Thomas Niedermayr
Clinical Associate Professor, Radiation Oncology - Radiation Physics
All Publications
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Radio-luminescent imaging for rapid, high resolution eye plaque loading verification.
Medical physics
2022
Abstract
BACKGROUND: Eye plaque brachytherapy (EPB) is currently an optimal therapy for intraocular cancers. Due to the lack of an effective and practical technique to measure the seed radioactivity distribution, current quality assurance (QA) practice according to the AAPM TG129 only stipulates that the plaque assembly be visually inspected. Consequently, uniform seed activity is routinely adopted to avoid possible loading mistakes of differential seed loading. However, modulated dose delivery, which represents a general trend in radiotherapy to provide more personalized treatment for a given tumor and patient, requires differential activities in the loaded seeds.PURPOSE: In this study, a fast and low-cost radio-luminescent imaging and dose calculating system to verify the seed activity distribution for differential loading was developed.METHODS: A proof-of-concept system consisting of a thin scintillator sheet coupled to a camera/lens system was constructed. A seed-loaded plaque can be placed directly on the scintillator surface with the radioactive seeds facing the scintillator. The camera system collects the radioluminescent signal generated by the scintillator at its opposite side. The predicted dose distribution in the scintillator's sensitive layer was calculated using a Monte Carlo simulation with the planned plaque loading pattern of I-125 seeds. Quantitative comparisons of the distribution of relative measured signal intensity and that of the relative predicted dose in the sensitive layer were performed by gamma analysis, similar to IMRT QA.RESULTS: Data analyses showed high gamma (3%/0.3mm, global, 20% threshold) passing rates for correct seed loadings and low passing rates with distinguished high gamma value area for incorrect loadings, indicating that possible errors may be detected. The measurement and analysis only required a few extra minutes, significantly shorter than the time to assay the extra verification seeds the physicist already must perform as recommended by TG129.CONCLUSIONS: Radio-luminescent QA can be used to facilitate and assure the implementation of intensity modulated, customized plaque loading. This article is protected by copyright. All rights reserved.
View details for DOI 10.1002/mp.16003
View details for PubMedID 36183146
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Applying 3D-Printed Templates in High-Dose-Rate Brachytherapy for Cervix Cancer: Simplified Needle Insertion for Optimized Dosimetry.
International journal of radiation oncology, biology, physics
2022
Abstract
PURPOSE: In cervical cancer brachytherapy, adding interstitial (IS) needles to intracavitary (IC) applicators can enhance dosimetry by improving target coverage while limiting normal tissue dose. However, its use is limited to a subset of practitioners with appropriate technical skill. We designed Tandem Anchored Radially Guiding Interstitial Templates (TARGITs) with a 3D printing workflow to optimize needle placement and facilitate greater ease-of-use of IC/IS technique. This study compared dosimetry and procedure characteristics between tandem-and-ovoid implants (T&O) using TARGIT technique versus non-TARGIT technique.METHODS AND MATERIALS: This single-institution retrospective cohort study included patients undergoing T&O brachytherapy as part of definitive radiation treatment for cervical cancer between February 2017 and January 2021. TARGIT technique was implemented from November 2019 onwards; all prior procedures involved non-TARGIT technique using a No Needle (NN) or Freehand Needle (FN) approach. Target coverage, dose to organs-at-risk, and procedure times were evaluated and compared between TARGIT technique and non-TARGIT technique.RESULTS: The cohort included 70 patients with cervical cancer who underwent 302 T&O procedures: 68 (23%) with TARGIT technique and 234 (77%) with non-TARGIT technique, which included 133 NN and 101 FN implants. TARGIT implants involved longer average procedure times (+6.3 minutes, p<0.0001). TARGIT implants achieved a higher mean high-risk CTV V100% than non-TARGIT implants (+4.4%, p=0.001) including for large tumors 30 cc or greater (+8.1%, p=0.002). Average D90 was 4.6 Gy higher and average D98 was 3.2 Gy higher for TARGIT technique compared to non-TARGIT technique (p=0.006 and p=0.02). Total treatment doses to rectum, bowel, and bladder were not significantly different for TARGIT versus non-TARGIT technique.CONCLUSION: The 3D-printed TARGIT approach to T&O brachytherapy achieved greater tumor coverage while sparing normal tissues, particularly for large tumor volumes, with only a slight increase in average procedure time. TARGIT represents a creative technological solution for increasing accessibility of advanced IC/IS brachytherapy technique for cervical cancer definitive radiation treatment.
