kVp Beam Research Articles

SU-FF-I-77: Diagnostic X-Ray Beam Hardening as a Function of Patient Size

Purpose: To investigate changes in x‐ray energy spectrum as a function of depth in phantoms representing lean and overweight individuals. Methods: Three phantom torsos comprised of tissue equivalent material were modeled as elliptical cylinders using MCNPX. Two x‐ray beams used in planar radiography (80 kVp HVL = 2.42 mm Al; 140 kVp HVL = 5.15 mm Al) and one beam used in CT (140 kVp HVL =9.09 mm Al) irradiated each phantom and normalized photon spectrum was examined at several depths. Separate comparisons were also made with data presented by Birch et al who examined spectral changes as a function of depth but did not include Compton scatteredphotons.Results: Good agreement with data from Birch et al is obtained for the lean phantom when Compton scatteredphotons are not included. These results show substantial beam hardening as evidenced by a shift in the peak beam energy from 32 keV (entrance) to 54 keV (exit) for the 80 kVp beam and 40 keV to 64 keV for the 140 kVp (HVL=5.15 mm Al) beam. However, substantial beam hardening is not observed when Compton scatteredphotons are included in the measurement. For the 80 kVp beam, the average energy of the photons traversing the phantom changes by only 15 % in traversing the lean, 20 cm diameter thick phantom, and by only 18% in traversing the very overweight 45 cm thick phantom. For the hardest beam (140 kVp HVL=9.09 mm Al), average beam energy changes by only 7.4% in traversing through 45 cm tissue.Conclusion: Under conditions of broad beam geometry such as radiography, spectral hardening of the radiography and CT x‐ray beams examined is not substantial, even after traversal through very thick phantoms. However, when narrow beam geometry is approximated, spectral beam hardening is significant.

SU‐FF‐T‐302: Response of OneDoseTM and TLD100 Dosimeters Over 6MV to Pd‐103 Equivalent Energies

Purpose: Two widely available in vivo dosimeters are TLD and MOSFET detectors. Both detectors are significantly energy dependent, especially in the keV energy region. The purpose of this work is to measure the energy dependence of LiF:Mg,Ti TLD‐100 and OneDose™ (Sicel) MOSFET detectors with particular emphasis on low energy x‐rays and gamma‐rays. Methods: Eleven beams, covering an energy range 6MV down to equivalent of 103Pd x‐rays have been employed for this work. All detectors were irradiated in air to ∼50 cGy water‐equivalent dose. Other than the 60Co, beams were calibrated with Farmer ionization chambers just prior to detector irradiations. Dose rate for 60Co was based on clinical calibration data. For the 60Co and 137Cs beams, detectors were “sandwiched” between 5mm and 1.2mm buildups respectively. Thicknesses of the detectors were adequate to establish electronic equilibrium for the lower energy beams. To reduce uncertainty in the mean response, twelve samples were irradiated with each beam. The OneDose™ detectors have dimensions 6×33×1mm. TLD samples in the form of flat packs (10×10×0.3mm) were made by sealing ∼22mg powder in thin (0.06mm) poyethylene. Results: Since OneDose™ detectors were factory‐calibrated with 60Co, both OneDose™ and TLD responses were normalized to 60Co. For OneDose™, over‐response relative to 60Co is observed to be largest (3.50±0.04) at 100 kVp beam (HVL 4.14 mmAl), and negligible at high energies (137Cs to 6MV). For TLD, over‐ response at 75kVp beam (HVL 2.09 mmAl) is observed to be 1.53. More data at lower energies and at 192Ir energy are in progress. Conclusion: Response of TLD and OneDose™ relative to 60Co has been measured over a wide energy range (6MV down to equivalent of 103Pd x‐rays). For TLD, our measured results agree with the recently published values fairly closely. OneDose™ results appear reasonable when compared with other type of MOSFET detectors.

