kV Beam Research Articles

Structural characterization of He ion microscope platinum deposition and sub-surface silicon damage

In this paper we studied helium ion beam induced deposition (HIBID) of Pt on a silicon wafer using the recently commercialized helium ion microscope (HIM) at 25 kV and low beam currents. The motivation of this work was to understand the impact of light, inert helium ions on deposition rate and structure purity, with some implications on the usefulness of HIM nano-machining for circuit modification. Two Pt-rich deposits with sub-micron dimensions were grown with HIBID at different ion beam currents. The pillar and substrate structure were studied using bright and dark field TEM images. The authors analyzed metal purity profile of the HIBID deposit on height using energy dispersive x-ray spectroscopy. The maximum Pt content measured reached 41%, which is the highest measured metal content of a HIBID-grown structure. TEM studies of the sub-surface damage to the Si shows more damage below the deposit grown at a higher beam current. The differences in amorphization layer thickness between the two different beam currents are discussed. A comparison to Pt deposition by Ga FIB and electron beam induced deposition is provided, along with conclusions regarding the usage of HIBID technology for circuit modification.

SU-E-T-163: Thin-Film Organic Photocell (OPV) Properties in MV and KV Beams for Dosimetry Applications.

To characterize dosimetric properties of low-cost thin film organic-based photovoltaic (OPV) cells to kV and MV x-ray beams for their usage as large area dosimeter for QA and patient safety monitoring device. A series of thin film OPV cells of various areas and thicknesses were irradiated with MV beams to evaluate the stability and reproducibility of their response, linearity and sensitivity to absorbed dose. The OPV response to x-rays of various linac energies were also characterized. Furthermore the practical (clinical) sensitivity of the cells was determined using IMRT sweeping gap test generated with various gap sizes. To evaluate their potential usage in the development of low cost kV imaging device, the OPV cells were irradiated with kV beam (60-120 kVp) from a fluoroscopy unit. Photocell response to the absorbed dose was characterized as a function of the organic thin film thickness and size, beam energy and exposure for kV beams as well. In addition, photocell response was determined with and without thin plastic scintillator. Response of the OPV cells to the absorbed dose from kV and MV beams are stable and reproducible. The photocell response was linearly proportional to the size and about slightly decreasing with the thickness of the organic thin film, which agrees with the general performance of the photocells in visible light. The photocell response increases as a linear function of absorbed dose and x-ray energy. The sweeping gap tests performed showed that OPV cells have sufficient practical sensitivity to measured MV x-ray delivery with gap size as small as 1 mm. With proper calibration, the OPV cells could be used for online radiation dose measurement for quality assurance and patient safety purposes. Their response to kV beam show promising potential in development of low cost kV radiation detection devices.

SU-E-J-39: Minimizing IGRT Imaging Exposures: KV Radiograph Vs. KV-CBCT Vs. MV Portal Images.

To compare the IGRT doses from MV, kV and CBCT images. kV imaging systems integrated into Varian Trilogy and TrueBeam accelerators were modeled using BEAMnrc/DOSXYZnrc Monte Carlo codes and the dose to calibration phantoms for a variety of kV beams(kVp, bow-tie filters, etc.) were calculated. The doses to the same phantoms and kV beams were then measured experimentally using calibrated ion-chambers.The "calibrated" Monte Carlo kV beams were used to calculate dose to CT images of patients. Organ doses were analyzed using DVHs. The doses to the prostate are 0.015 and 2.2cGy using AP kV and MV images; are 0.06 and 2.3cGy using lateral kV and MV images; and, are 1.7 cGy using CBCT images. For head and neck images, the doses to the eye are 0.08 and 0.001 cGy using AP and PA kV images; are 2.3 and 1.8cGy using AP and PA MV images; are 0.001 and 2.4cGy for lateral kV and MV images; and, are 0.2 cGy for CBCT images. For kV radiographs, organ doses can be further reduced, by over 30%, by using bow-tie filters. CBCT doses to the prostate are 1.6 and 0.9cGy for OBI and TrueBeam pelvis scans; a >40% dose reduction for the same image quality. For OBI CBCT head scans the doses to the eye and brain stem are 0.2 and 2.8cGy, respectively. Due to the low penetration of kV beams,selecting beam angles so that the sensitive organs are near the beam exit, and/or using bow-tie filters, can substantially reduce organ doses when using kV radiographs. For daily positioning of pediatric brain patient with a set of orthogonal kV images, a CBCT scan, or a set of orthogonal MV images, the doses to the eyes are 0.1, 0.2, and 4.7 cGy, respectively.

