prospective Study.

Three-dimensional rotational angiography (3D-RA) is increasingly used for the evaluation of intracranial aneurysms (IAs); however, radiation exposure to the lens is a concern. We investigated the effect of head off-centering by adjusting table height on the lens dose during 3D-RA and its feasibility in patient examination. The effect of head off-centering during 3D-RA on the lens radiation dose at various table heights was investigated using a RANDO head phantom (Alderson Research Labs). We prospectively enrolled 20 patients (58.0 ± 9.4 years) with IAs who were scheduled to undergo bilateral 3D-RA. In all patients' 3D-RA, the lens dose-reduction protocol involving elevation of the examination table was applied to one internal carotid artery, and the conventional protocol was applied to the other. The lens dose was measured using photoluminescent glass dosimeters (GD-352M, AGC Techno Glass Co., LTD), and radiation dose metrics were compared between the two protocols. Image quality was quantitatively analyzed using source images for image noise, signal-to-noise ratio, and contrast-to-noise ratio. Additionally, three reviewers qualitatively assessed the image quality using a five-point Likert scale. The phantom study showed that the lens dose was reduced by an average of 38% per 1 cm increase in table height. In the patient study, the dose-reduction protocol (elevating the table height by an average of 2.3 cm) led to an 83% reduction in the median dose from 4.65 mGy to 0.79 mGy (P < 0.001). There were no significant differences between dose-reduction and conventional protocols in the kerma area product (7.34 vs. 7.40 Gy·cm², P = 0.892), air kerma (75.7 vs. 75.1 mGy, P = 0.872), and image quality. The lens radiation dose was significantly affected by table height adjustment during 3D-RA. Intentional head off-centering by elevation of the table is a simple and effective way to reduce the lens dose in clinical practice.

HyperArc VMAT SRT for Locally Recurrent Previously-Irradiated Head and Neck Cancers: Plan Quality, Treatment Delivery Accuracy and Efficiency

To demonstrate the feasibility and efficacy of HyperArc VMAT treatments for locally recurrent, locally advanced or previously irradiated head and neck cancers treated with stereotactic radiotherapy (SRT).First, an anthropomorphic head phantom from the MD Anderson's IROC credentialing laboratory containing a 1.9 cm diameter spherical target and in vivo-dosimetry system was imaged, planned and irradiated (25 Gy in 1 fraction) using HyperArc VMAT module with 10MV-FFF beam. Second, RANDO head phantom was imaged, planned and irradiated (35 Gy in 5 fractions) by generating eight HyperArc VMAT plans (4 right, 4 left neck tumors) at different anatomical locations (C1-C4). Average tumor volume was 21.7 cc (13.0-32.3 cc). Distance to isocenter from the central marker of the Encompass device down to neck was 25.8 cm (23.8-28.0 cm) and 24.3 cm (22.0-27.1 cm) for the left and right neck tumors and 9 cm (in) from the both lateral markers that defined the patient protection zone. Third, two head and neck cancer patients with recurrent large mass of 56 cc (4.8 cm, diameter) on right maxillary sinus and 159 cc (6.7 cm, diameter) on right ethmoid sinus were imaged, planned and treated (35 Gy in 5 fractions) with HyperArc SRT. Plan quality, treatment delivery accuracy and efficiency were reported.Phantom irradiation results met all the compliance criteria set forth by the IROC for HyperArc SRS. For end-to-end tests, the average conformity index, Paddick conformation number, gradient and heterogeneity indices, and dose to 2 cm away from the target were 1.08 ± 0.05 (1.02-1.14), 0.85 ± 0.02 (0.83-0.89), 2.71 ± 0.27 (2.28-2.92), 0.14 ± 0.02 (0.11-0.17) and 23.5 ± 1.5% (21.4-25.0%). Average maximal dose to skin and spinal cord was 12.8 ± 3.5 Gy (9.2-16.7 Gy) and 4.6 ± 0.8 Gy (4.0-5.6 Gy). Mean values of modulation factor, beam on time and treatment time was 2.96 ± 0.5 (2.39-3.78), 3.1 ± 0.2 min (2.67-3.78 min), and 13.96 ± 0.50 min (13.39-14.78 min) with no collision issue. Pretreatment portal dosimetry quality assurance results demonstrated a high pass rate of 99.1 ± 1.6% for clinical gamma passing criteria of 2%/2 mm. Similar results were found on the patient's plans including highly conformal target coverage, sharp dose fall-off (50% dose fall-off within 5 mm from the target) and sparing adjacent critical organs including spinal cord (< 5 Gy). Perfect pitch couch correction average was < 1.5 mm and 2o. Beam on time was about 3.02 min, and patient's treatment completed within 15 min treatment slot.For recurrent head and neck SRT treatments, HyperArc VMAT provided highly conformal dose distribution, rapid dose fall-off, excellent sparing of adjacent critical organs, highly precise and accurate treatment that could be delivered to as low as C4 level. It could potentially allow HyperArc SRT to glomus tumors to those patients who may not tolerate frame-based SRS. Clinical follow up of these patients is ongoing to confirm the therapeutic benefits of this novel and attractive head and neck SRT.D. Pokhrel: None. M.E. Bernard: None. D. Lockhart: None. J. Jeremiah: None. D. Fabian: None. M.R. Kudrimoti: None.