MO‐FF‐A1‐03: Investigation of Discrepancy in Reports On GafChromic EBT Energy Dependence

Purpose: Radiochromic films, such as GafChromic EBT, provide a convenient and versatile dosimeter, and are particularly useful for small field dosimetry. Due to previous reports showing energy independence, GafChromic EBT films were implemented as dosimeters across a wide range of energies. However, in recent years, reports showing energy dependence have been presented, contradicting previously published data. This discrepancy was investigated by comparing two batches of film manufactured 4 years ago to two recently manufactured batches. Method and Materials: Small pieces of film were irradiated to 100 cGy using a 6 MV and a 105 kVp beams. A 30 cm × 30 cm × 4.5 cm phantom equipped with optical fibers was used, with appropriate amount of Solid Water as backscatter. Net optical density was measured by two completely independent methods: a 10 nm range at main absorbance peak was used for real‐time measurements, and a red channel of an Epson 1680 flatbed scanner was used for the measurements performed 24 hours after irradiation. Results: The ratios of optical density (105 kVp to 6 MV) were markedly different between the old and new batches of film, both for real‐time and scanner measurements. While the ratio of the older films demonstrated energy independent within 10%, as previously reported, the newer films show approximately a 25% drop in response when orthovoltage energies were used. Conclusion: A statistically significant difference in energy dependence exists between older and newer GafChromic EBT films, confirming both the publications demonstrating energy independence and those showing a decrease in sensitivity at low energies. Chemical analysis is underway to investigate the potential causes of the observed results.

TH-D-351-06: Comparison Between 2D Monte Carlo Modeled and Experimental Cone-Beam CT X-Ray Projections

Purpose: Fast and accurate modeling of cone-beam CT (CBCT) x-ray projection data can improve cone-beam CT (CBCT) image quality either by conditioning projection data prior to image reconstruction or by supporting rigorous comparative simulation studies of competing image reconstruction and processing algorithms. In this study, we compare Monte Carlo- computed x-ray projections with projections experimentally acquired from our Varian Trilogy CBCT imaging system for phantoms of known design. Method and Materials: Our recently developed Monte Carlo photon-transport code, PTRAN, was used to compute primary and scatter projections for cylindrical phantoms of known diameter (CatPhan and NA model 76-410) with and without bow-tie filter and antiscatter grid for both full- and half-fan geometries. The simulations were based upon measured 120 kVp spectra, beam profiles, and flat-panel detector (4030CB) point-spread functions. The beam-stop array method was used to acquire scatter and SPR distributions from the OBI images. The biasing of scatter measurements due to the long detector PSF tails was corrected either by a lead mask or by deconvolution. Computed projections were compared to flat- and dark-field corrected 4030CB images. Results: The simulated primary profiles agree with experiment within 3%, while the simulated scatter profiles agree within 8–10%. Both PSF measurements and mask measurements indicate that scatter radiation values can be biased by as much as 7% detector PSF tails. Conclusion: In agreement with the literature, the difference between simulated and measured projection data is of the order of 6–8%. Higher accuracy can be achieved mainly by improving the beam modeling and correcting the non linearities induced by the detector PSF. This project was supported in part by grants from Varian Medical Systems and NCI (R01 CA 75371).

SU‐GG‐J‐70: Development of An Image‐Guided Conformal Small Animal Irradiation Platform

Purpose: To develop an image‐guided conformal small animal irradiation system for pre‐clinical studies. The initial dosimetric and imaging properties of the system have been characterized. Method and Materials: The system consists of a 225 kVp x‐ray tube (GE Isovolt, maximum power 3 kW, focal spot=0.4mm–3.0mm), and 20.5×20.5 cm amorphous silicon flat panel detector (512×512 pixels, 7 frames‐per‐second) mounted on a rotating gantry. The system can rotate 360°, at a maximum speed of 3 RPM. The source‐isocenter distance is 31 cm, and the source‐imager distance is 62 cm. Cone‐beam CT images are generated from projections over half or full rotation, and reconstructed using filtered‐back projection. A motorized stage allows translational movement of specimen. Initial dosimetric characterization of the system has been done using EBT Gafchromic film in a solid water phantom at 2 cm depth. The unit is free standing and self‐shielded. Results: The maximum dose rate achievable at isocenter is 4.5 Gy/min for 220 kVp unfiltered beam, and 2 Gy/min for 220 kVp beam with 0.5mm added Cu filtration (HVL = 1.0mm Cu). The maximum dose rate for 100 kVp is 2.5 Gy/min (unfiltered) and 1.5 Gy/min with 2mm added Al filtration (HVL = 3.1 mm Al). Cone‐beam CT images are generated with a resolution of 0.2×0.2×0.2 mm. Conclusion: Image‐guided conformal pre‐clinical irradiation systems will enable novel translational research in radiation oncology and radiobiology. The presented system has been demonstrated to provide the dose rate, mechanical stability and image‐guidance capabilities necessary to facilitate such research.