TH‐C‐BRA‐02: A Novel Technology for Volume‐Of‐Interest KV Cone Beam CT

Purpose: In this work we describe the development of a new technology allowing volume‐of‐interest (VOI) kV CBCT. A device for dynamic kV beam collimation is described and initial imaging and dosimetric results are given. Methods: We have manufactured a prototype of an iris aperture capable of tracking a chosen VOI during kV CBCT imaging. The aperture and housing is compact and occupies an area comparable to the bow‐tie filter when mounted to the source of an on‐board imaging system. The system allows two‐dimensional translation of the aperture as a function of gantry angle and dynamic adjustment of iris diameter. Iris leaves are non‐overlapping and therefore can be made sufficiently thick to attenuate higher energy kV beams used. The device was tested for functionality on a Varian OBI system. Dose reduction in the VOI was measured and compared to full‐field imaging, for planar and CBCT imaging protocols, as a function of iris dimension. Initial projection and CBCT images of a head phantom were acquired. Results: The system provides a versatile tool for dynamic collimation of the kV beam during CBCT. While dose reduction in VOI CBCT is achieved primarily by blocking of non‐VOI anatomy, the dose inside the imaged VOI is also reduced substantially, by 5% to 60% for 20 cm and 2 cm iris diameters, respectively. The magnitude of dose reduction increases slightly with increased tube potential, between 65 and 125 kVp. Projection images demonstrate that the iris produces effective collimation with minimal leakage. Reconstructed CBCT images are affected by truncation artefact, but this may be remedied by data filling techniques or other (e.g. pi‐line) reconstruction algorithms. Conclusions: The technology presented is a novel approach to kV CBCT, localizing imaging dose to a chosen VOI while, compared to full‐field CBCT, reducing dose both inside and outside of the VOI. No conflict of interest exists.

SU-E-J-03: Positioning Errors of Metal Localization Devices with Motion Artifacts on KV and MV Cone Beam CT.

To investigate motion artifacts of kV CBCT and MV CBCT images on metal localization devices for image guided radiation therapy. 8 MU pelvis CBCT template for Siemens Artiste MVision and Pelvis template for Varian IX on-board Exact Arms kV were used to acquire CBCT images in this study. Images from both CBCT modalities were compared in CNRs, metal landmark absolute positions, and image volume distortion on three different planes of view. The images were taken on a breathing-simulated thoracic phantom in which several typical metal localization devices were implanted, including clips and wires for breast patients, gold seeds for prostate patients, and BBs as skin marks. To magnify the artifacts, a 4cm diameter metal ball was also implanted in the thoracic phantom to mimic the metal artifacts. The amplitude of the sinusoidal breathing was 1cm, and the period varies from 2sec, 4sec to 8sec. For MV CBCT, the CNR at 4sec breathing cycle with 1cm breathing amplitude was 5.0, 3.4 and 4.6 for clips, gold seeds and BBs, respectively while it was 1.5, 2.0 and 1.6 for kV CBCT. On the images, kV CBCT showed symmetric streaking artifacts both in the transverse and longitudinal directions relative to the motion direction. kV CBCT images predicted 89% of the expected volume, while MV CBCT images predicted 95% of the expected volume. Simulated soft tissue observed in MVCT cannot be detected in kVCT. MV CBCT images showed better volume prediction, less streaking effects and better CNRs of a moving metal target, i.e. clips, BBs, gold seeds and metal balls than kV CBCT images. MV CBCT was more advantageous compared to kV CBCT with less motion artifacts for metal localization devices.

SU-E-I-106: Description of Energy Dose Deposition Kernel for the Diagnostic Beam.