View details for DOI 10.1016/j.ijrobp.2022.05.027
View details for PubMedID 35654306
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Dose Prediction for Cervical Cancer Brachytherapy Using 3-D Deep Convolutional Neural Network
IEEE TRANSACTIONS ON RADIATION AND PLASMA MEDICAL SCIENCES
2022; 6 (2): 214-221
View details for DOI 10.1109/TRPMS.2021.3098507
View details for Web of Science ID 000750257400011
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Limited Time Penalty for Improved Dosimetry: Simplified Needle Insertion in Combined Tandem and Ovoid plus Interstitial Cases with Custom Templates
LIPPINCOTT WILLIAMS & WILKINS. 2021: S14-S15
View details for Web of Science ID 000701779700025
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MR to Ultrasound Image Registration with Segmentation-Based Learning for HDR Prostate Brachytherapy
WILEY. 2021
View details for Web of Science ID 000673145402120
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MR to ultrasound image registration with segmentation-based learning for HDR prostate brachytherapy.
Medical physics
2021
Abstract
Propagation of contours from high-quality magnetic resonance (MR) images to treatment planning ultrasound (US) images with severe needle artifacts is a challenging task, which can greatly aid the organ contouring in high dose rate (HDR) prostate brachytherapy. In this study, a deep learning approach was developed to automatize this registration procedure for HDR brachytherapy practice.Because of the lack of training labels and difficulty of accurate registration from inferior image quality, a new segmentation-based registration framework was proposed for this multi-modality image registration problem. The framework consisted of two segmentation networks and a deformable registration network, based on the weakly-supervised registration strategy. Specifically, two 3D V-Nets were trained for the prostate segmentation on the MR and US images separately, to generate the weak supervision labels for the registration network training. Besides the image pair, the corresponding prostate probability maps from the segmentation were further fed to the registration network to predict the deformation matrix, and an augmentation method was designed to randomly scale the input and label probability maps during the registration network training. The overlap between the deformed and fixed prostate contours was analyzed to evaluate the registration accuracy. Three datasets were collected from our institution for the MR and US image segmentation networks, and the registration network learning, which contained 121, 104 and 63 patient cases, respectively.The mean Dice similarity coefficient (DSC) results of the two prostate segmentation networks are 0.86±0.05 and 0.90±0.03, for MR images and the US images after the needle insertion, respectively. The mean DSC, center-of-mass (COM) distance, Hausdorff distance (HD) and averaged symmetric surface distance (ASSD) results for the registration of manual prostate contours were 0.87±0.05, 1.70±0.89 mm, 7.21±2.07 mm, 1.61±0.64 mm, respectively. By providing the prostate probability map from the segmentation to the registration network, as well as applying the random map augmentation method, the evaluation results of the four metrics were all improved, such as an increase of DSC from 0.83±0.08 to 0.86±0.06 and from 0.86±0.06 to 0.87±0.05, respectively.A novel segmentation-based registration framework was proposed to automatically register prostate MR images to the treatment planning US images with metal artifacts, which not only largely saved the labor work on the data preparation, but also improved the registration accuracy. The evaluation results showed the potential of this approach in HDR prostate brachytherapy practice.
View details for DOI 10.1002/mp.14901
View details for PubMedID 33905566
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Intensity modulated Ir-192 brachytherapy using high-Z 3D printed applicators.
Physics in medicine and biology
2020
Abstract
Gynecologic cancers are often asymmetric, yet current Ir-192 brachytherapy techniques provide only limited radial modulation of the dose. The shielded solutions investigated here solve this by providing the ability to modulate between highly asymmetric and radially symmetric dose distributions at a given location. To find applicator designs that can modulate between full dose and less than 50% dose, at the dimensions of the urethra, a 2D calculation algorithm was developed to narrow down the search space. Two shielding design types were then further investigated using Monte Carlo and Boltzmann-solver dose calculation algorithms. 3D printing techniques using ISO10993 certified biocompatible plastics and 3D printable tungsten-loaded plastics were tested. It was also found that shadowing effects set by the shape of the shielding cannot be easily modulated out, hence careful design is required. The shielded applicator designs investigated here, allow for reduction of the dose by over 50% at 5 mm from the applicator surface in desired regions, while also allowing radially symmetric dose with isodose line (IDL) deviations less than 0.5 mm from circular. The shielding designs were also chosen with treatment delivery time in mind. Treatment times for these shielded designs were found to be less than 1.4 times longer than a six-channel unshielded cylinder for the equivalent fully symmetric dose distribution. The 2D calculation methods developed here provide a simple way to rapidly evaluate shielding designs, while the 3D printing techniques also allow for devices with novel shapes to be rapidly prototyped. Both TOPAS Monte Carlo and Acuros BV calculations show that significant dose shaping, and organ at risk (OAR) sparing can be achieved without significantly compromising the plan in regions that require the full dose.