Measurement of radiotherapy superficial X-ray dose under eye shields with radiochromic film

Accurate measurement and knowledge of dose delivered under eye shield during superficial X-ray radiotherapy is required for patient peripheral dose assessment. Critical structures can include the cornea, lens and retina. Measurement of dose under eye shields has been historically performed with Thermoluminescent Dosimeters (TLD's) due to their small size and design. Restrictions include the energy dependence and the fact that they only provide a point dose assessment. This note investigates the use of a low energy dependence radiochromic thin film for measurement of dose under eye shields in a phantom and compares results to theoretical calculation of dose. Results have shown a good match between predicted and experimentally measured results at the centre of an eye shield irradiated with 50 kVp and 150 kVp beams. The added advantage of radiochromic film compared to TLD measurements is the two dimensional dose map which is recorded for the assessment of dose providing not only an assessment at the site of the cornea, lens and retina in a phantom but in other areas as well. Radiochromic film has been found to accurately measure dose under eye shield in phantom treatments.

SU-FF-T-333: Orthovoltage Dosimetry: Percent Depth Dose Multiplicity From Two Energies

Purpose: To produce a multiplicity of depth dose curves on a two‐kVp orthovoltage system by appropriate combination of the 75 kVp and 225 kVp x‐ray beams. Method and Materials: Using tabulated percent depth dose (PDD) curves at 75 and 225 kVp for five rectangular applicators (4×6, 6×8, 8×10, 10×15, and 15×20 cm2) for our Philips RT‐250 unit, we calculated the mixed‐beam PDDs for each applicator using w 75 ⋅ PDD 75 + w 225 ⋅ PDD 225 , where w denotes the beam component weight, the subscript indicates the component kVp, and w 75 +w 225 =1 . By specifying a dose D0 at the surface and an isodose level at a particular depth, we calculated the required beam weights; incorporating the tissue dose rates then allows us to calculate the beam‐on times for the 75 and 225 kVp beams, respectively. All single‐ and mixed‐energy PDDs were verified in 20×20 cm2 blocks of solid water (Gammex RMI, Middleton, WI) using a field standard electrometer and ion chamber set. The ion chamber fit snugly at the centre of a 2.0 cm thick block. The depth doses were measured at solid water depths of 1.0 to 15.0 cm keeping the source‐to‐surface distance constant at 50.0 cm, and using 10.0 cm of backscatter material. The PDD values were calculated by scaling the measurements relative to the tabulated or theoretical values at a depth of 1.0 cm. Results: The ion chamber measurements at 75 and 225 kVp were within 2 percentage points of the tabulated values for all rectangular applicators. Similarly, the mixed‐energy PDD curves for f 75={0.25, 0.5, 1.0} were also within 2 percentage points of the theoretical expectations. Conclusion: A firm theoretical formalism to create PDDs combining the 75 kVp and 225 kVp beams was formulated and verified experimentally using ion chamber measurements. Our mixed‐energy methodology allows one to synthesize a multiplicity of PDDs between any two orthovoltage energies.

SU-EE-A2-02: Evaluation of Radiation Dosimeters for Kilovoltage X-Ray Beam Dosimetry