Over the recent years, the employment of kV x-rays as a diagnostic tool into clinical routine has resulted in a significant increase in the patient's exposure to ionizing radiation. The accurate determination of the absorbed dose to patients during diagnosis is therefore necessary to avoid unnecessary exposure to the patient. This study presents an analytical model of the energy deposition kernel for the monoenergetic and polyenergetic kV beams for the fast calculation of dose in radiography. The analytical model is based on the pencil beam kernels derived from Monte Carlo simulations. DOSXYZnrc code from the EGSnrc family was employed to simulate the pencil beam of 0.1 cm width for 80, 100 and 120 keV mono-energetic and polyenergetic beams. The lateral dose profiles were calculated at different depths within a homogenous water phantom of size 50×50×50 cm3 . The evaluated dose profiles showed a high amplitude primary component at the central axis and a long range low amplitude scatter component spanning a considerable distance from the central axis. The profiles were fitted analytically with a triple exponential decay function with an offset. All coefficients of the exponential function were further fitted with appropriate analytical functions to represent their behavior relative to depth and photon energy. The accuracy of the obtained kernel was checked by the convolution of a rectangular fluence profile and comparing the calculated dose distribution with the Monte Carlo simulated dose profiles for 2×2 cm2 field size. In a homogeneous phantom, the comparisons of the convolution method and Monte Carlo simulations showed sufficient agreement except for largest depths (deviation approx. 15%). Future developments will focus on an implementation of the method for dose calculation in the patient.

SU-E-I-45: Measurement of CT Dose to An HDPE Phantom Using Calorimetry: A Feasibility Study.

Radiation dose in CT is traditionally evaluated using an ionization chamber calibrated in terms of air kerma in a phantom of specific dimensions. The radiation absorbed dose, J/kg, can also be realized directly by measuring the temperature rise in the medium. We investigate using this primary method to determine the CT dose at a point (a few mm), using the recently proposed (APMM TG220) high density polyethylene (HDPE) phantom as a medium. The calorimeter detection scheme is adapted from the second generation NIST water calorimeter using sensitive thermistors in a Wheatstone bridge powered by a lock-in amplifier. The temperature sensitivity is about 3 microK. The expected temperature rise in PE is about 0.6 mK per Gy. The thermistor sensors were placed inside a 26 cm dia. × 10 cm HDPE phantom. Two preliminary tests were made: at a linear accelerator with a 6 MV photon beam, and at a 16-slice CT scanner with a 120 kV beam, each with the thermal sensor and with a calibrated ionization chamber. The 6 MV photon beam with 10 on/off cycles at 60 s each yielded the (uncorrected) run-to-run average dose of 3.06 Gy per cycle (sdm 0.3%), about 8% higher than the Result from the ionization chamber (calibrated in terms of absorbed to water). The CT measurements were also made in the middle section of the TG200 30 cm phantom. Twenty consecutive axial scans at 250 mA, which delivers a nominal accumulated dose (CTDIvol) of 705 mGy in 50 s at three axial and three radial locations were measured. The accumulated dose measured by the ionization chamber at the center of the smaller phantom was 347 mGy. The calorimeter data show qualitative tracking of the chamber measurements. Detailed thermal and electrical analysis of the system are planned to obtain quantitative results.

A megavoltage scatter correction technique for cone-beam CT images acquired during VMAT delivery

Kilovoltage cone-beam CT (kV CBCT) can be acquired during the delivery of volumetric modulated arc therapy (VMAT), in order to obtain an image of the patient during treatment. However, the quality of such CBCTs is degraded by megavoltage (MV) scatter from the treatment beam onto the imaging panel. The objective of this paper is to introduce a novel MV scatter correction method for simultaneous CBCT during VMAT, and to investigate its effectiveness when compared to other techniques. The correction requires the acquisition of a separate set of images taken during VMAT delivery, while the kV beam is off. These images—which contain only the MV scatter contribution on the imaging panel—are then used to correct the corresponding kV/MV projections. To test this method, CBCTs were taken of an image quality phantom during VMAT delivery and measurements of contrast to noise ratio were made. Additionally, the correction was applied to the datasets of three VMAT prostate patients, who also received simultaneous CBCTs. The clinical image quality was assessed using a validated scoring system, comparing standard CBCTs to the uncorrected simultaneous CBCTs and a variety of correction methods. Results show that the correction is able to recover some of the low and high-contrast signal to noise ratio lost due to MV scatter. From the patient study, the corrected CBCT scored significantly higher than the uncorrected images in terms of the ability to identify the boundary between the prostate and surrounding soft tissue. In summary, a simple MV scatter correction method has been developed and, using both phantom and patient data, is shown to improve the image quality of simultaneous CBCTs taken during VMAT delivery.