Set-up error validation with EPID images: Measurements vs Egs_cbct simulation.

In this study, the egs_cbct code's ability to replicate an electronic portal imaging device (EPID) is explored. We have investigated head and neck (H&N) setup verification on an Elekta Precise linear accelerator. It is equipped with an electronic portal imaging device (EPID) that can capture a set of projection images over different gantry angles. Cone-beam computed tomography (CBCT) images were reconstructed from projection images of two different setup scenarios. Projections of an Anthropomorphic Rando head phantom were also simulated by using the egs_cbct Monte Carlo code for comparison with the measured projections.Afterwards, CBCT images were reconstructed from this data. Image quality was evaluated against a metric defined as the image acquisition interval (IAI). It determines the number of projection images to be used for CBCT image reconstruction. From this results it was established that phantom shifts could be determined within 2 mm and rotations within one degree accuracy using only 20 projection images (IAI = 10 degrees). Similar results were obtained with the simulated data. In this study it is demonstrated that a head and neck setup can be verified using substantially fewer projection images. Bony landmarks and air cavities could still be observed in the reconstructed Rando head phantom. The egs_cbct code can be used as a tool to investigate setup errors without tedious measurements with an EPID system.

150. Dosimetric verification of stereotactic head treatments performed with Helical Tomotherapy using alanine dosimeters and gafchromic films

PurposeThe aim of this work is the dosimetric verification of stereotactic head treatments performed with Helical Tomotherapy [1] using an anthropomorphic phantom, the Rando Head Alderson, combined with alanine [2] dosimeters and EBT3 gafchromic films [3]. MethodsA CT scan of Rando Head Phantom was used to simulate five different clinical situations regarding “tumor” dimension, position and proximity relations with Organs at Risk (OAR). All five treatment plans were computed using 1 cm beam width, pitch values lower than 0.215 and modulation factor between 1.6 and 2.5. In all cases, the dose constraints required for OAR have been met. During treatment delivery, the alanine dosimeters were placed in the phantom housings to perform dose-specific measurements for either the PTV and OAR, and gafchromic films were inserted between contiguous slices of the phantom to achieve bidimensional dose distributions on the transverse plane. ResultsDose distributions measured by gafchromic films were compared to those calculated using the 3% – 3 mm criterion for the gamma function using the Tomotherapy DQA module. Also the results of point measurements made with alanine agree with the dose values calculated by TPS within the experimental uncertainties. ConclusionsThe data show an excellent agreement between calculated and measured dose not only for PTV but also for the OAR, receiving significantly lower doses.

Comparison of manual and dose reduction modes of a MORITA R100 CBCT.

To compare effective dose differences when acquiring (1) dose reduction mode and (2) manual mode in a MORITA R100 CBCT. 24 exposure protocols with different technique factors were performed in both the dose reduction mode and the manual mode in a Veraviewepocs 3D R100 cone beam CT device. 48 TLD were placed in a RANDO head phantom at 24 different sites. Effective doses were calculated according to the formalism published in the 103rd ICRP report. Effective doses for the dose reduction mode protocols ranged from 14 to 156 µSv [mean = 57 µSv, standard deviation (SD) = 37 µSv], whereas effective doses for the manual mode protocols ranged from 22 to 267 µSv (mean = 94 µSv, SD = 65 µSv). Thus, across all protocols, the dose reduction mode leads to a drop of the effective dose by 38 % (SD = 6 %). The estimated effective doses are significantly lower if dose reduction protocols are acquired.

Commissioning an in-room mobile CT for adaptive proton therapy with a compact proton system.