Purpose: To evaluate radiation detectors for the dosimetry of kilovoltage x-ray beams. Method and Materials: The study evaluated cylindrical and parallel plate ionization chambers provided with the PTW MP3 water tank system. Kilovoltage x-ray beams were generated by a Pantak DXT300 x-ray unit with energies ranging from 50 to 280 kVp. The following data was measured: percentage depth dose curves in water, relative detector response both in air and in solid water with the 6 cm diameter applicator. The reference detector was an NE 2571 ionization chamber that had been calibrated in a primary standards laboratory. The EGSnrc Monte Carlo code V4.2.2 was used to calculate depth dose curves in water for comparison with measured data. The x-ray spectrum of primary beam was determined using an analytical program, XRAYBEAM, and subsequently verified by calculation of the half value layer. Results: For the measured depth doses, the PTW 0.3 cc thimble chamber, NACP, Markus and Roos chamber were in good agreement with the Monte Carlo calculated depth dose data. The agreement was better than 3% in most cases, and a maximum deviation of 4% for the 50 kVp x-ray beam. The M31002 pinpoint chamber gave a greater deviation, up to 9% for the 50 kVp beam. The relative detector response, as compared to the NE 2571, indicated that thimble chambers had a small variation of up to 5% over the energy range. However the relative detector response for the parallel plate chambers was large, with the deviation increasing as the x-ray beam energy decreased, up to 70% for the Markus chamber. Similar large response changes also occurred for the pinpoint chamber. Conclusion: The parallel plate chambers (Markus, Roos, NACP) were found to be suitable for depth dose measurements in water for kilovoltage x-ray beams.

Dual-energy cardiac imaging: an image quality and dose comparison for a flat-panel detector and x-ray image intensifier

This study presents a comparison of dual-energy imaging with an x-ray image intensifier and flat-panel detector for cardiac imaging. It also investigates if the wide dynamic range of the flat-panel detector can improve dual-energy image quality while reducing patient dose. Experimental contrast-to-noise (CNR) measurements were carried out in addition to simulation studies. Patient entrance exposure and system tube loading were also recorded. The studied contrast objects were calcium and iodine. System performance was quantified with a figure-of-merit (FOM) defined as the image CNR2 over patient entrance exposure. The range of thickness studied was from 10 to 30 cm of Lucite (PMMA). Detector dose was initially set to 140 nGy (16 µR)/frame. The high-energy 120 kVp beam was filtered by an additional 0.8 mm silver filter. Keeping the same filament current, the kVp for the low-energy beam was adjusted as a function of thickness until 140 nGy was achieved. System performance was found to be similar for both systems, with the x-ray image intensifier performing better at lower thicknesses and the flat-panel detector performing better at higher thicknesses. This requirement of fixed detector entrance exposure was then relaxed and the kVp for the low-energy beam was allowed to vary while the mAs of the x-ray tube remained fixed to study changes in dual-energy image quality, patient dose and FOM with the flat-panel detector. It was found that as the kVp for the low-energy beam was reduced, system performance would rise until reaching a maximum while simultaneously lowering patient exposure. Suggested recommendations for optimal dual-energy imaging implementation are also provided.

SU‐FF‐T‐138: Comparison of Change in Optical Density Between Three Radiochromic Films Due to 100 CGy Dose‐To‐Water Delivered by X‐Rays in the 75 KVp to 18 MV Range

Purpose: Over the last few decades, optical fiber‐based dosimeters for in vivo measurements have been investigated. One of the attractive features of optical dosimetry is the potential for improved water‐equivalency, compared to methods that use electrical signals. The materials considered thus far have been scintillating and optically stimulated luminescent sensors, doped and scintillating optical fibers, and radiochromic films. Several of the radiochromic films, including GafChromic MD‐55 and HS, have shown a lower response for a given dose when low‐energy (orthovoltage) x‐ray beams were used, compared to a Co‐60 source. This study continues the investigation of suitability of another radiochromic film, GafChromic EBT, for real‐time in vivo point‐based dosimetry by considering its response to a given dose delivered by beams varying in nominal energy. Method and Materials: 1 cm × 1cm pieces of MD‐55, HS and EBT film were irradiated within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibers for real‐time measurements. Each exposure delivered a dose of 100 cGy, using x‐ray beams ranging in energy from 75 kVp to 18 MV. Changes in optical density (ΔOD) were compared for the three types of film across all beam energies used. Results: Both MD‐55 and HS films showed a decrease of approximately 35% in ΔOD for a 75 kVp beam compared to a Co‐60 beam. Decrease in response for 225 kVp beam was less pronounced (approximately 15%). Both HS and MD‐55 exhibited this trend, confirming previous reports and validating our technique. The EBT film showed no decrease in response through the orthovoltage range. Conclusion: Due to a near‐constant response of EBT film for a given dose delivered using 75 kVp to 18 MV x‐ray beams, EBT medium is more suitable for in vivo measurements than MD‐55 and HS films.