H− radio frequency source development at the Spallation Neutron Source

The Spallation Neutron Source (SNS) now routinely operates nearly 1 MW of beam power on target with a highly persistent ∼38 mA peak current in the linac and an availability of ∼90%. H(-) beam pulses (∼1 ms, 60 Hz) are produced by a Cs-enhanced, multicusp ion source closely coupled with an electrostatic low energy beam transport (LEBT), which focuses the 65 kV beam into a radio frequency quadrupole accelerator. The source plasma is generated by RF excitation (2 MHz, ∼60 kW) of a copper antenna that has been encased with a thickness of ∼0.7 mm of porcelain enamel and immersed into the plasma chamber. The ion source and LEBT normally have a combined availability of ∼99%. Recent increases in duty-factor and RF power have made antenna failures a leading cause of downtime. This report first identifies the physical mechanism of antenna failure from a statistical inspection of ∼75 antennas which ran at the SNS, scanning electron microscopy studies of antenna surface, and cross sectional cuts and analysis of calorimetric heating measurements. Failure mitigation efforts are then described which include modifying the antenna geometry and our acceptance∕installation criteria. Progress and status of the development of the SNS external antenna source, a long-term solution to the internal antenna problem, are then discussed. Currently, this source is capable of delivering comparable beam currents to the baseline source to the SNS and, an earlier version, has briefly demonstrated unanalyzed currents up to ∼100 mA (1 ms, 60 Hz) on the test stand. In particular, this paper discusses plasma ignition (dc and RF plasma guns), antenna reliability, magnet overheating, and insufficient beam persistence.

Position detection accuracy of a novel linac-mounted intrafractional x-ray imaging system

The authors have developed a system that monitors intrafractional target motion perpendicular to the treatment beam with the aid of radioopaque markers by means of separating kV image and megavoltage (MV) treatment field on a single flat-panel detector. They equipped a research Siemens Artiste linear accelerator (linac) with a 41 × 41 cm(2) a-Si flat-panel detector underneath the treatment head. The in-line geometry allows kV (imaging) and MV (treatment) beams to share closely aligned beam axes. The kV source, usually mounted directly across from the flat-panel imager, was retracted toward the gantry by 13 cm to intentionally misalign kV and MV beams, resulting in a geometric separation of MV treatment field and kV image on the detector. Two consecutive images acquired within 140 ms (the first with MV-only and the second with kV and MV signal) were subtracted to generate a kV-only image. The images were then analyzed "online" with an automated threshold-based marker detection algorithm. They employed a 3D and a 4D phantom equipped with either a single radioopaque marker or three Calypso beacons to mimic respiratory motion. Measured room positions were either cross-referenced with a phantom voltage signal (single marker) or the Calypso system. The accuracy of the back-projection (from detected marker positions into room coordinates) was verified by a simulation study. A phantom study has demonstrated that the imaging framework is capable of automatically detecting marker positions and sending this information to the tracking tool at an update rate of 7.14 Hz. The system latency is 86.9 ± 1.0 ms for single marker detection in the absence of MV radiation. In the presence of a circular MV field of 5 cm diameter, the latency is 87.1 ± 0.9 ms. The total RMS position detection accuracy is 0.20 mm (without MV radiation) and 0.23 mm (with MV). Based on the evaluated motion patterns and MV field size, the positional accuracy and system latency indicate that this system is suitable for real-time adaptive applications.

Stability of hydrogen incorporated in ZnO nanowires by plasma treatment

The stability of hydrogen in ZnO is studied using hydrogenated nanowires by plasmatreatment. Enhanced near band edge UV emission and reduced defect level green emissionis observed after hydrogen plasma treatment. Through thermal stability tests,this effect is found to be stable at room temperature and nearly stable up to ∼ 500 K, but begins to deteriorate at higher temperature. The study of the irradiation stability ofthe hydrogen in ZnO nanowires shows that the hydrogen is stable under an electron beamwith an accelerating voltage lower than 5 kV, but is not stable under 10 kV or underan intensive laser beam. The results could benefit the further understanding ofthe role of hydrogen in ZnO and light-emitting devices based on hydrogenatedZnO.

