Daniel W. Mundy

Analysis of environmental influences in nuclear half-life measurements exhibiting time-dependent decay rates

Abstract In a recent series of papers evidence has been presented for correlations between solar activity and nuclear decay rates. This includes an apparent correlation between Earth–Sun distance and data taken at Brookhaven National Laboratory (BNL), and at the Physikalisch-Technische Bundesanstalt (PTB). Although these correlations could arise from a direct interaction between the decaying nuclei and some particles or fields emanating from the Sun, they could also represent an “environmental” effect arising from a seasonal variation of the sensitivities of the BNL and PTB detectors due to changes in temperature, relative humidity, background radiation, etc. In this paper, we present a detailed analysis of the responses of the detectors actually used in the BNL and PTB experiments, and show that sensitivities to seasonal variations in the respective detectors are likely too small to produce the observed fluctuations.

Additional experimental evidence for a solar influence on nuclear decay rates

Abstract Additional experimental evidence is presented in support of the recent hypothesis that a possible solar influence could explain fluctuations observed in the measured decay rates of some isotopes. These data were obtained during routine weekly calibrations of an instrument used for radiological safety at The Ohio State University Research Reactor using 36 Cl. The detector system used was based on a Geiger–Muller gas detector, which is a robust detector system with very low susceptibility to environmental changes. A clear annual variation is evident in the data, with a maximum relative count rate observed in January/February, and a minimum relative count rate observed in July/August, for seven successive years from July 2005 to June 2011. This annual variation is not likely to have arisen from changes in the detector surroundings, as we show here.

Cutaneous sympathetic neural responses to body cooling in type 2 diabetes mellitus.

In humans, sympathetic vasoconstrictor nerves in the skin contribute to resting vascular tone and mediate reflex vasoconstrictor responses to body cooling. Although it is well recognized that type 2 diabetes mellitus (T2DM) is associated with peripheral neurovascular changes, it is unclear to what extent the thermal responsiveness of the cutaneous vasoconstrictor system is altered in individuals with relatively uncomplicated T2DM. We tested the hypothesis that skin sympathetic nerve activity (SSNA) is decreased at baseline and during body cooling in individuals with T2DM compared to healthy controls (C) of similar age and body size. We measured SSNA (microneurography) and skin blood flow (laser-Doppler flowmetry) in the innervated area in 8 T2DM and 12 C subjects at baseline and during 3-4min of rapid whole body cooling via a water-perfused suit. SSNA (total integrated activity) increased, and cutaneous vascular conductance decreased in both groups during body cooling (P<0.01 for both). However, SSNA was not different between groups during either baseline or body cooling conditions (P=NS). The deltas in SSNA between baseline and body cooling were similar between groups: T2DM: 55±27 and C: 57±12 units (P=NS). We conclude that reflex cutaneous sympathetic and vascular responses to rapid whole body cooling are preserved in relatively healthy individuals with T2DM.

Analysis of automatic match results for cone-beam computed tomography localization of conventionally fractionated lung tumors.

PURPOSE To evaluate the dependence of an automatic match process on the size of the user-defined region of interest (ROI), the structure volume of interest (VOI), and changes in tumor volume when using cone-beam computed tomography (CBCT) for tumor localization and to compare these results with a gold standard defined by a physicians manual match. METHODS AND MATERIALS Daily CBCT images for 11 patients with lung cancer treated with conventionally fractionated radiation therapy were retrospectively matched to a reference CT image using the Varian On Board Imager software (Varian, Palo Alto, CA) and a 3-step automatic matching protocol. Matches were performed with 3 ROI sizes (small, medium, large), with and without a structure VOI (internal target volume [ITV] or planning target volume [PTV]) used in the last step. Additionally, matches were performed using an intensity range that isolated the bony anatomy of the spinal column. All automatic matches were compared with a manual match made by a physician. RESULTS The CBCT images from 109 fractions were analyzed. Automatic match results depend on ROI size and the structure VOI. Compared with the physicians manual match, automatic matches using the PTV as the structure VOI and a small ROI resulted in differences ≥ 5 mm in 1.8% of comparisons. Automatic matches using no VOI and a large ROI differed by ≥ 5 mm in 30.3% of comparisons. Differences between manual and automatic matches using the ITV as the structure VOI increased as tumor size decreased during the treatment course. CONCLUSIONS Users of automatic matching techniques should carefully consider how user-defined parameters affect tumor localization. Automatic matches using the PTV as the structure VOI and a small ROI were most consistent with a physicians manual match, and were independent of volumetric tumor changes.