PurposeTo describe the commissioning of AIRO mobile CT system (AIRO) for adaptive proton therapy on a compact double scattering proton therapy system.MethodsA Gammex phantom was scanned with varying plug patterns, table heights, and mAs on a CT simulator (CT Sim) and on the AIRO. AIRO‐specific CT‐stopping power ratio (SPR) curves were created with a commonly used stoichiometric method using the Gammex phantom. A RANDO anthropomorphic thorax, pelvis, and head phantom, and a CIRS thorax and head phantom were scanned on the CT Sim and AIRO. Clinically realistic treatment plans and nonclinical plans were generated on the CT Sim images and subsequently copied onto the AIRO CT scans for dose recalculation and comparison for various AIRO SPR curves. Gamma analysis was used to evaluate dosimetric deviation between both plans.Results AIRO CT values skewed toward solid water when plugs were scanned surrounded by other plugs in phantom. Low‐density materials demonstrated largest differences. Dose calculated on AIRO CT scans with stoichiometric‐based SPR curves produced over‐ranged proton beams when large volumes of low‐density material were in the path of the beam. To create equivalent dose distributions on both data sets, the AIRO SPR curve's low‐density data points were iteratively adjusted to yield better proton beam range agreement based on isodose lines. Comparison of the stoichiometric‐based AIRO SPR curve and the “dose‐adjusted” SPR curve showed slight improvement on gamma analysis between the treatment plan and the AIRO plan for single‐field plans at the 1%, 1 mm level, but did not affect clinical plans indicating that HU number differences between the CT Sim and AIRO did not affect dose calculations for robust clinical beam arrangements.ConclusionBased on this study, we believe the AIRO can be used offline for adaptive proton therapy on a compact double scattering proton therapy system.

Experimental Assessment of Proton Dose Calculation Accuracy in Small-Field Delivery Using a Mevion S250 Proton Therapy System.

Purpose:Dose calculation accuracy of the Varian Eclipse treatment planning system (TPS) is empirically assessed for small-aperture fields using a Mevion S250 double scattering proton therapy system.Materials and Methods:Five spherical pseudotumors were modeled in a RANDO head phantom. Plans were generated for the targets with apertures of 1, 2, 3, 4, or 5 cm diameter using one, two, and three beams. Depth-dose curves and lateral profiles of the beams were taken with the planned blocks and compared to Eclipse calculations. Dose distributions measured with EBT3 films in the phantom were also compared to Eclipse calculations. Film quenching effect was simulated and considered.Results:Depth-dose scans in water showed a range pullback (up to 2.0 mm), modulation widening (up to 9.5 mm), and dose escalation in proximal end and sub-peak region (up to 15.5%) when compared to the Eclipse calculations for small fields. Measured full width at half maximum and penumbrae for lateral profiles differed <1.0 mm from calculations for most comparisons. In the phantom study, Eclipse TPS underestimated sub-peak dose. Gamma passing rates improved with each beam added to the plans. Greater range pullback and modulation degradation versus water scans were observed due to film quenching, which became more noticeable as target size increased.Conclusions:Eclipse TPS generates acceptable target coverage for small targets with carefully arranged multiple beams despite relatively large dose discrepancy for each beam. Surface doses higher than Eclipse calculations can be mitigated with multiple beams. When using EBT3 film, the quenching effect should be considered.

Design of a head phantom produced on a 3D rapid prototyping printer and comparison with a RANDO and 3M lucite head phantom in eye dosimetry applications

An anthropomorphic head phantom including eye inserts allowing placement of TLDs 3 mm below the cornea has been produced on a 3D printer using a photo-cured acrylic resin to best allow tissue equivalence. Thus Hp(3) can be determined in radiological and interventional photon radiation fields. Eye doses and doses to the forehead have been compared to an Alderson RANDO head and a 3M Lucite skull phantom in terms of surface dose per incident air kerma for frontal irradiation since the commercial phantoms do not allow placement of TLDs 3 mm below the corneal surface. A comparison of dose reduction factors (DRFs) of a common lead glasses model has also been performed. Eye dose per incident air kerma were comparable between all three phantoms (printed phantom: 1.40, standard error (SE) 0.04; RANDO: 1.36, SE 0.03; 3M: 1.37, SE 0.03). Doses to the forehead were identical to eye surface doses for the printed phantom and the RANDO head (ratio 1.00 SE 0.04, and 0.99 SE 0.03, respectively). In the 3M Lucite skull phantom dose on the forehead was 15% lower than dose to the eyes attributable to phantom properties. DRF of a sport frame style leaded glasses model with 0.75 mm lead equivalence measured were 6.8 SE 0.5, 9.3 SE 0.4 and 10.5 SE 0.5 for the RANDO head, the printed phantom, and the 3M Lucite head phantom, respectively, for frontal irradiation. A comparison of doses measured in 3 mm depth and on the surface of the eyes in the printed phantom revealed no difference larger than standard errors from TLD dosimetry. 3D printing offers an interesting opportunity for phantom design with increasing potential as printers allowing combinations of tissue substitutes will become available. Variations between phantoms may provide a useful indication of uncertainty budgets when using phantom measurements to estimate individual personnel doses.

Comparison of Online 6 Degree-of-Freedom Image Registration of Varian TrueBeam Cone-Beam CT and BrainLab ExacTrac X-Ray for Intracranial Radiosurgery.