Acquisition of MV-scatter-free kilovoltage CBCT images during RapidArc™ or VMAT

PurposeTo perform kilovoltage (kV) cone beam computed tomography (CBCT) imaging concomitant with the delivery of megavoltage (MV) RapidArc treatment, and demonstrate the feasibility of obtaining MV-scatter-free kV CBCT images. Methods and materialsRapidArc/CBCT treatment and imaging plans are designed, and delivered on the Varian TrueBeam, using its Developer Mode. The plan contains 250 control points for MV-radiation delivery, each over an arc of 0.4–0.7o. Interlaced between successive MV delivery control points are imaging control points, each over an arc of 0.7–1.1o. During the 360o gantry rotation for the RapidArc delivery, CBCT projections of a phantom are acquired at 11 frames per second. The kV projections with minimal MV-scatter are selected, based on gantry angle, and the CBCTs image reconstructed. For comparison, a reference CBCTr image is acquired in the normal way. In addition, to examine the effect of MV-scatter we acquire CBCTc using the same treatment plan without the imaging control points, i.e. with continuous MV delivery during the 360o rotation. Quantitative evaluation of image qualities is performed based on the concepts of CNR (contrast-to-noise ratio) and NSTD (normalized standard deviation). ResultsThe different types of CBCT images were reconstructed, evaluated, and compared. Visual comparison indicates that the image quality of CBCTs is similar to that of the reference CBCTr, and that the quality of CBCTc is significantly degraded by the MV-scatter. Quantitative evaluation of the image quality indicates that MV-scatter significantly decreases the CNR of CBCT (from ∼7 to ∼3.5 in one comparison). Similarly, MV-scatter significantly increases the inhomogeneity of image intensity, e.g. from ∼0.03 to ∼0.06 in one comparison. ConclusionWe have developed a method to acquire MV-scatter-free kV CBCT images concomitant with the delivery of RapidArc treatment. Engineering development is necessary to improve the process, e.g. by synchronization of the MV and kV beams.

TH-E-BRC-10: Accuracy of Monte Carlo Dose Calculations with Kilovoltage Photon Beams

Purpose: To assess the importance of tissue segmentation for Monte Carlo(MC)dose calculations with kilovoltage photonbeams and to suggest a new segmentation method based on effective atomic number differences. Methods: MCdose to 34 ICRU‐44 tissues was calculated in a phantom irradiated by a number of kilovoltage beams using the EGSnrc/BEAMnrc and DOSXYZnrc codes. Photonbeams currently used in small animal radiotherapy were studied: a microCT 120 kV beam, two 225 kV beams filtered with either 4 mm of Al or 0.5 mm of Cu, a heavily filtered 320 kV beam, and a 192Ir beam.MCdose was calculated for a five 120 kV beam treatment plan for a mouse with lungcancer. The microCT images were segmented using three different segmentation schemes — with 4 (the DOSXYZnrc default), 8, and 39 tissue types. The 4‐tissue and 8‐tissue segmentation schemes were based only on mass density differences. The new advanced 39‐tissue segmentation scheme is based on the combination of both mass density differences and atomic number differences. Results: Using our model we showed that mis‐assignment of adipose to cartilage caused dose calculation differences of 36%, 24%, and 14% for the 120 kV beam and the 225 kV beams filtered with 4 mm Al and 0.5 mm Cu, respectively. A second order polynomial approximated well the absorbed dose to tissue as a function of the effective number for all beams. The small animal study showed that the doses to adipose and bone were overestimated by 30–50% when the 4‐tissue segmentation was used compared to the 8‐ tissue and 39‐tissue segmentation schemes. The dose to the lungs and bone were underestimated by 5–10% with the 39‐tissue segmentation compared to the 8‐tissue segmentation. Conclusions: A new tissue segmentation method was proposed for a more accurate MCdose calculations with kilovoltage beams.