View details for DOI 10.1088/1361-6560/ab9b54
View details for PubMedID 32521512
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Factor 10 Expedience of Monthly Linac Quality Assurance via an Ion Chamber Array and Automation Scripts.
Technology in cancer research & treatment
2019; 18: 1533033819876897
Abstract
PURPOSE: While critical for safe and accurate radiotherapy, monthly quality assurance of medical linear accelerators is time-consuming and takes physics resources away from other valuable tasks. The previous methods at our institution required 5 hours to perform the mechanical and dosimetric monthly linear accelerator quality assurance tests. An improved workflow was developed to perform these tests with higher accuracy, with fewer error pathways, in significantly less time.METHODS: A commercial ion chamber array (IC profiler, Sun Nuclear, Melbourne, Florida) is combined with automation scripts to consolidate monthly linear accelerator QA. The array was used to measure output, flatness, symmetry, jaw positions, gated dose constancy, energy constancy, collimator walkout, crosshair centering, and dosimetric leaf gap constancy. Treatment plans were combined with automation scripts that interface with Sun Nuclear's graphical user interface. This workflow was implemented on a standard Varian clinac, with no special adaptations, and can be easily applied to other C-arm linear accelerators.RESULTS: These methods enable, in 30 minutes, measurement and analysis of 20 of the 26 dosimetric and mechanical monthly tests recommended by TG-142. This method also reduces uncertainties in the measured beam profile constancy, beam energy constancy, field size, and jaw position tests, compared to our previous methods. One drawback is the increased uncertainty associated with output constancy. Output differences between IC profiler and farmer chamber in plastic water measurements over a 6-month period, across 4 machines, were found to have a 0.3% standard deviation for photons and a 0.5% standard deviation for electrons, which is sufficient for verifying output accuracy according to TG-142 guidelines. To minimize error pathways, automation scripts which apply the required settings, as well as check the exported data file integrity were employed.CONCLUSIONS: The equipment, procedure, and scripts used here reduce the time burden of routine quality assurance tests and in most instances improve precision over our previous methods.
View details for DOI 10.1177/1533033819876897
View details for PubMedID 31707931
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Optimizing efficiency and safety in external beam radiotherapy using automated plan check (APC) tool and six sigma methodology.
Journal of applied clinical medical physics
2019; 20 (8): 56–64
Abstract
To develop and implement an automated plan check (APC) tool using a Six Sigma methodology with the aim of improving safety and efficiency in external beam radiotherapy.The Six Sigma define-measure-analyze-improve-control (DMAIC) framework was used by measuring defects stemming from treatment planning that were reported to the departmental incidence learning system (ILS). The common error pathways observed in the reported data were combined with our departmental physics plan check list, and AAPM TG-275 identified items. Prioritized by risk priority number (RPN) and severity values, the check items were added to the APC tool developed using Varian Eclipse Scripting Application Programming Interface (ESAPI). At 9 months post-APC implementation, the tool encompassed 89 check items, and its effectiveness was evaluated by comparing RPN values and rates of reported errors. To test the efficiency gains, physics plan check time and reported error rate were prospectively compared for 20 treatment plans.The APC tool was successfully implemented for external beam plan checking. FMEA RPN ranking re-evaluation at 9 months post-APC demonstrated a statistically significant average decrease in RPN values from 129.2 to 83.7 (P < .05). After the introduction of APC, the average frequency of reported treatment-planning errors was reduced from 16.1% to 4.1%. For high-severity errors, the reduction was 82.7% for prescription/plan mismatches and 84.4% for incorrect shift note. The process shifted from 4σ to 5σ quality for isocenter-shift errors. The efficiency study showed a statistically significant decrease in plan check time (10.1 ± 7.3 min, P = .005) and decrease in errors propagating to physics plan check (80%).Incorporation of APC tool has significantly reduced the error rate. The DMAIC framework can provide an iterative and robust workflow to improve the efficiency and quality of treatment planning procedure enabling a safer radiotherapy process.
View details for DOI 10.1002/acm2.12678
View details for PubMedID 31423729
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A multichannel superconducting tunnel junction detector for high-resolution X-ray spectroscopy of magnesium diboride films
Applied Superconductivity Conference
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. 2003: 1114–19
View details for DOI 10.1109/TASC.2003.814169
View details for Web of Science ID 000184241700259