Purpose:The study was aimed to compare online 6 degree-of-freedom image registrations of TrueBeam cone-beam computed tomography and BrainLab ExacTrac X-ray imaging systems for intracranial radiosurgery.Methods:Phantom and patient studies were performed on a Varian TrueBeam STx linear accelerator (version 2.5), which is integrated with a BrainLab ExacTrac imaging system (version 6.1.1). The phantom study was based on a Rando head phantom and was designed to evaluate isocenter location dependence of the image registrations. Ten isocenters at various locations representing clinical treatment sites were selected in the phantom. Cone-beam computed tomography and ExacTrac X-ray images were taken when the phantom was located at each isocenter. The patient study included 34 patients. Cone-beam computed tomography and ExacTrac X-ray images were taken at each patient’s treatment position. The 6 degree-of-freedom image registrations were performed on cone-beam computed tomography and ExacTrac, and residual errors calculated from cone-beam computed tomography and ExacTrac were compared.Results:In the phantom study, the average residual error differences (absolute values) between cone-beam computed tomography and ExacTrac image registrations were 0.17 ± 0.11 mm, 0.36 ± 0.20 mm, and 0.25 ± 0.11 mm in the vertical, longitudinal, and lateral directions, respectively. The average residual error differences in the rotation, roll, and pitch were 0.34° ± 0.08°, 0.13° ± 0.09°, and 0.12° ± 0.10°, respectively. In the patient study, the average residual error differences in the vertical, longitudinal, and lateral directions were 0.20 ± 0.16 mm, 0.30 ± 0.18 mm, 0.21 ± 0.18 mm, respectively. The average residual error differences in the rotation, roll, and pitch were 0.40°± 0.16°, 0.17° ± 0.13°, and 0.20° ± 0.14°, respectively. Overall, the average residual error differences were <0.4 mm in the translational directions and <0.5° in the rotational directions. ExacTrac X-ray image registration is comparable to TrueBeam cone-beam computed tomography image registration in intracranial treatments.

SU‐D‐BRA‐03: Analysis of Systematic Errors with 2D/3D Image Registration for Target Localization and Treatment Delivery in Stereotactic Radiosurgery

Purpose:Determine systematic deviations between 2D/3D and 3D/3D image registrations with six degrees of freedom (6DOF) for various imaging modalities and registration algorithms on the Varian Edge Linac.Methods:The 6DOF systematic errors were assessed by comparing automated 2D/3D (kV/MV vs. CT) with 3D/3D (CBCT vs. CT) image registrations from different imaging pairs, CT slice thicknesses, couch angles, similarity measures, etc., using a Rando head and a pelvic phantom. The 2D/3D image registration accuracy was evaluated at different treatment sites (intra‐cranial and extra‐cranial) by statistically analyzing 2D/3D pre‐treatment verification against 3D/3D localization of 192 Stereotactic Radiosurgery/Stereotactic Body Radiation Therapy treatment fractions for 88 patients.Results:The systematic errors of 2D/3D image registration using kV‐kV, MV‐kV and MV‐MV image pairs using 0.8 mm slice thickness CT images were within 0.3 mm and 0.3° for translations and rotations with a 95% confidence interval (CI). No significant difference between 2D/3D and 3D/3D image registrations (P&gt;0.05) was observed for target localization at various CT slice thicknesses ranging from 0.8 to 3 mm. Couch angles (30, 45, 60 degree) did not impact the accuracy of 2D/3D image registration. Using pattern intensity with content image filtering was recommended for 2D/3D image registration to achieve the best accuracy. For the patient study, translational error was within 2 mm and rotational error was within 0.6 degrees in terms of 95% CI for 2D/3D image registration. For intra‐cranial sites, means and std. deviations of translational errors were −0.2±0.7, 0.04±0.5, 0.1±0.4 mm for LNG, LAT, VRT directions, respectively. For extra‐cranial sites, means and std. deviations of translational errors were ‐ 0.04±1, 0.2±1, 0.1±1 mm for LNG, LAT, VRT directions, respectively. 2D/3D image registration uncertainties for intra‐cranial and extra‐cranial sites were comparable.Conclusion:The Varian Edge radiosurgery 6DOF‐based system, can perform 2D/3D image registration with high accuracy for target localization in image‐guided stereotactic radiosurgery.The work was supported by a Research Scholar Grant, RSG‐15‐137‐01‐CCE from the American Cancer Society.