SU‐E‐T‐668: Comparison of the Physical Characteristics of Secondary Electrons and Dose Enhancement from X‐Ray Irradiation of Gold Nanoparticles Using Monte Carlo Simulation

Purpose: To investigate secondary electron production in gold nanoparticle (GNP) aided radiotherapy through Monte Carlo simulations, thereby advance the understanding of dose enhancements and their role in improving cell kill. Methods: Using the Geant4 toolkit, simulations were performed with four polyenergetic sources, namely 50 kVp, 250 kVp, Cobalt‐60, and 6 MV, to irradiate a gold sphere of diameters 2, 50, or 100 nm in water. The energy of the secondary electrons and the frequency and type of physics interaction that created them were tracked. This allowed calculation of the absorbed dose and energy deposition inside and outside of the nanoparticle. Results: The presence of a GNP can enhance the production of electrons by approximately 3 orders of magnitude at kV beam energies. For MV beams, the increase in electron production was approximately 10 folds. Considerable dose enhancement occurred when gold was present, at approximately 1000 folds for kV beams, and between 2 to 7 folds for MV beams. The energy deposited was calculated to compare how many additional electrons were generated and take into account their energies. In the kV beams, the addition of a GNP caused significantly greater deposited energy surrounding the nanoparticle by 2 to 3 orders of magnitude. For the MV beams, the increase was approximately 5 folds. The proportion of energy deposition inside the GNP versus the outside was also analyzed. At greater nanoparticle diameters, a larger portion of the overall deposited energy resided within the nanoparticle. Conclusions: We present simulation results that show the presence of GNPˈs can considerably increase the dose and energy deposition outside the nanoparticle, especially at kV energies. This enhancement is also present for MV beams, although to a lesser degree.

TU-C-214-08: Position Detection Accuracy of a Novel Linac-Mounted Intra-Fractional X-Ray Imaging System

Purpose: We have developed a system that monitors intra‐fractional target motion perpendicular to the treatment beam with the aid of metallic markers by means of separating kV image and MV treatment field on a single flat‐ panel detector. Methods: We equipped a research Siemens Artiste linac with a 41×41cm2 a‐Si flat‐panel detector underneath the treatment head. The inline geometry allows kV (imaging) and MV (treatment) beams to share closely aligned beam axes. The kV source, usually mounted directly across from the flat‐panel imager, was retracted towards the gantry by 13cm to intentionally misalign kV and MV beams, resulting in a geometric separation of MV treatment field and kV image on the detector. Two consecutive frames acquired within 140ms (the first with MV‐only and the second with kV and MV) were subtracted to generate a kV‐only image. The images were then analyzed ‘online’ with an automated threshold‐based marker detection algorithm. We employed a 2D phantom equipped with either a single metallic marker or three Calypso beacons to mimic respiratory motion. Measured room positions were either cross‐referenced with a phantom voltage signal (single marker) or the Calypso system. Results: A phantom study has demonstrated that our imaging framework is capable of automatically detecting marker positions and sending this information to our tracking tool at an update rate of 7.14Hz. The system latency is 111.35+/−2.6ms for single marker detection in the absence of MV radiation. In the presence of a 7×7cm2 300MU/min MV field, the latency increases to 117.23+−12.57ms. The position accuracy is −0.19+/−0.21mm (without MV radiation) and −0.26+/−0.27mm (with MV). The detection of three markers increased latency by 10–15ms. Conclusions: Positional accuracy and system latency strongly indicate that this system is suitable for real‐time adaptive applications. In the future, we hope to utilize the full imageinformation and substitute metallic with anatomical markers. Supported by Siemens Healthcare/CR.