Uncertainty analysis of a Compton camera imaging system for radiation therapy dose reconstruction

PURPOSE Improved radiotherapy dose delivery techniques over the past decade have increased the necessity for accurate, independent verification of delivered dose. Compton camera imaging (CCI) systems may have the potential to quantitatively reconstruct three-dimensional dose delivered to the patient with little or no a priori information. METHODS In this work, the adequacy of a Compton camera imaging system for application to radiotherapy dose reconstruction is explored using analytical models of system spatial and dosimetric resolution. The effects of scatter and absorption detector energy resolution, initial photon energy, and detector separation distance on system performance were calculated with the goal of determining whether current detector technology is adequate for such an application. RESULTS Results indicate that the energy and spatial resolutions associated with current Si and Ge double-sided strip detectors in a planar configuration is sufficient to determine dose deposition to within an average of 1.9 mm and 2.5%. Minimum values of less than 0.5 mm and 1% are achievable under certain conditions. As the energy of the photon incident on the patient increases from 1.0 to 10 MeV, system performance improves at the expense of the range of patient and detector scattering angles over which the system is capable of reconstructing dose deposition to within the acceptable upper limits of 5 mm and 5%. System performance also improves with increasing distance between the scatter and absorption detectors, but is acceptable throughout the range of values likely to be associated with a gantry-mounted system (2-20 cm). CONCLUSIONS The results indicate that Compton camera imaging systems based on current solid-state detector technology have the potential to provide independent verification of dose delivered to a patient during radiation therapy. Further consideration must be given to detector efficiency and image reconstruction algorithms for this application of CCI systems.

An accelerated threshold-based back-projection algorithm for compton camera image reconstruction.

PURPOSE Compton camera imaging (CCI) systems are currently under investigation for radiotherapy dose reconstruction and verification. The ability of such a system to provide real-time images during dose delivery will be limited by the computational speed of the image reconstruction algorithm. In this work, the authors present a fast and simple method by which to generate an initial back-projected image from acquired CCI data, suitable for use in a filtered back-projection algorithm or as a starting point for iterative reconstruction algorithms, and compare its performance to the current state of the art. METHODS Each detector event in a CCI system describes a conical surface that includes the true point of origin of the detected photon. Numerical image reconstruction algorithms require, as a first step, the back-projection of each of these conical surfaces into an image space. The algorithm presented here first generates a solution matrix for each slice of the image space by solving the intersection of the conical surface with the image plane. Each element of the solution matrix is proportional to the distance of the corresponding voxel from the true intersection curve. A threshold function was developed to extract those pixels sufficiently close to the true intersection to generate a binary intersection curve. This process is repeated for each image plane for each CCI detector event, resulting in a three-dimensional back-projection image. The performance of this algorithm was tested against a marching algorithm known for speed and accuracy. RESULTS The threshold-based algorithm was found to be approximately four times faster than the current state of the art with minimal deficit to image quality, arising from the fact that a generically applicable threshold function cannot provide perfect results in all situations. The algorithm fails to extract a complete intersection curve in image slices near the detector surface for detector event cones having axes nearly parallel to the image plane. This effect decreases the sum of the image, thereby also affecting the mean, standard deviation, and SNR of the image. All back-projected events associated with a simulated point source intersected the voxel containing the source and the FWHM of the back-projected image was similar to that obtained from the marching method. CONCLUSIONS The slight deficit to image quality observed with the threshold-based back-projection algorithm described here is outweighed by the 75% reduction in computation time. The implementation of this method requires the development of an optimum threshold function, which determines the overall accuracy of the method. This makes the algorithm well-suited to applications involving the reconstruction of many large images, where the time invested in threshold development is offset by the decreased image reconstruction time. Implemented in a parallel-computing environment, the threshold-based algorithm has the potential to provide real-time dose verification for radiation therapy.

SU-F-T-169: A Periodic Quality Assurance Program for a Spot-Scanning Proton Treatment Facility

PURPOSE To develop daily and monthly quality assurance (QA) programs in support of a new spot-scanning proton treatment facility using a combination of commercial and custom equipment and software. Emphasis was placed on efficiency and evaluation of key quality parameters. METHODS The daily QA program was developed to test output, spot size and position, proton beam energy, and image guidance using the Sun Nuclear Corporation rf-DQA™3 device and Atlas QA software. The program utilizes standard Atlas linear accelerator tests repurposed for proton measurements and a custom jig for indexing the device to the treatment couch. The monthly QA program was designed to test mechanical performance, image quality, radiation quality, isocenter coincidence, and safety features. Many of these tests are similar to linear accelerator QA counterparts, but many require customized test design and equipment. Coincidence of imaging, laser marker, mechanical, and radiation isocenters, for instance, is verified using a custom film-based device devised and manufactured at our facility. Proton spot size and position as a function of energy are verified using a custom spot pattern incident on film and analysis software developed in-house. More details concerning the equipment and software developed for monthly QA are included in the supporting document. Thresholds for daily and monthly tests were established via perturbation analysis, early experience, and/or proton system specifications and associated acceptance test results. RESULTS The periodic QA program described here has been in effect for approximately 9 months and has proven efficient and sensitive to sub-clinical variations in treatment delivery characteristics. CONCLUSION Tools and professional guidelines for periodic proton system QA are not as well developed as their photon and electron counterparts. The program described here efficiently evaluates key quality parameters and, while specific to the needs of our facility, could be readily adapted to other proton centers.

