Average Gamma Research Articles

Independent validation of a dedicated commissioning software and investigation of the direction dependence of the field symmetry for the LIAC intraoperative electron radiotherapy accelerator

Commissioning of the LIAC intraoperative electron radiotherapy accelerator relies heavily on the manufacturer's SWL-LIAC simulation software, which substantially reduces the number of difficult-to-set-up measurements needed. The aim of this study was to independently validate the accuracy of SWL-LIAC's calculated outputs, namely, percentage depth-doses (PDDs), off-axis dose profiles and relative output factors (ROFs), against experimental data. For all the available energies (6, 8, 10, 12 MeV), PDD measurements were made for all sizes of the flat-ended applicators (3–10 cm) and several beveled applicators. Profiles and ROFs were measured for the more frequently used applicators in clinical practice and the reference 10 cm diameter applicator. Meticulous measurements were made using a p-type electron diode in a water phantom, aided by inhouse custom-made positioning tools and confirmed using radiochromic film dosimetry. The measured and calculated scans were compared using multi-criteria gamma analysis. Eighty-five percent of the calculated ROFs showed a 2% or better agreement with the measurements. The average gamma passing rate for PDDs was 100% using a 2mm/1% (global dose difference) criterion. Repeated measurements showed that the measured in-plane profiles were somewhat tilted, which was confirmed by the manufacturer. The average gamma passing rate of all profiles was 95.5% (2mm/3%); Without considering in-plane profiles, this value was 97.6%. It can be concluded that the accuracy of the SWL-LIAC calculations may be deemed acceptable for most clinical applications. However, assuming isotropic off-axis profiles is unwarranted and measuring profiles in both directions for all energies at installation and acceptance testing is indicated.

Generation of clinical 177Lu SPECT/CT images based on Monte Carlo simulation with GATE.

Patient-specific dosimetry in MRT relies on quantitative imaging, pharmacokinetic assessment and absorbed dose calculation. The DosiTest project was initiated to evaluate the uncertainties associated with each step of the clinical dosimetry workflow through a virtual multicentric clinical trial. This work presents the generation of simulated clinical SPECT datasets based on GATE Monte Carlo modelling with its corresponding experimental CT image, which can subsequently be processed by commercial image workstations. This study considers a therapy cycle of 6.85GBq 177Lu-labelled DOTATATE derived from an IAEA-Coordinated Research Project (E23005) on "Dosimetry in Radiopharmaceutical therapy for personalised patient treatment". Patient images were acquired on a GE Infinia-Hawkeye 4 gamma camera using a medium energy (ME) collimator. Simulated SPECT projections were generated based on experimental time points and validated against experimental SPECT projections using flattened profiles and gamma index. The simulated projections were then incorporated into the patient SPECT/CT DICOM envelopes for processing and their reconstruction within a commercial image workstation. Gamma index passing rate (2% - 1 pixel criteria) between 95 and 98% and average gamma between 0.28 and 0.35 among different time points revealed high similarity between simulated and experimental images. Image reconstruction of the simulated projections was successful on HERMES and Xeleris workstations, a major step forward for the initiation of a multicentric virtual clinical dosimetry trial based on simulated SPECT/CT images. Realistic 177Lu patient SPECT projections were generated in GATE. These modelled datasets will be circulated to different clinical departments to perform dosimetry in order to assess the uncertainties in the entire dosimetric chain.

Development and performance assessment of an advanced Lucas-Kanade algorithm for dose mapping of cervical cancer external radiotherapy and brachytherapy plans.