SU‐E‐I‐152: CCD Vs CdTe X‐Ray Methods to Estimate Fat Content in Breast Biopsies

Purpose: This work focuses on analyzing CCD vs CdTe x‐ray methods to estimate fat in breast biopsies. Methods: A digital specimen radiography system consisting of a 2″× 2″ phosphor coupled to a 1″ × 1″ CCD was used. We obtained ADU signals as a function of the energy incident signals for different plastic biopsies each 5 mm in diameter and of varying thicknesses. Simulations were performed for 231 combinations of polycarbonate (lexan), lucite and polyethylene (polyet). These plastics mimic respectively fibrous tissue, cancer, and fat. The thickness for heterogeneous biopsies was restricted to 5mm. The average ADU and sigma calculated for 21 subgroups of % polyet reveal significant overlaps between groups. The ADUmean values provide a way to estimate the % polyet experimentally. 26 kV (0.3 mA) beams and exposure times of 3.8 seconds gave X=7.5E‐4 C/kg. Region of interests (ROIs) consisting of 15960 pixels were generated and corresponded to a 1.36 mm diameter lesion in the object plane. Using a single pixel CdTe detector, the thickness of polyet can be estimated by assuming the biopsies are composed of two materials, namely polyet and a 50:50 mixture of lexan and lucite. 15 kV (0.15 mA) beams and 1 minute exposures yielded X=1.9E‐4 C/kg per measurement. The diameter of the beam at the sample is 1.36 mm. The weighted means of the estimated thicknesses were evaluated using data from 8 keV to 10.6 keV in steps of 0.2 keV. Results: The root mean square between estimated and actual % polyet for the CCD was 13.8 and 9.2 for the CdTe detector. Conclusions: The preliminary analysis shows a minor advantage using the CdTe based system. Extending the study to a pixilated CdTe (or CZT) detector could allow larger ROIs to be analyzed. We anticipate better results with breast tissue since contrast is higher.

SU‐E‐I‐70: Characterisation of the Parameters Defining a Kilovoltage Source for Accurate Dose Computation

Purpose: To perform a sensitivity analysis of the parameters that define beam quality for a kilovoltage (kV) x‐ray sourceMethods: Using a previously developed hybrid approach to calculate radiation dose deposited by x‐ray beams of energies <150 kVp, we computed dose inside a simulated heterogeneous phantom for varying beam spectra to evaluate the sensitivity of calculated dose to half‐value layer (HVL). The approach involves computing the primary photon component deterministically and scattered component stochastically, accounting for the real micro cross sections of the materials involved. We characterized the spectrum of the Varian® On‐Board Imaging® units at our institution by HVL measurements made using of a Farmer‐type Capintec ion chamber (0.06cc) in air. We compared doses computed with our characterized source spectrum with measurements inside water‐equivalent Gammex® Solid Water® phantom. Measurements were done using a 10×10 cm2 field at 100 cm SSD at every centimeter for depths 1 to 12 cm. Results: We found that measuring kV and HVL gives us sufficient beam quality information for accurate [>0.5%] x‐ray dose calculations. We also showed that HVL varies by less than 0.1 mm Al for field sizes over 5×5 cm2, allowing us to use one HVL irrespective of field size. Agreement between calculations and experimental measurements was better than 2% for a transverse profile and the central axis depth dose. This implies that our approach to characterizing the beam quality of a kV source is suitable for ensuring accurate results Conclusions: We have determined the parameters we need to characterize the source in order to obtain good agreement between theoretical calculations and experimental measurements. This is a crucial step in developing an independent tool to calculate patient dose from kV beams such as cone‐beam CT and brings us closer to our goal of calculating patient‐specific dose from imaging procedures.

MO‐F‐110‐04: Experimental Validation of a Novel Approach to Fast and Accurate Kilovoltage Dose Computation

Purpose:To validate a novel hybrid method for the calculation of kilovoltage (kV) x‐ray dose. Methods: We performed experimental validation of a novel hybrid approach for calculating dose deposited by imaging beams (<150 kVp). The approach involves computing the primary photon component deterministically and scattered component stochastically, accounting for the real micro cross sections of the materials involved. Depth dose and profile measurements for the Varian® On‐Board Imaging® unit in the radiographic mode using a 125 kVp x‐ray beam (4.6 mm Al half‐value layer) were performed. The measurements were made using a Farmer‐type Capintec ion chamber (0.06cc) in homogeneous and heterogeneous phantoms. A phantom consisting of Certified Therapy‐Grade Solid Water® (Gammex 457) was used to measure the dose under homogeneous conditions. The heterogeneous phantom was constructed using lung‐ equivalent material (Gammex 455) and bone‐equivalent material (Gammex 450). Results: Measurements in the homogeneous phantom agreed with theoretical calculations within 2% for depth doses and within 2% of the central axis dose for profiles. Local agreement in highly attenuated regions such as depths >10 cm or outside the beam edge were about 10%. However, the dose differences in these regions were less than 2% of the maximum dose. Conclusions: This work provides experimental validation of our hybrid calculation method. The next step will be to test the algorithm using more complex phantoms, such as the anthropomorphic Rando® phantom, which mimics patient geometry as well as inhomogeneities. This is a crucial step in the validation of an independent tool to calculate patient dose from kV beams such as cone‐beam CT and brings us closer to our goal of calculating patient‐specific dose from imaging procedures.

