TB risk by time since U.S. entry among non-U.S.-born residents of Washington State, USA.

BACKGROUND: Progress towards TB elimination in the United States will require improved detection and treatment of latent TB infection among non-U.S.-born residents who remain at disproportionate risk of TB disease. To inform targeted testing efforts, we evaluated risk of TB disease among non-U.S.-born residents of Washington State, USA, by region of origin and time from U.S. entry.METHODS: We conducted a retrospective cohort study among non-U.S.-born residents diagnosed with TB disease in Washington State from 2005 to 2014, for which country-specific population estimates were also available. The risk of TB disease among non-U.S.-born residents was estimated by time since U.S. entry, World Bank region of origin, and WHO TB incidence category.RESULTS: Risk of TB disease for non-U.S.-born residents was highest within the first year after U.S. entry. Among persons from countries with high TB incidence who had resided in the United States for more than 20 years, risk for TB remained elevated.CONCLUSION: Elevated risk of developing TB disease among individuals not born in the United States persisted long after U.S. entry, particularly among persons originating from certain regions and from high-burden countries. These findings contribute to evidence supporting a refinement of existing screening guidelines.

Evaluation of the spatial dependence of the point spread function in 2D PET image reconstruction using LOR-OSEM.

PURPOSE: The use of positron emission tomography (PET) imaging has proved beneficial in the staging and diagnosis of several cancer disease sites. Additional applications of PET imaging in treatment planning and the evaluation of treatment response are limited by the relatively low spatial resolution of PET images. Including point spread function (PSF) information in the system matrix (SM) of iterative reconstruction techniques has been shown to produce improved spatial resolution in PET images. METHODS: In this study, the authors sampled the spatially variant PSF at over 6000 locations in the field of view for a General Electric Discovery ST PET/CT (General Electric Healthcare, Waukesha, WI) scanner in 2D acquisition mode. The authors developed PSF blurred SMs based on different combinations of the radial, depth, and azimuthal spatial dependencies to test the overall spatial dependence of the PSF on image quality. The PSF blurred SMs were included in a LOR-OSEM reconstruction algorithm and used for image reconstruction of geometric phantoms. The authors also examined the effect of sampling density on PSF characterization to design a more efficient sampling scheme. RESULTS: The authors found that depth dependent change in the amplitude of the detector response was the most important factor affecting image quality. A SM created from a PSF that introduced r (perpendicular to the LOR), d (parallel to the LOR), or r and d dependent blurring across the radial lines of response led to visually identifiable improvements in spatial resolution and contrast in reconstructed images compared to images reconstructed with a purely geometric SM with no PSF blurring. Images reconstructed using a SM with r and d dependent blurring across the radial lines of response showed improved spatial resolution and contrast-noise ratios compared to images reconstructed with a SM that had only r dependent blurring. Additionally, the authors determined that the PSF could be adequately characterized with roughly 85% fewer samples through the use of a better optimized sampling scheme. CONCLUSIONS: PET image reconstruction using a SM made from an accurately characterized PSF that accounts for r and d dependencies results in improved spatial resolution and contrast-noise relations, which may aid in lesion boundary detection for treatment planning or quantitative assessment of treatment response.

Monte Carlo modelling of energy deposition in trabecular bone.

This study includes the design and testing of a program that creates quadric-based geometric models of the trabecular region, designed specifically for use with the 2005 version of the Monte Carlo radiation transport code PENELOPE. Our model was tested, by comparison with published data, in two aspects: the distributions of path lengths throughout the geometry and absorbed fraction values from the monoenergetic emission of electrons from within our geometry. In both comparisons, our results show a close agreement with published methods.

Heavy ion track structure simulations in liquid water at relativistic energies.

Interaction cross sections for bare heavy charged (HZE) particles are obtained from proton interaction cross sections by scaling laws. Proton interaction cross sections are calculated within the (relativistic) plane wave Born approximation and the modelled dielectric response function of liquid water. Relativistic polarisation effects (Fermi density effect) are discussed. The interaction model is implemented into the biophysical track structure simulation code PARTRAC.

SU-E-T-125: High-Resolution Linear Diode Array for Use in Stereotactic Beam Commissioning.