PurposeThe aim of this study was to verify the possibility of summing the dose distributions of combined radiotherapeutic treatment of cervical cancer using the extended Lucas‐Kanade algorithm for deformable image registration.Materials and methodsFirst, a deformable registration of planning computed tomography images for the external radiotherapy and brachytherapy treatment of 10 patients with different parameter settings of the Lucas‐Kanade algorithm was performed. By evaluating the registered data using landmarks distance, root mean square error of Hounsfield units and 2D gamma analysis, the optimal parameter values were found. Next, with another group of 10 patients, the accuracy of the dose mapping of the optimized Lucas‐Kanade algorithm was assessed and compared with Horn‐Schunck and modified Demons algorithms using dose differences at landmarks.ResultsThe best results of the Lucas‐Kanade deformable registration were achieved for two pyramid levels in combination with a window size of 3 voxels. With this registration setting, the average landmarks distance was 2.35 mm, the RMSE was the smallest and the average gamma score reached a value of 86.7%. The mean dose difference at the landmarks after mapping the external radiotherapy and brachytherapy dose distributions was 1.33 Gy. A statistically significant difference was observed on comparing the Lucas‐Kanade method with the Horn‐Schunck and Demons algorithms, where after the deformable registration, the average difference in dose was 1.60 Gy (P‐value: 0.0055) and 1.69 Gy (P‐value: 0.0012), respectively.ConclusionLucas‐Kanade deformable registration can lead to a more accurate model of dose accumulation and provide a more realistic idea of the dose distribution.

Dosimetric analysis on the effect of tumor motion in IMRT for liver cancer: comparison of TomoTherapy and VMAT using the Delta4 Hexa-Motion system

The internal target volume (ITV)-based IMRT method can be used to reduce the effect of respiratory tumor motion. In this study, the effect of tumor motion on the dosimetric accuracy during ITV-based IMRT was investigated. ITV-based IMRT plans for a total of ten cases of liver cancer were prepared in TomoTherapy (Tomo) and volumetric modulated arc therapy (VMAT). Delivery quality assurance (DQA) plans were created using Delta4 phantom for the verification of dosimetric accuracy of the established Tomo and VMAT plans. DQA measurements were performed in both the static mode and the Hexa-Motion mode by using the Delta4 Hexa-Motion system in Tomo and VMAT. In the static mode, the average gamma passing rate (GPR) using a 3% dose difference/3 mm distance-to-agreement criteria was 99.61% in Tomo and 99.35% in VMAT, which confirmed sufficient dosimetric accuracy in both IMRT methods. For the Hexa-Motion measurement, the average GRP result of 90.93% in Tomo was higher than the result of 82.58% obtained in VMAT. In addition, the difference in the GPR results between the static and the Hexa-motion modes in Tomo was 8.68%, which is approximately 8% less than the 16.77% difference in VMAT. This study verifies that the dosimetric error of ITV-based IMRT due to the tumor motion was larger in VMAT than in Tomo. Also, Tomo can be considered to be more suitable than the conventional LINAC-based VMAT method in terms of the dosimetric accuracy of ITV-based IMRT for the treatment of a moving tumor.

Measurement of the CP-violating phase phi _s in B_{s}^{0} rightarrow J/psi phi decays in ATLAS at 13\xa0TeV

A measurement of the B_{s}^{0} rightarrow J/psi phi decay parameters using 80.5, mathrm {fb^{-1}} of integrated luminosity collected with the ATLAS detector from 13 text {Te}text {V} proton–proton collisions at the LHC is presented. The measured parameters include the CP-violating phase phi _{s} , the width difference Delta Gamma _{s} between the B_{s}^{0} meson mass eigenstates and the average decay width Gamma _{s}. The values measured for the physical parameters are combined with those from 19.2, mathrm {fb^{-1}} of 7 and 8 text {Te}text {V} data, leading to the following: ϕs=-0.087±0.036(stat.)±0.021(syst.)radΔΓs=0.0657±0.0043(stat.)±0.0037(syst.)ps-1Γs=0.6703±0.0014(stat.)±0.0018(syst.)ps-1\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$\\begin{aligned} \\phi _{s}= & {} -0.087 \\pm 0.036 ~\\mathrm {(stat.)} \\pm 0.021 ~\\mathrm {(syst.)~rad} \\\\ \\Delta \\Gamma _{s}= & {} 0.0657 \\pm 0.0043 ~\\mathrm {(stat.)}\\pm 0.0037 ~\\mathrm {(syst.)~ps}^{-1} \\\\ \\Gamma _{s}= & {} 0.6703 \\pm 0.0014 ~\\mathrm {(stat.)}\\pm 0.0018 ~\\mathrm {(syst.)~ps}^{-1} \\end{aligned}$$\\end{document}Results for phi _{s} and Delta Gamma _{s} are also presented as 68% confidence level contours in the phi _{s} – Delta Gamma _{s} plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. phi _{s} and Delta Gamma _{s} measurements are in agreement with the Standard Model predictions.