SU‐E‐J‐181: Evaluation of Motion Artifacts of Metal Localization Devices on KV and MV Cone Beam CT

Purpose: To investigate the strengths and weaknesses in motion effect of kV CBCT and MV CBCT for image guided radiation therapy with metal localization devices. Method and Materials: 15 MU pelvis CBCT template for Siemens Artiste MVision and Pelvis template for Varian IX on‐board Exact Arms kV were used to acquire CBCT images in this study. Images from both CBCT modalities were compared in CNRs, image landmark coordinates, and image volume distortion on three different planes of view. The images were taken on a breathing‐simulated thoracic phantom in which several typical metal localization devices were implanted, including clips and wires for breast patients, gold seeds for prostate patients, and BBs as skin marks. To magnify the artifacts, a 4cm diameter metal ball was also implanted in the thoracic phantom to mimic the metal artifacts. The thoracic phantom was placed on a home‐made moving phantom performing a sinusoidal motion in the patient superior‐inferior direction. The amplitude of the sinusoidal waves was 1cm, and the period varies from 2sec, 4sec to 8sec. Results: For MV CBCT, the CNR at 4sec breathing cycle with 1cm breathing amplitude was 5.0, 3.4 and 4.6 for clips, gold seeds and BBs, respectively while it was 1.5, 2.0 and 1.6 for kV CBCT. On the images, kV CBCT showed symmetric streaking artifacts both in the transverse and longitudinal directions relative to the motion direction. kV CBCT images predicted 89% of the expected volume, while MV CBCT images predicted 95% of the expected volume. Conclusion: MV CBCT images showed better volume prediction, less streaking effects and better CNRs of a moving metal target, i.e. clips, BBs, gold seeds and metal balls than kV CBCT images. MV CBCT was more advantageous compared to kV CBCT with less motion artifacts for metal localization devices.

SU‐C‐214‐03: Monte Carlo Simulation of Varian OBI Cone Beam CT (CBCT) and Dose Distribution in Head Phantom in 125kVp and 100kVp

Purpose: (1) To validate a Monte Carlo model for Varianˈs OBI CBCT in head scan mode and calculate the dose inside a Rando head phantom; (2) To compare the dose in head phantom for 125 kVp and 100 kVp scans. Methods: The BEAMnrcMP Monte Carlo package was employed and modified to simulate Varianˈs OBI CBCT system operated at 125kVp and 100kVp in full‐fan full‐bowtie mode for the same mAs and 200 deg scans only. Percentage depth dose (PDD) of a single KV beam was measured with a cc01 micro ion chamber in water tank. Dose profiles of the single KV beam at multiple depths including 0.5 cm were also measured with the same micro chamber. After the validation of the Monte‐Carlo model with the measured beam data, dose calculations were performed on a Rando head phantom. Results: The Monte Carlo simulated PDD curves agree very well with the measurements. Also, the simulated dose profiles matches well to the measured ones in both in‐plane and cross‐plane directions. Monte Carlo dose calculation made on a Rando head phantom indicated that the post right side of head get higher dose, due to the posterior KV scan from gantry 290 deg to 90 deg. The dose ratio between bone and soft tissue were ∼4.4 and ∼4.9 for 125kVp and 100kVp, respectively. The dose ratio between 125kVp and 100kVp is about 1.7, close to the theoretical value of 1.252=1.56. Conclusions: Our Monte Carlo model has been validated by measurements to simulate Varianˈs OBI CBCT system. It was shown that CBCT dose in bone is four times of that in soft tissue, such that the accumulated bone dose during a treatment course could be up to >100cGy in posterior area. Investigation on patient dose deposition due to kV CBCT is in progress.