Dose and energy dependence of response of Gafchromic® XR-QA film for kilovoltage x-ray beams

There is a growing interest in Gafchromic® films for patient dosimetry in radiotherapy and in radiology. A new model (XR-QA) with high sensitivity to low dose was tested in this study. The response of the film to different x-ray beam energies (range 28–145 kVp with various filtrations, dose range 0–100 mGy) and to visible light was investigated, together with the after exposure darkening properties. Exposed films were digitized with a commercially available, optical flatbed scanner. A single functional form for dose versus net pixel value variation has been determined for all the obtained calibration curves, with a unique fit parameter different for each of the used x-ray beams. The film response was dependent on beam energy, with higher colour variations for the beams in the range 80–140 kVp. Different sources of uncertainties in dose measurements, governed by the digitalization process, the film response uniformity and the calibration curve fit procedure, have been considered. The overall one-sigma dose measurement uncertainty depended on the beam energy and decreased with increasing absorbed dose. For doses above 10 mGy and beam energies in the range 80–140 kVp the total uncertainty was less than 5%, whereas for the 28 kVp beam the total uncertainty at 10 mGy was about 10%. The post-exposure colour variation was not negligible in the first 24 h after the exposure, with a consequent increase in the calculated dose of about 10%. Results of the analysis of the sensitivity to visible light indicated that a short exposure of this film to ambient and scanner light during the measurements will not have a significant impact on the radiation dosimetry.

Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy

Kilovoltage cone-beam computerized tomography (kV-CBCT) systems integrated into the gantry of linear accelerators can be used to acquire high-resolution volumetric images of the patient in the treatment position. Using on-line software and hardware, patient position can be determined accurately with a high degree of precision and, subsequently, set-up parameters can be adjusted to deliver the intended treatment. While the patient dose due to a single volumetric imaging acquisition is small compared to the therapy dose, repeated and daily image guidance procedures can lead to substantial dose to normal tissue. The dosimetric properties of a clinical CBCT system have been studied on an Elekta linear accelerator (Synergy RP, XVI system) and additional measurements performed on a laboratory system with identical geometry. Dose measurements were performed with an ion chamber and MOSFET detectors at the center, periphery, and surface of 30 and 16-cm-diam cylindrical shaped water phantoms, as a function of x-ray energy and longitudinal field-of-view (FOV) settings of 5,10,15, and 26 cm. The measurements were performed for full 360 degrees CBCT acquisition as well as for half-rotation scans for 120 kVp beams using the 30-cm-diam phantom. The dose at the center and surface of the body phantom were determined to be 1.6 and 2.3 cGy for a typical imaging protocol, using full rotation scan, with a technique setting of 120 kVp and 660 mAs. The results of our measurements have been presented in terms of a dose conversion factor fCBCT, expressed in cGy/R. These factors depend on beam quality and phantom size as well as on scan geometry and can be utilized to estimate dose for any arbitrary mAs setting and reference exposure rate of the x-ray tube at standard distance. The results demonstrate the opportunity to manipulate the scanning parameters to reduce the dose to the patient by employing lower energy (kVp) beams, smaller FOV, or by using half-rotation scan.

Feasibility of real time dual-energy imaging based on a flat panel detector for coronary artery calcium quantification