DETERMINATION OF RA-226, TH-232, K-40 AND CS-137 ACTIVITIES IN SOILS AND BEACH SANDS AND RELATED EXTERNAL GAMMA DOSES IN ARIKLI MINERALIZATION AREA (AYVACIK/TURKEY)

Uranium mineralization areas may jeopardize public health and surrounding natural life. In the current study for radioactivity and geochemical analyses, 43 soil, 5 rock and 10 beach sand samples were collected in and around Arıklı (Ayvacık, Turkey) uranium mineralization area. Radioactivity levels in the soils (up to Ra-226: 629, Th-232: 240, K-40: 3669Bq/kg) were found higher than the that of beach sands. Besides, radioactivity of the beach sand samples is found at normal levels (avg: Ra-226: 31, Th-232: 31, K-40: 542Bq/kg) when compared to worldwide average soil. Dose values indicate that the region have ~three times higher average gamma radioactivity than the corresponding world average although measured gamma doses ranged up to 687nGy/h. The high measured values are related to uranium mineralization, alteration zones, faults and Arıklı tuff.

Radiation dose calculation in 3D heterogeneous media using artificial neural networks.

External beam radiotherapy (EBRT) treatment planning requires a fast and accurate method of calculating the dose delivered by a clinical treatment plan. However, existing methods of calculating dose distributions have limitations. Monte Carlo (MC) methods are accurate but can take too long to be clinically viable. Deterministic approaches are quicker but can be inaccurate under certain conditions, particularly near heterogeneities and air interfaces. Neural networks trained on MC-derived data have the potential to reproduce dose distributions that agree closely with the MC method while being significantly quicker to deploy. In this work we present a framework for training machine learning models capable of directly calculating the dose delivered to a point in three-dimensional (3D) heterogeneous media given only spatially local information. The framework consists of three parts. First, we describe a novel method of randomly generating 3D heterogeneous geometries using simplex noise. Dose distributions for training were obtained by importing these geometries into a MC simulation. The second and third parts of the framework are precalculated data channels, aligned with the patient computed tomography (CT) image, to be used as input to the model. These data channels are a computationally efficient way of encoding the parameters of an incident radiation beam while also allowing the model to learn from data that would otherwise be outside of its receptive field. We demonstrate the viability of the framework by a training small, fully connected neural network model to reproduce dose distributions from megavoltage photon beams. The trained network displayed excellent agreement with MC dose distributions in randomly generated geometries with an average gamma index (3%/3mm) pass rate of 94.7% and an average error of 1.45% of peak dose. Finally, the network was used to calculate the dose in a patient CT image, on which the network was not trained, producing similarly impressive results. A novel method of generating training data for learned radiation dosimetry models has been introduced, along with preprocessing steps that allow even simple models to reproduce accurate dose distributions for EBRT. More importantly, we have demonstrated that a model trained using the proposed framework can generalize from the training data to predicting the therapeutic dose in realistic media.

Gamma dose monitoring to assess the excess lifetime cancer risk in western Himalaya

The present work focussed on demarcation of areas with cancer development risk through excess lifetime cancer risk (ELCR) by assessing spatial variability of gamma dose in outdoor and indoor environment in western Himalaya. Average outdoor gamma dose and outdoor annual effective dose exceed the corresponding world averages. An indoor gamma dose (barring Budgam, Ganderbal and Kashmir University of Kashmir division and Reasi city of Jammu division) also exceed the world average. The probability of cancer development is higher in main-Shopian (3.65 × 10−3), Mala Bagh (2.80 × 10−3), Bundoda (2.88 × 10−3) and Dhadpeta (2.89 × 10−3), as total ELCR exceeds the world average.