PURPOSE: Task Group 106 discusses detector arrays and their usefulness in "soft-wedge" profile measurements. This study verifies an extension of their use; where a high-resolution linear diode array is used to measure profile scans for use in commissioning stereotactic beam. Small-field data is acquired with the LDA-99sc (IBA Dosimetry America, Memphis, TN), an array of 99 two-mm-diameter Hi-pSi diodes. These data are compared to data collected during serial scanning with an IBA SFD stereotactic diode. METHODS: Field measurements were taken at varying depths using an LDA-99sc in 1×1 cm2 , 2×2cm2 , and 3×3cm2 fields of a Trilogy accelerator (Varian Medical Systems, Palo Alto, CA). These scans were duplicated in resolution and sample rate with an SFD scanning diode. Both techniques acquired point data in 1.0mm increments. RESULTS: In identical conditions, the LDA-99sc provided readings with significantly less signal fluctuation than with traditional diode scanning; which is attributed to the reduction of water motion associated with detector travel during traditional scanning. This smoother data precluded the use of smoothing and filtration algorithms; allowing the beam to be modeled using raw data. The LDA-99sc measured an average penumbra of 0.1 mm wider than that which was measured with the diode. The LDA-99sc also measured an average field size of 0.3mm wider than that which was measured with the diode. With respect to time, a single profile scan (30cm-depth, 3×3cm2 field) using the LDA-99sc took 31 seconds, while a diode scan took 229 seconds. CONCLUSIONS: The LDA99 provided accurate commissioning-grade small-field beam profile data, as verified by comparison with diode scans, with a reduction of uncertainty in readings and a substantial reduction in time. Variation in penumbra and beam widths were deemed acceptable and can be attributed to the LDA-99sc data being raw, while the diode data was processed. Research supported in-part by IBA Dosimetry America.

SU-E-J-40: Lung Lesion Tracking Using Fixed-Spaced Non-Migrating Fiducial Markers in Robotic Radiosurgery.

PURPOSE: With lung lesion treatment being a major indication of the use of the CyberKnife (CK) robotic radiosurgery system (Accuray Inc, Sunnyvale, CA), the ability for the CK's stereoscopic kV imaging system to accurately track implanted fiducial markers becomes vital in the accurate delivery of therapeutic radiation. This study examines a novel fixed-space fiducial marker delivery system which is capable of delivering two VISICOIL (IBA Dosimetry America, Memphis, TN) non-migrating fiducial markers simultaneously at a fixed spacing through a single 20-gauge needle. More specifically, presented herein is a preliminary study which tests the CK'streatment localization system's (TLS) ability to track markers of varying size and spacing, comparing subsequent stereoscopic kV imaging to DRR's generated during the planning stages. METHODS: Three markers were placed in an XLT Lung Phantom (CIRS Inc, Norfolk, VA); two markers inserted along a diagonal line in a coronal plane, separated by biocompatible spacersof varying size, and a common third marker being placed in a non-varyinglocation in a coronal plane anterior to the marker pair. This third marker allows the calculation of rotational and translation corrections. Differentcombinations were scanned, planned, and simulated; 3.5mm- and 5.0mm- long markers, each 5mm in diameter, were separated by 15mm, 17mm, and 20mm spacers. RESULTS: The TLS system was able to track each of the aforementioned configurations with standard lung imaging parameters.Longer markers were not included in the study since earlier studies showed that without the natural deformation that would occur upon implantation, the length would induce false tracking. CONCLUSIONS: This is a necessary firststep in determining the minimum spacing with which the CK's TLS can track, a study which can now proceed with the use of phantom treatments elivered to orthogonally-overlapping radiochromic film, bisecting a tumor volume which is implanted with this fiducial marker system. Research supported in-part by IBA Dosimetry America.

SU-E-T-468: Gamma Knife Perfexion Dosimetry: A Monte Carlo Model of One Sector.

PURPOSE: We have implemented a Monte Carlo (MC) based dose computation model of one sector of the Gamma Knife Perfexion (GK PFX) using the Penelope MC dosimetry codes. The single sector simulation was rotated about the z-axis to model all eight GK sectors. GK dosimetric aspects examined include: 1) output factors (OF) for each of the three GK collimator sizes (4, 8, 16 mm), 2) OFs for each source row and collimator size, and 3) dose distribution profiles along the x- and z-axes, compared to film measurements and dose calculations from the Leksell GammaPlan (LGP) workstation. METHODS: We defined the internal GK PFX geometry in Penelope with the aid of vendor-supplied proprietary information. A single source per row was modeled for five rows for each of the 3 collimators (15 beams modeled). MC simulations were carried out on a Linux cluster. Phase space files (PSFs) were collected for the 15 modeled collimators then rotated about the z-axis to model the sector of 24 sources per collimator. 3D dose distributions from the MC model, film, and LGP DICOM-RT dose exports were analyzed using Matlab. For OF calculations, a 16 cm diameter dosimetry sphere was modeled with a virtual detector volume at its center. RESULTS: Good agreement is found for row- and total-output factors (greatest deviation of any type < 4%) compared to reference values. Off-axis factors closely follow LGP predicted dose distributions along the x-axis and differ on the inferior side of the z-axis. CONCLUSIONS: Detailed geometric representations (radiation source, device components) of the GK PFX are required for high fidelity MC simulations. Calculated GK PFX OF values depend on the simulated detector volume size (4 mm OF most dependent). Our model shows strong agreement for the GK PFX OFs and dose profile curves compared to reference values. Non-disclosure agreement for proprietary information with Elekta AB. No financial contribution.