Clinical Implementation of a Proton Dose Verification System Utilizing a GPU Accelerated Monte Carlo Engine

Purpose To develop a clinical infrastructure that allows for routine Monte Carlo dose calculation verification of spot scanning proton treatment plans and includes a simple biological model to aid in normal tissue protection. Materials and Methods A graphical processing unit accelerated Monte Carlo dose engine was used as the calculation engine for dose verification on spot scanning proton plans. An infrastructure was built around this engine that allows for seamless exporting of treatment plans from the treatment planning system and importing of dose distribution from the Monte Carlo calculation via DICOM (digital imaging and communications in medicine). An easy-to-use Web-based interface was developed so that the application could be run from any computer. In addition to the standard relative biological effectiveness = 1.1 for proton therapy, a simple linear equation dependent on dose-weighted linear energy transfer was included. This was used to help detect possible high biological dose in critical structures. Results More than 270 patients were treated at our proton center in the first year of operation. Because most plans underwent multiple iterations before final approval, more than 1000 plans have been run through the system from multiple users with minimal downtime. The average time from plan export to importing of the Monte Carlo doses was less than 15 minutes. Treatment plans have been modified based on the nominal Monte Carlo dose or the biological dose. Conclusion Monte Carlo dose calculation verification of spot scanning proton treatment plans is feasible in a clinical environment. The 3-dimensional dose verification, particularly near heterogeneities, has resulted in plan modifications. The biological dose data provides actionable feedback for end of range effects, especially in pediatric patients.

Automation of Routine Elements for Spot-Scanning Proton Patient-Specific Quality Assurance

PURPOSE At our institution, all proton patient plans undergo patient-specific quality assurance (PSQA) prior to treatment delivery. For intensity-modulated proton beam therapy, quality assurance is complex and time consuming, and it may involve multiple measurements per field. We reviewed our PSQA workflow and identified the steps that could be automated and developed solutions to improve efficiency. METHODS We used the treatment planning systems (TPS) capability to support C# scripts to develop an Eclipse scripting application programming interface (ESAPI) script and automate the preparation of the verification phantom plan for measurements. A local area network (LAN) connection between our measurement equipment and shared database was established to facilitate equipment control, measurement data transfer, and storage. To improve the analysis of the measurement data, a Python script was developed to automatically perform a 2D-3D γ-index analysis comparing measurements in the plane of a two-dimensional detector array with TPS predictions in a water phantom for each acquired measurement. RESULTS Device connection via LAN granted immediate access to the plan and measurement information for downstream analysis using an online software suite. Automated scripts applied to verification plans reduced time from preparation steps by at least 50%; time reduction from automating γ-index analysis was even more pronounced, dropping by a factor of 10. On average, we observed an overall time savings of 55% in completion of the PSQA per patient plan. CONCLUSIONS The automation of the routine tasks in the PSQA workflow significantly reduced the time required per patient, reduced user fatigue, and frees up system users from routine and repetitive workflow steps allowing increased focus on evaluating key quality metrics.

Spot scanning proton therapy plan assessment: design and development of a dose verification application for use in routine clinical practice

The use of radiation therapy for the treatment of cancer has been carried out clinically since the late 1800’s. Early on however, it was discovered that a radiation dose sufficient to destroy cancer cells can also cause severe injury to surrounding healthy tissue. Radiation oncologists continually strive to find the perfect balance between a dose high enough to destroy the cancer and one that avoids damage to healthy organs. Spot scanning or “pencil beam” proton radiotherapy offers another option to improve on this. Unlike traditional photon therapy, proton beams stop in the target tissue, thus better sparing all organs beyond the targeted tumor. In addition, the beams are far narrower and thus can be more precisely “painted” onto the tumor, avoiding exposure to surrounding healthy tissue. To safely treat patients with proton beam radiotherapy, dose verification should be carried out for each plan prior to treatment. Proton dose verification systems are not currently commercially available so the Department of Radiation Oncology at the Mayo Clinic developed its own, called DOSeCHECK, which offers two distinct dose simulation methods: GPU-based Monte Carlo and CPU-based analytical. The three major components of the system include the web-based user interface, the Linux-based dose verification simulation engines, and the supporting services and components. The architecture integrates multiple applications, libraries, platforms, programming languages, and communication protocols and was successfully deployed in time for Mayo Clinic’s first proton beam therapy patient. Having a simple, efficient application for dose verification greatly reduces staff workload and provides additional quality assurance, ultimately improving patient safety.