Radiation levels following the Beirut seaport explosion: environmental survey and public dose exposure.

On August 4, 2020, an explosion at the Port of Beirut, Lebanon, ripped the city to shreds, killing more than 200 people, wounding 6500 others, and leaving about 300,000 residents temporarily homeless. The explosion raised many concerns regarding the possible presence of radioactive material and the associated public health risks. Accordingly, and after opening the affected sites for public access, an external radiological survey of the hit area was conducted. The dose rate reading in Beirut's seaport (ground zero) varied between 58 and 100nSv/h. The detected levels were within the average worldwide annual gamma dose rate of 50 to 53nSv/h set by the United Nations Scientific Committee on the Effects of Atomic Radiation. The recorded values were not significantly different from those measured by the Lebanese Atomic Energy Commission (LAEC) at the National Council for Scientific Research (CNRS-L) in 2009 and 2010 at three locations in Beirut's seaport (minimum value 50nSv/h and maximum value 140nSv/h). Besides, in the surveyed area, radiological scans were conducted on 360 injured patients reported to the American University of Beirut Medical Center (AUBMC), showing no signs of radiation exposure due to the explosion.

Improving proton dose calculation accuracy by using deep learning

Pencil beam (PB) dose calculation is fast but inaccurate due to the approximations when dealing with inhomogeneities. Monte Carlo (MC) dose calculation is the most accurate method but it is time consuming. The aim of this study was to develop a deep learning model that can boost the accuracy of PB dose calculation to the level of MC dose by converting PB dose to MC dose for different tumor sites. The proposed model uses the PB dose and computed tomography image as inputs to generate the MC dose. We used 290 patients (90 head and neck, 93 liver, 75 prostate and 32 lung) to train, validate, and test the model. For each tumor site, we performed four numerical experiments to explore various combinations of training datasets. Training the model on data from all tumor sites together and using the dose distribution of each individual beam as input yielded the best performance for all four tumor sites. The average gamma passing rate (1 mm/1%) between the converted and the MC dose was 92.8%, 92.7%, 89.7% and 99.6% for head and neck, liver, lung, and prostate test patients, respectively. The average dose conversion time for a single field was less than 4 s. The trained model can be adapted to new datasets through transfer learning. Our deep learning-based approach can quickly boost the accuracy of PB dose to that of MC dose. The developed model can be added to the clinical workflow of proton treatment planning to improve dose calculation accuracy.

Radiation treatment planning study to investigate feasibility of delivering Immunotherapy in Combination with Ablative Radiosurgery to Ultra-High DoSes (ICARUS).

PurposeImmune checkpoint inhibitors improve survival in metastatic diseases for some cancers. Multisite SBRT with pembrolizumab (SBRT + Pembro) was shown to be safe with promising local control using biologically effective doses (BEDs) = 95–120 Gy. Increased BED may improve response rate; however, SBRT doses are limited by surrounding organs at risk (OARs). The purpose of this work was to develop and validate methods for safe delivery of ultra‐high doses of radiation (BED10 > 300) to be used in future clinical trials.Methods and MaterialsThe radiation plans from 15 patients enrolled on a phase I trial of SBRT + pembro were reanalyzed. Metastatic disease sites included liver (8/15), inguinal region (1/15), pelvis (2/15), lung (1/15), abdomen (1/15), spleen (1/15), and groin (1/15). Gross tumor volumes (GTVs) ranged from 80 to 708 cc. Following the same methodology used in the Phase I trial on which these patients were treated, GTVs > 65 cc were contracted to a 65 cc subvolume (SubGTV) resulting in only a portion of the GTV receiving prescription dose. Volumetric modulated arc therapy (VMAT) was used to plan treatments BED10 = 360 Gy. Plans utilizing both 6FFF and 10FFF beams were compared to clinical plans delivering BED10 = 112.50 Gy. The target primary goal was V100% > 95% with a secondary goal of V70% > 99% and OAR objectives per the trial. To demonstrate feasibility, plans were delivered to a diode array phantom and evaluated for fidelity using gamma analysis.ResultsAll 30 plans met the secondary coverage goal and satisfied all OAR constraints. The primary goal was achieved in 12/15 of the 6FFF plans and 13/15 of the 10FFF plans. Average gamma analysis passing rate using criteria of 3% dose difference and 3, 2, and 1 mm were 99.1 ± 1.0%, 98.5 ± 1.6%, and 95.1 ± 3.8%, respectively.ConclusionNovel VMAT planning approaches with clinical treatment planning software and linear accelerators prove capable of delivering radiation doses in excess of 360 Gy BED10 to tumor subvolumes, while maintaining safe OAR doses.