SU‐F‐J‐42: Comparison of Varian TrueBeam Cone‐Beam CT and BrainLab ExacTrac X‐Ray for Cranial Radiotherapy

Purpose:To compare online image registrations of TrueBeam cone‐beam CT (CBCT) and BrainLab ExacTrac x‐ray imaging systems for cranial radiotherapy.Method:Phantom and patient studies were performed on a Varian TrueBeam STx linear accelerator (Version 2.5), which is integrated with a BrainLab ExacTrac imaging system (Version 6.1.1). The phantom study was based on a Rando head phantom, which was designed to evaluate isocenter‐location dependence of the image registrations. Ten isocenters were selected at various locations in the phantom, which represented clinical treatment sites. CBCT and ExacTrac x‐ray images were taken when the phantom was located at each isocenter. The patient study included thirteen patients. CBCT and ExacTrac x‐ray images were taken at each patient's treatment position. Six‐dimensional image registrations were performed on CBCT and ExacTrac, and residual errors calculated from CBCT and ExacTrac were compared.Results:In the phantom study, the average residual‐error differences between CBCT and ExacTrac image registrations were: 0.16±0.10 mm, 0.35±0.20 mm, and 0.21±0.15 mm, in the vertical, longitudinal, and lateral directions, respectively. The average residual‐error differences in the rotation, roll, and pitch were: 0.36±0.11 degree, 0.14±0.10 degree, and 0.12±0.10 degree, respectively. In the patient study, the average residual‐error differences in the vertical, longitudinal, and lateral directions were: 0.13±0.13 mm, 0.37±0.21 mm, 0.22±0.17 mm, respectively. The average residual‐error differences in the rotation, roll, and pitch were: 0.30±0.10 degree, 0.18±0.11 degree, and 0.22±0.13 degree, respectively. Larger residual‐error differences (up to 0.79 mm) were observed in the longitudinal direction in the phantom and patient studies where isocenters were located in or close to frontal lobes, i.e., located superficially.Conclusion:Overall, the average residual‐error differences were within 0.4 mm in the translational directions and were within 0.4 degree in the rotational directions.

SU‐E‐J‐47: Comparison of Online Image Registrations of Varian TrueBeam Cone‐Beam CT and BrainLab ExacTrac Imaging Systems

PurposeTo compare online image registrations of TrueBeam cone‐beam CT (CBCT) and BrainLab ExacTrac imaging systems.MethodsTests were performed on a Varian TrueBeam STx linear accelerator (Version 2.0), which is integrated with a BrainLab ExacTrac imaging system (Version 6.0.5). The study was focused on comparing the online image registrations for translational shifts. A Rando head phantom was placed on treatment couch and immobilized with a BrainLab mask. The phantom was shifted by moving the couch translationally for 8 mm with a step size of 1 mm, in vertical, longitudinal, and lateral directions, respectively. At each location, the phantom was imaged with CBCT and ExacTrac x‐ray. CBCT images were registered with TrueBeam and ExacTrac online registration algorithms, respectively. And ExacTrac x‐ray image registrations were performed. Shifts calculated from different registrations were compared with nominal couch shifts.ResultsThe averages and ranges of absolute differences between couch shifts and calculated phantom shifts obtained from ExacTrac x‐ray registration, ExacTrac CBCT registration with default window, ExaxTrac CBCT registration with adjusted window (bone), Truebeam CBCT registration with bone window, and Truebeam CBCT registration with soft tissue window, were: 0.07 (0.02–0.14), 0.14 (0.01–0.35), 0.12 (0.02–0.28), 0.09 (0–0.20), and 0.06 (0–0.10) mm, in vertical direction; 0.06 (0.01–0.12), 0.27 (0.07–0.57), 0.23 (0.02–0.48), 0.04 (0–0.10), and 0.08 (0– 0.20) mm, in longitudinal direction; 0.05 (0.01–0.21), 0.35 (0.14–0.80), 0.25 (0.01–0.56), 0.19 (0–0.40), and 0.20 (0–0.40) mm, in lateral direction.ConclusionThe shifts calculated from ExacTrac x‐ray and TrueBeam CBCT registrations were close to each other (the differences between were less than 0.40 mm in any direction), and had better agreements with couch shifts than those from ExacTrac CBCT registrations. There were no significant differences between TrueBeam CBCT registrations using different windows. In ExacTrac CBCT registrations, using bone window led to better agreements than using default window.