The feasibility of a real-time dual-energy imaging technique with dynamic filtration using a flat panel detector for quantifying coronary arterial calcium was evaluated. In this technique, the x-ray beam was switched at 15 Hz between 60 kVp and 120 kVp with the 120 kVp beam having an additional 0.8 mm silver filter. The performance of the dynamic filtration technique was compared with a static filtration technique (4 mm Al+0.2 mm Cu for both beams). The ability to quantify calcium mass was evaluated using calcified arterial vessel phantoms with 20-230 mg of hydroxylapatite. The vessel phantoms were imaged over a Lucite phantom and then an anthropomorphic chest phantom. The total thickness of Lucite phantom ranges from 13.5-26.5 cm to simulate patient thickness of 16-32 cm. The calcium mass was measured using a densitometric technique. The effective dose to patient was estimated from the measured entrance exposure. The effects of patient thickness on contrast-to-noise ratio (CNR), effective dose, and the precision of calcium mass quantification (i.e., the frame to frame variability) were studied. The effects of misregistration artifacts were also measured by shifting the vessel phantoms manually between low- and high-energy images. The results show that, with the same detector signal level, the dynamic filtration technique produced 70% higher calcium contrast-to-noise ratio with only 4% increase in patient dose as compared to the static filtration technique. At the same time, x-ray tube loading increased by 30% with dynamic filtration. The minimum detectability of calcium with anatomical background was measured to be 34 mg of hydroxyapatite. The precision in calcium mass measurement, determined from 16 repeated dual-energy images, ranges from 13 mg to 41 mg when the patient thickness increased from 16 to 32 cm. The CNR was found to decrease with the patient thickness linearly at a rate of (-7%/cm). The anatomic background produced measurement root-mean-square (RMS) errors of 13 mg and 18 mg when the vessel phantoms were imaged over a uniform (over the rib) and nonuniform (across the edge of rib) bone background, respectively. Misregistration artifacts due to motions of up to 1.0 mm between the low- and high-energy images introduce RMS error of less than 4.3 mg, which is much smaller than the frame to frame variability and the measurement error due to anatomic background. The effective dose ranged from 1.1 to 6.6 microSv for each dual-energy image, depending on patient thickness. The study shows that real-time dual-energy imaging can potentially be used as a low dose technique for quantifying coronary arterial calcium.

TH-C-I-611-07: Quantum Efficiency of An MCP Detector: Monte Carlo Calculation

Purpose: To calculate quantum efficiency (QE) of the microchannel plate (MCP) detector at different x‐ray tube voltages using Monte Carlo method.Method and Materials:Monte Carlo calculation of the QE was performed using GEANT4. The MCPs with channel diameters of 2–25 μm and lead contents of 50%, 38%, 20%, 8% and 0% were considered. QE calculations were performed using GEANT4 for monochromatic photons at 200 energy points in the 0–200 keV range with 1 keV steps. QE of MCP was calculated for 35, 45, 90, and 120 kVp photon beams from tungsten target x‐ray tube, passed through corresponding soft tissue equivalent filters. QE for polychromatic photon beams were calculated by weighting the monochromatic QE with the photon energy distribution of the filtered x‐ray spectra. Results: The QE for 35 kVp beam was 83% and 86% for MCPs with 2 μm channel diameter and 50% lead content, and 5 μm channel MCP with no lead in the material, respectively. Efficiency was decreased to 51% at 35 kVp for MCP with 50% lead content and 5 μm channel diameter. The efficiencies for 45 kVp photon beams were 68% and 76% for MCPs with 50% lead and no lead, respectively, and 5 μm channel diameter. QE was increased at 45 kVp to 91% for 2 μm channel MCP with 50% lead. The QE of the 90 kVp and 120 kVp beams were in the 81–92% ranges except the lead free MCP, for which it was decreased to 20–30%. Conclusion:Monte Carlo calculations of the quantum efficiency of the edge on MCP detector shown that the smaller the MCP channel diameter, the higher the QE. Commercially available MCPs with 5–6 μm channel diameters can provide sufficiently high quantum efficiency for medical x‐ray imaging applications.

TU-FF-A1-05: Limitations of the Photon and Electron Transport Algorithms in GEANT4 for Radiotherapy Applications

Purpose: To examine the limitations in the photon and electron transport algorithms in GEANT4, so as to identify its suitability for radiotherapy applications. Method and Materials: A set of simulations were performed to access the accuracy of GEANT4. We calculated depth dose distributions of photon and electron beams incident on a water phantom, and examined the effects of dose perturbation in the presence of a lead interface. The results are compared to the EGSnrc Monte Carlo code. We attempt to provide explanations for the differences observed. An investigation of the problems in the condensed history algorithm is done through a series of cavity simulations under Fano conditions. Results: In homogeneous water, the depth dose distribution of a 10 MeV incident electron beam depends on the electron step size limitations and secondary particle production thresholds. The disagreement with BEAMnrc is over 4%, which is partly due to differences in cross section data among the EM models. For incident photon beams of 400 keV and 6 MeV, GEANT4 agrees with EGSnrc to within 0.4% and 2% respectively beyond the depth of dose maximum. Larger differences are found in the buildup regions due to problems in electron transport. In a Pb/water geometry, we observe an underdose of up to 84% upstream of the interface for a 100 kVp beam. This is mainly due to the neglect of spin effects in electron elastic scattering. In the Fano cavity simulations, problems in electron step and boundary crossing algorithms result in an underestimation of cavity dose by up to 39%. Accurate cavity response can be obtained using severe step size restrictions. Conclusion: GEANT4 is unsuitable for simulations where electron disequilibrium exists. Improvements in boundary crossing and backscattering are needed, and spin effects should be modeled.