Inverse treatment planning for an electronic brachytherapy system delivering anisotropic radiation therapy

An inverse radiation treatment planning algorithm for Sensus Healthcare’s SculpturaTM electronic brachytherapy system has been designed. The algorithm makes use of simulated annealing to optimize the conformation number (CN) of the treatment plan. The highly anisotropic dose distributions produced by the SculpturaTM x-ray source empower the inverse treatment planning algorithm to achieve highly conformal treatment plans for a wide range of prescribed planning target volumes. Over a set of 10 datasets the algorithm achieved an average CN of 0.79 ± 0.08 and an average gamma passing rate of 0.90 ± 0.10 at 5%/5 mm. A regularization term that encouraged short treatment plans was used, and it was found that the total treatment time could be reduced by 20% with only a nominal reduction in the CN and gamma passing rate. It was also found that downsampling the voxelized volume (from 3203 to 643 voxels) prior to optimization resulted in a 150× speedup in the optimization time (from 2 + minutes to < 1 s) without affecting the quality of the treatment plan.

Commissioning of GPU–Accelerated Monte Carlo Code FRED for Clinical Applications in Proton Therapy

We present commissioning and validation of Fred, a graphical processing unit (GPU)–accelerated Monte Carlo code, for two proton beam therapy facilities of different beam line design: CCB (Krakow, IBA) and EMORY (Atlanta, Varian). We followed clinical acceptance tests required to approve the certified treatment planning system for clinical use. We implemented an automated and efficient procedure to build a parameter library characterizing the clinical proton pencil beam. Beam energy, energy spread, lateral propagation model, and a dosimetric calibration factor were parametrized based on measurements performed during the facility start-up. The Fred beam model was validated against commissioning and supplementary measurements performed with and without range shifter. We obtained 1) submillimeter agreement of Bragg peak shapes in water and lateral beam profiles in air and slab phantoms, 2) &lt;2% dose agreement for spread out Bragg peaks of different ranges, 3) average gamma index (2%/2 mm) passing rate of &gt;95% for &gt;1000 patient verification measurements using a two-dimensional array of ionization chambers, and 4) gamma index passing rate of &gt;99% for three-dimensional dose distributions computed with Fred and measured with an array of ionization chambers behind an anthropomorphic phantom. The results of example treatment planning study on &gt;100 patients demonstrated that Fred simulations in computed tomography enable an accurate prediction of dose distribution in patient and application of Fred as second patient quality assurance tool. Computation of a patient treatment in a CT using 104 protons per pencil beam took on average 2′30 min with a tracking rate of 2.9×105p+/s. Fred was successfully commissioned and validated against the clinical beam model, showing that it could potentially be used in clinical routine. Thanks to high computational performance due to GPU acceleration and an automated beam model implementation method, the application of Fred is now possible for research or quality assurance purposes in most of the proton facilities.