SU‐E‐J‐30: The Effect of Planning CT Slice Thickness On Evaluation of SRS Target Localization Accuracy

Purpose:To evaluate the effect of planning CT slice thickness on SRS target localization accuracy on Varian Edge SRS treatment platform.Methods:A metal BB of diameter 5 mm was inserted in a Rando head phantom. Planning CTs were scanned with 0.8, 2 and 3 mm slice thickness respectively. The center of BB was setup as isocenter and tattoos were marked on phantom surface. The phantom was initially aligned using laser and tattoos, then a CBCT was acquired and 6‐degree of freedom (DOF) image registration was performed based on bony anatomy. The 6D couch correction was applied. A verification CBCT was taken to evaluate couch corrections. Three orthogonal pairs of images (MV‐MV, KV‐KV and MVKV), were also taken to verify the position of the phantom using a 2D‐3D registration algorithm. Winston‐Lutz tests were taken at 12 combinations of gantry, collimator and couch angles using 1×1cm MLC aperture and 10mm circular cone.Results:The vector lengths of the residual error measured with verification CBCT were 0.3, 0.2 and 0.2 mm for the planning CT of 0.8, 2 and 3 mm slice thicknesses respectively. Residual setup error vector lengths for 0.8, 2, and 3 mm CT measured by orthogonal pair were as following: a) MV‐MV: 0.2, 0.5, 0.5mm; b) KV‐KV: 0.2, 0.3, 0.3mm; c)MV‐KV: 0.1, 0.6, 0.3mm respectively. The vector lengths for the Winston‐Lutz tests were: a) MLC: 0.55, 0.32, and 0.2mm; b) CONE: 0.16, 0.17, and 0.29mm for slice thickness of 0.8, 2, and 3mm respectively.Conclusion:The residual setup error in this study measured by the 2D‐3D algorithm was comparable to that of the 3D‐3D algorithm. The planning CT slice thickness in the range of 0.8 to 2 mm has no significant impact on the evaluation of the residual setup error and the Result of Winston‐Lutz test.

An Investigation of the Feasibility and Utility of a Low-dose Cone-beam Computed Tomography Scan Protocol for Head and Neck Cancer Patients

IntroductionRoutine use of cone-beam computed tomography (CBCT) scan protocols as part of the image guidance process (image-guided radiation therapy) has become an integral part of the practice of radiation therapists (RTs). Concerns regarding imaging dose as well as increased in-room time for patients led to reluctance among site group members to adopt CBCT for all radical head and neck cancer (HNC) patients at our institution. This investigation set out to assess the feasibility and utility of a revised CBCT scan protocol with the aim of supporting daily CBCT for HNC patients receiving radiation therapy. MethodsThe project was performed in three phases. Phase 1 involved the experimental adjustment of CBCT scan protocol parameters in clinical use for HNC patients at our institution. An Elekta Synergy linear accelerator with kilovoltage CBCT capability and a RANDO head phantom were used for scan acquisition procedures. Image registration using bony anatomy was performed on two image sets generated using the current clinical scan protocol (HNS20) and an experimental modified scan protocol (MHNS20). Image registration results were compared by two investigators. Measurements of scan doses using a metal–oxide–semiconductor field-effect transistor and a Unidose meter were performed. Catphan phantom images were acquired using HNS20 and MHNS20 protocols. In phase 2, ten volunteer RTs performed image registration and matching processes on two image sets performed using HNS20 and MHNS20 protocols. RTs were unaware of the scan protocols used for image acquisition. A threshold of 3 mm was set (the current maximum couch shift allowance in the clinical HNC IGRT protocol) to compare the image registration data from HNS20 and MHNS20. In phase 3, after research ethics board approval, 10 HNC patients consented to the study. Two pretreatment CBCT scans were performed: scan 1 was acquired using MHNS20 protocol, and scan 2 was acquired using the HNS20 protocol. A threshold of 2 mm was set to compare the differences in couch shift data resulting from the image registration of the two image sets. Comparison of HNS20 and MHNS20 based on image registration results was performed. ResultsIn phase 1, radiation doses measured by the investigators on the left optical lens using a metal–oxide–semiconductor field-effect transistor and a Unidose meter indicated that the MHNS20 protocol would result in a lower dose to the left optical lens. In phase 2, shifts of the treatment table to achieve the planned isocentre, which were recorded after the image matching process, were within 3 mm in 80% of the RT procedures. In the y-axis (superior/inferior direction), 100% of the procedures were within 3 mm. In the z-axis (anterior/posterior) and x-axis (lateral), 90% of the procedures were within 3 mm. Qualitative data from a questionnaire completed by RTs after the image matching indicated that 50% of the RTs had no preference between the images sets in terms of visibility of structures. Forty percent of RTs had no preference regarding speed of matching or preference for registration between the image sets. When a preference was indicated, the HNS20 scan protocol was chosen by the RTs. In phase 3, couch shift data recorded after each CBCT scan were compared. All results in all three planes for 10 patients included in the study were within the 2-mm threshold. ConclusionsThe feasibility and clinical utility of a potential lower-dose CBCT scan protocol has been investigated. The modified protocol (MHNS20) produced image data acceptable within current practice using bony anatomy for registration purposes. The MHNS20 protocol also delivered lower doses to the left optical lens and therefore potentially to other pertinent structures. The actual delivered doses to patients during IGRT procedures using the MHNS20 may be different than those measured during this investigation.

Assessment of the effective dose in supine, prone, and oblique positions in the maxillofacial region using a novel combined extremity and maxillofacial cone beam computed tomography scanner.

The objectives of this study were to assess the organ and effective doses (International Commission on Radiological Protection [ICRP] 103 standard) resulting from supine, prone, and oblique phantom positions in the maxillofacial region using a novel cone beam computed tomography (CBCT) device and to compare the results with conventional dental CBCT and multislice computed tomography (MSCT) devices. Measurements were performed using an anthropomorphic RANDO head phantom (Radiation Analogue Dosimetry System) with 20 MOSFET (metal-oxide-semiconductor field-effect transistor) dosimeters placed in the most radiosensitive maxillofacial organs. Effective doses were measured in 3 phantom positions using a combined extremity and maxillofacial CBCT device: the Planmed Verity CBCT scanner. Reference values were measured in the upright position with 2 CBCT devices and in the prone position with one MSCT scanner. The Planmed Verity CBCT scanner effective doses were 247 μSv in supine, 192 μSv in prone, and 134 μSv in oblique position. The effective dose with ProMax 3D MAX CBCT was 168 μSv; with i-CAT Next Generation, 170 μSv; and with Philips Brilliance 64 MSCT, 781 μSv. Head positioning has an important effect on the organ and effective doses. The Planmed Verity CBCT scanner effective dose results were comparable with those attained on 2 conventional CBCT devices and were considerably lower than the MSCT scanner results.

SU‐E‐J‐123: Targeting Accuracy of Image‐Guided Radiosurgery for Intracranial Lesions

Purpose:To evaluate the setup accuracies of image‐guided intracranial radiosurgery across several different linear accelerator platforms.Methods:A CT scan with a slice thickness of 1.0 mm was acquired of a Rando head phantom (The Phantom Laboratory) in a U‐frame mask (BrainLAB AG). The phantom had three embedded BBs, simulating a central, left, and anterior lesion. The phantom was setup with each BB placed at the radiation isocenter under image guidance. Four different setup procedures were investigated: (1) NTX_ExacTrac: 6 degree‐of‐freedom (6D) correction on a Novalis Tx (BrainLAB AG) with ExacTrac localization (BrainLAB AG); (2) NTX_CBCT: 4D correction on the Novalis Tx with cone‐beam computed tomography (CBCT); (3) TrueBeam_CBCT: 4D correction on a TrueBeam (Varian) with CBCT; (4) Edge_CBCT: 6D correction on an Edge (Varian) with CBCT. The experiment was repeated 5 times with different initial setup error at each BB location on each platform, and the mean (μ) and one standard deviation (σ) of the residual error was compared. The congruence between radiation and imaging isocenters on each platform was evaluated by acquiring Winston Lutz (WL) images of a WL jig followed by imaging using ExacTrac or CBCT. The difference in coordinates of the jig relative to radiation and imaging isocenters was then recorded.Results:Averaged over all three BB locations, the residual vector setup errors (μ±σ) of the phantom in mm were 0.6±0.2, 1.0±0.5, 0.2±0.1, and 0.3±0.1 on NTX_ExacTrac, NTX_CBCT, TrueBeam_CBCT, and Edge_CBCT, with their ranges in mm being 0.4∼1.1, 0.4∼1.9, 0.1∼0.5, and 0.2∼0.6, respectively. And imaging isocenter was found stable relative to radiation isocenter, with the congruence to radiation isocenter in mm being 0.6±0.1, 0.7±0.1, 0.3±0.1, 0.2±0.1, respectively, on the four systems in the same order.Conclusion:Millimeter accuracy can be achieved with image‐guided radiosurgery for intracranial lesions based on this set of experiments.

Phantom based KV-CBCT dose plan calculations for re-planning decision: Comparison with CT-based dose plans

Objective Kilovoltage cone-beam computed tomography scans (kVCBCT) are helpful for patients positioning and viewing anatomical changes. This study investigates, on an anthropomorphic phantom, with no movement and anatomical changes, the possibility and accuracy of using CBCT-based dosimetry for re-planning decision. Materials and method Rando phantom's brain, Head and Neck (H&N), thorax and pelvis were scanned. Targets and main organ at risks (OAR) were contoured. IMRT was used for H&N and VMAT for brain, lung and pelvis. Dose distributions were calculated using Eclipse 10.0 AAA Algorithm (Varian Medical System). CBCT scans were acquired per region on a Novalis Tx. After registration with the reference CT, structures were copied and dosimetric plans were recalculated on the CBCT. The dose volume histograms (HVH) for targets and OAR and 2D dose maps were compared. Furthermore the Hounsfield units to electron density (HU to ED) curves for the different CBCT protocols were obtained using a Catphan® 504 phantom. To end examples of patients with anatomical changes are presented. Results PTV and OAR average mean dose differences for the brain, H&N, lungs and pelvic region were −1.1 ± 0.7%, −0.6 ± 0.4%, −0.9 ± 1.0% and 0.1 ± 0.5% respectively. Passing rate superior than 98% was achieved (gamma index criteria of 2%/1 mm) for absolute 2D CBCT and CT-based dose distributions. CBCT HU to ED curves were within ±30 HU compared with the reference HU of the materials contained inside phantom, amply inside requested CT tolerance values. Conclusion Decision to re-plan VMAT or IMRT treatments seems feasible using CBCT data. Future investigations on the HU to ED curve and curve stability will be done with the help of a CBCT electron density phantom (CIRS).