Portal Dosimetry as a patient specific Quality Assurance tool for Volumetric Arc Radiotherapy

Introduction: Volumetric Arc Radiotherapy (VMAT) is an advanced technique. Calculations of VMATplans are not so accurate even with State-of-Art dose calculation algorithms due to their complexity.Hence pre-treatment patient specific Quality Assurance (QA) of each VMAT plan is required. In thepresent study Electronic Portal Imaging Device (EPID) based portal dosimetry system was used forpre-treatment patient specific QA. Material and Methods: A total of 50 patients were chosen inthis study. Verification plans of each patient were calculated for portal dosimetry then executed onthe EPID system to measure the spatial distribution of radiation dose. Calculated and measured dosedistribution were compared to evaluate Gamma Index (GI) passing criteria of Dose Difference (DD)of 3% and Distance–to-Agreement (DTA) of 3mm, Area Gamma (γ% ≤1) &gt;95%, Average Gamma(gAve) &lt;0.5% and Maximum Gamma (gMax) &lt;3.5%. Results: The mean values of Area Gamma (γ%≤1) were observed to be varied from 99.14±0.23% to 99.87±0.18%. The Mean Values of AverageGamma (gAve) are found to vary from 0.19±0.05% to 0.15±0.04% and the mean values ofMaximum Gamma (gMax) found to be varied from 1.94±0.37% to 1.59±0.41%. All the plans werepassed the gamma index criteria with very good agreement. Thus the use of Portal Dosimetry forpre-treatment patient QA is found to be a very useful, quick, precise, efficient and effective pre-treatment patient specific QA tool for VMAT treatment. Conclusion: Portal Dosimetry can be utilizedfor routine use for patient specific quality assurance for Volumetric Arc Radiotherapy treatment.

Correlation Between Average Segment Width and Gamma Passing Rate as a Function of MLC Position Error in Volumetric Modulated Arc Therapy.

Objective:This study analyzed the correlation between the average segment width (ASW) and gamma passing rate according to the multi-leaf collimator (MLC) position error.Method:To evaluate the changes in the gamma passing rate according to the MLC position error, 21 volumetric modulated arc therapy (VMAT) plans were generated using pelvic lymph node metastatic prostate cancer patient's data which is sensitive to MLC position errors as they involve several long, narrow, irregular fields. The ASW for each VMAT plan was calculated using our own code developed using Visual Basic for Applications (VBA). The gamma passing rate of the VMAT plan according to the MLC position error was evaluated using ArcCHECK (Sun Nuclear, Melbourne, FL, USA) while inducing symmetric MLC position errors in 0.25 mm intervals from −1 mm to +1 mm in the infinity medical linear accelerator (Elekta AB, Stockholm, Sweden). Finally, we examined the correlation between the change in the passing rate () due to the MLC position error and the ASW in VMAT through linear regression analysis using the least squares method.Results:The ASW and were found to have a linear correlation according to the MLC position error, and the coefficient of determination was 0.88. For a 1 mm position error of MLC in VMAT, the gamma passing rate improved by approximately 11.9% as the ASW increased by 10 mm.Conclusion:These results are expected to be employed as guidelines to minimize the dose uncertainty due to MLC position error in VMAT.

Оценка радиационных рисков злокачественных новообразований среди российских участников ликвидации последствий аварии на Чернобыльской АЭС

The paper considers radiation risks of solid cancer incidence and mortality, as well as risk of leu-kemia incidence (other than chronic lymphocytic leukemia) among Russian Chernobyl cleanup workers (liquidators). The study of the cohort of liquidators carried out at the National Radiation Epidemiological Registry (NRER) was based on the follow-up data collected from 1992 over 2019. The size of the Chernobyl cleanup workers cohort exceeded 65 thousand people, their av-erage age at the time of entering the exclusion zone was 34 years, the average external gamma radiation dose received by liquidators during their cleanup work was about 0.133 Gy. Radiation-induced risks of solid cancer incidence and mortality in the study cohort were statistically signifi-cant, the risk magnitude rose with increasing the follow-up length. For the maximum follow-up period, from 1992 over 2019, the excess relative risk coefficient for solid cancer incidence was ERR/Gy=0.62, 95% CI (0.29; 0.98), and excess relative risk coefficient for solid cancer mortality was ERR/Gy=0.74, 95% CI (0.32; 1.22), the estimated coefficients were in good agreement with similar coefficients calcu-lated for the Russian liquidators with the use of ICRP radiation risk models. Non-parametric esti-mates of relative radiation risk within the same dose intervals for solid cancers and for leukemias in the cohort of liquidators were statistically significant for radiation doses above 0.150 Gy. For radiation doses below 0,150 Гр the linear non-threshold model is conservative, i.e. there was ev-idence for statistically significant radiation risk of leukemia incidence among liquidators during the first 11 years after the accident, from 1986 over 1997, ERR/Gy=4.41, 95% CI (0.24; 14.23). In later years, until 2018 there was no evidence of radiation-related risk of leukemia incidence. Out-comes of future studies will impact on optimization of radiological protection, development of reference levels for Russian general public exposure and improvement of the system for delivery of targeted medical care to people exposed to radiation.

Validation of the dosimetric and geometric accuracy of MR-only treatment planning solution for prostate cancer radiotherapy.

IntroductionThe aim was to validate the dosimetric and geometric accuracy of radiotherapy treatment plans for prostate cancer based on magnetic resonance (MR) imaging only and a solution based on computed tomography (CT) supported by MR imaging.Material and methodsWe used CT and MR images of ten prostate cancer patients implanted with three fiducial markers (FM) in the prostate gland. Rigid registration based on FM was performed to assess the fusion accuracy between MR and CT images. The differences between prostate contours (clinical target volume – CTV) on CT (CTVCT) and MR (CTVMR) images were scored using the Dice similarity coefficient and directly comparing the outlined volumes. The volumetric modulated arc therapy plans were designed and optimised on synthetic CT (sCT) to obtain the dose distribution for the MR-only solution. In the next step, the sCT images were replaced by conventional CT images and the plans were recalculated. The doses obtained on sCT and CT were compared by direct dose subtraction and the gamma method.ResultsThe averaged fiducial registration error was equal to 0.5 mm. All CTVCT volumes were significantly bigger than corresponding CTV delineated on MR images (p = 0.005). The direct dose comparison shows that for 97.1% of patients’ bodies, the differences were smaller than 0.1%. The average gamma passing rates were higher than 0.970.ConclusionsMR imaging allows for a more precise delineation of the prostate compared to CT imaging. The workflow of plan preparation based on MR and CT is burdened with an FM registration error that is eliminated by an MR-only solution with no compromise on dose distribution.