Effective dose span of ten different cone beam CT devices

Evaluation and reduction of dose are important issues. Since cone beam CT (CBCT) has been established now not just in dentistry, the number of acquired examinations continues to rise. Unfortunately, it is very difficult to compare the doses of available devices on the market owing to different exposition parameters, volumes and geometries. The aim of this study was to evaluate the spans of effective doses (EDs) of ten different CBCT devices. 48 thermoluminescent dosemeters were placed in 24 sites in a RANDO(®) head phantom. Protocols with lowest exposition parameters and protocols with highest exposition parameters were performed for each of the ten devices. The ED was calculated from the measured energy doses according to the International Commission on Radiological Protection 2007 recommendations for each protocol and device, and the statistical values were evaluated afterwards. The calculation of the ED resulted in values between 17.2 µSv and 396 µSv for the ten devices. The mean values for protocols with lowest and highest exposition parameters were 31.6 µSv and 209 µSv, respectively. It was not the aim of this study to evaluate the image quality depending on different exposition parameters but to define the spans of EDs in which different CBCT devices work. There is a wide span of ED for different CBCT devices depending on the selected exposition parameters, required spatial resolution and many other factors.

Fast polyenergetic forward projection for image formation using OpenCL on a heterogeneous parallel computing platform

Simulated projection images of digital phantoms constructed from CT scans have been widely used for clinical and research applications but their quality and computation speed are not optimal for real-time comparison with the radiography acquired with an x-ray source of different energies. In this paper, the authors performed polyenergetic forward projections using open computing language (OpenCL) in a parallel computing ecosystem consisting of CPU and general purpose graphics processing unit (GPGPU) for fast and realistic image formation. The proposed polyenergetic forward projection uses a lookup table containing the NIST published mass attenuation coefficients (μ∕ρ) for different tissue types and photon energies ranging from 1 keV to 20 MeV. The CT images of interested sites are first segmented into different tissue types based on the CT numbers and converted to a three-dimensional attenuation phantom by linking each voxel to the corresponding tissue type in the lookup table. The x-ray source can be a radioisotope or an x-ray generator with a known spectrum described as weight w(n) for energy bin E(n). The Siddon method is used to compute the x-ray transmission line integral for E(n) and the x-ray fluence is the weighted sum of the exponential of line integral for all energy bins with added Poisson noise. To validate this method, a digital head and neck phantom constructed from the CT scan of a Rando head phantom was segmented into three (air, gray∕white matter, and bone) regions for calculating the polyenergetic projection images for the Mohan 4 MV energy spectrum. To accelerate the calculation, the authors partitioned the workloads using the task parallelism and data parallelism and scheduled them in a parallel computing ecosystem consisting of CPU and GPGPU (NVIDIA Tesla C2050) using OpenCL only. The authors explored the task overlapping strategy and the sequential method for generating the first and subsequent DRRs. A dispatcher was designed to drive the high-degree parallelism of the task overlapping strategy. Numerical experiments were conducted to compare the performance of the OpenCL∕GPGPU-based implementation with the CPU-based implementation. The projection images were similar to typical portal images obtained with a 4 or 6 MV x-ray source. For a phantom size of 512 × 512 × 223, the time for calculating the line integrals for a 512 × 512 image panel was 16.2 ms on GPGPU for one energy bin in comparison to 8.83 s on CPU. The total computation time for generating one polyenergetic projection image of 512 × 512 was 0.3 s (141 s for CPU). The relative difference between the projection images obtained with the CPU-based and OpenCL∕GPGPU-based implementations was on the order of 10(-6) and was virtually indistinguishable. The task overlapping strategy was 5.84 and 1.16 times faster than the sequential method for the first and the subsequent digitally reconstruction radiographies, respectively. The authors have successfully built digital phantoms using anatomic CT images and NIST μ∕ρ tables for simulating realistic polyenergetic projection images and optimized the processing speed with parallel computing using GPGPU∕OpenCL-based implementation. The computation time was fast (0.3 s per projection image) enough for real-time IGRT (image-guided radiotherapy) applications.