Small Animal IMRT Using 3D-Printed Compensators

Preclinical radiation replicating clinical intensity modulated radiation therapy (IMRT) techniques can provide data translatable to clinical practice. For this work, treatment plans were created for oxygen-guided dose-painting in small animals using inverse-planned IMRT. Spatially varying beam intensities were achieved using 3-dimensional (3D)-printed compensators. Optimized beam fluence from arbitrary gantry angles was determined using a verified model of the XRAD225Cx treatment beam. Compensators were 3D-printed with varied thickness to provide desired attenuation using copper/polylactic-acid. Spatial resolution capabilities were investigated using printed test-patterns. Following American Association of Physicists in Medicine TG119, a 5-beam IMRT plan was created for a miniaturized (∼1/8th scale) C-shape target. Electron paramagnetic resonance imaging of murine tumor oxygenation guided simultaneous integrated boost (SIB) plans conformally treating tumor to a base dose (Rx1) with boost (Rx2) based on tumor oxygenation. The 3D-printed compensator intensity modulation accuracy and precision was evaluated by individually delivering each field to a phantom containing radiochromic film and subsequent per-field gamma analysis. The methodology was validated end-to-end with composite delivery (incorporating 3D-printed tungsten/polylactic-acid beam trimmers to reduce out-of-field leakage) of the oxygen-guided SIB plan to a phantom containing film and subsequent gamma analysis. Resolution test-patterns demonstrate practical printer resolution of ∼0.7 mm, corresponding to 1.0 mm bixels at the isocenter. The miniaturized C-shape plan provides planning target volume coverage (V95% = 95%) with organ sparing (organs at risk Dmax < 50%). The SIB plan to hypoxic tumor demonstrates the utility of this approach (hypoxic tumor V95%,Rx2 = 91.6%, normoxic tumor V95%,Rx1 = 95.7%, normal tissue V100%,Rx1 = 7.1%). The more challenging SIB plan to boost the normoxic tumor rim achieved normoxic tumor V95%,Rx2 = 90.9%, hypoxic tumor V95%,Rx1 = 62.7%, and normal tissue V100%,Rx2 = 5.3%. Average per-field gamma passing rates using 3%/1.0 mm, 3%/0.7 mm, and 3%/0.5 mm criteria were 98.8% ± 2.8%, 96.6% ± 4.1%, and 90.6% ± 5.9%, respectively. Composite delivery of the hypoxia boost plan and gamma analysis (3%/1 mm) gave passing results of 95.3% and 98.1% for the 2 measured orthogonal dose planes. This simple and cost-effective approach using 3D-printed compensators for small-animal IMRT provides a methodology enabling preclinical studies that can be readily translated into the clinic. The presented oxygen-guided dose-painting demonstrates that this methodology will facilitate studies driving much needed biologic personalization of radiation therapy for improvements in patient outcomes.

Geometric and dosimetric verification of a recurrent neural network algorithm to compensate for respiratory motion using an articulated robotic couch

The purpose of this study is to evaluate the performance of a recurrent neural network (RNN)-based prediction algorithm to compensate for respiratory movement using an articulated robotic couch system. A prototype of a real-time respiratory motion compensation couch was built using an optical 3D motion tracking system and a six-degree-of-freedom-articulated robotic system. To compensate for the system latency from motion detection to re-positioning of the system, RNN and double exponential smoothing (ES2) prediction algorithms were applied. Three aspects of performance were evaluated, simulation and experiments for geometric and dosimetric evaluations, using data from three liver and three lung patients who underwent stereotactic body radiotherapy. Overall, the RNN algorithm showed better geometric and dosimetric results than the other approaches. In simulation tests, RNN showed 82% average improvement ratio, compared with non-predicted results. In the geometric evaluation, RNN only showed average FWHM broadening of 1.5 mm, compared with the static case. In the dosimetric evaluation, RNN showed average gamma passing rates of 97.4 ± 1.0%, 89.0 ± 2.4% under the 3%/3 mm, 2%/2 mm respectively. It may be technically feasible to use the RNN prediction algorithm to compensate for respiratory motion with an articulated robotic couch system. The RNN algorithm could be widely used for motion compensation in patients undergoing radiotherapy.

Effects of Age-Related Stereotype Threat on Metacognition.

Previous work has shown that memory performance in older adults is affected by activation of a stereotype of age-related memory decline. In the present experiment, we examined whether stereotype threat would affect metamemory in older adults; that is, whether under stereotype threat they make poorer judgments about what they could remember. We tested older adults (M Age = 66.18 years) on a task in which participants viewed words paired with point values and “bet” on whether they could later recall each word. If they bet on and recalled a word, they gained those points, but if they bet on and failed to recall a word, they lost those points. Thus, this task required participants to monitor how much they could remember and prioritize high value items. Participants performed this task over six lists of items either under stereotype threat about age-related memory decline or not under stereotype threat. Participants from both groups performed similarly on initial lists, but on later lists, participants under stereotype threat showed impaired performance as indicated by a lower average point score and a lower average gamma coefficient. The results suggest that a modest effect of stereotype threat on recall combined with a modest effect on metacognitive judgments to result in a performance deficit. This pattern of results may reflect an effect of stereotype threat on executive control reducing the ability to strategically use memory.