TU‐D‐I‐611‐05: Monte Carlo Investigation of Scatter Contribution to Kilovoltage Cone‐Beam Computed Tomography Images

Purpose: In this work, we evaluate the scatter radiation present in projection images produced by kilovoltage (kV) cone‐beam computed tomography (CBCT) using Monte Carlo (MC) calculations. We also demonstrate that MC simulations can be used to reduce scatter in kV CBCT reconstructed images. Method and Materials: All images were acquired using a kV CBCT bench‐top system composed of an x‐ray tube, a rotation stage and a CsI flat‐panel imager. The EGSnrc MC code was used to fully model the flat‐panel imager and the x‐ray source for energies of 80, 100, and 120 kVp. The x‐ray source model was validated using first and second half‐value layer (HVL) and profile measurements. MC simulations and measured images were obtained for a cylindrical water phantom and for the head of the anthropomorphic Rando phantom. A modified version of the DOSXYZnrc program was used to score phase space files with identified scattered and non‐scattered particles after the phantoms. The cone angle and the source‐to‐detector distance (SDD) were varied during the simulations to determine their effect on scatter radiation in projection images. MC predictions of the scatter distribution were subtracted from the CBCT projection images prior to the reconstruction to improve the reconstructed image quality. Results: The simulated and measured first and second HVL agree within 2% except for the 80 kVp beam where the agreement is within 9%. The measured and simulated profiles agree within 4%. Simulated and measured projection images are in good agreement. Decreasing the cone angle and increasing the SDD decrease the amount of scatter. On the other hand, the phantom geometry has little effect on the scatter radiation distribution. Conclusion: MC simulations were used to characterize the scatter radiation from different geometries. It was shown that it is possible to use MC simulations to reduce the scatter in the reconstructed image.

Cassette-based digital mammography.

Over the past several years, digital mammography systems have been installed clinically across North America in small but growing numbers. A photostimulable phosphor-based full-field digital mammography image was evaluated in this investigation. Commonly known as computed radiography (CR), its use closely mimics the screen-film mammography paradigm. System performance using modulation transfer function (MTF) and detective quantum efficiency (DQE) metrics show MTF(2.5 mm(-1)) = 0.5, DQE(2.5 mm(-1)) = 0.3, and MTF(5.0 mm(-1)) = 0.2, DQE(5.0 mm(-1)) = 0.05, for a 26 kVp beam, 0.03 mm molybdenum tube filtration, 4.5 cm tissue attenuation, and 15 mR incident exposure to the detector. Slightly higher DQE values were measured at 32 kVp with 0.025 mm rhodium tube filtration. CR mammography advantages include the ability to use existing mammography machines, where multiple rooms can be converted to "digital" operation, which allows overall cost savings compared to integrated digital mammography systems. Chief disadvantages include the labor-intensive handling of the cassettes prior to and after the imaging exam, lack of a direct interface to the x-ray system for recording technique parameters, and relatively slow processing time. Clinical experience in an IRB-approved research trial has suggested that digital mammography with photostimulable storage phosphors and a dedicated CR reader is a viable alternative to conventional screen-film mammography.

Photon-counting radiography with the gas microstripdetector

We have built a proof-of-concept photon-counting x-ray imaging system using a Xe:CH4 gas microstrip detector (GMD) as the image receptor, and have used this system to demonstrate several advantages of photon counting over energy integration. Our experiments spanned x-ray spectra from 10 to 50 kVp and Xe:CH4 pressures from 1 to 4 atm. When photon counting is done, the energy deposited in the detector by each incident photon can be measured on adjacent anode strips and a centroid calculation can be used to provide spatial resolution significantly better than the anode strip pitch. We measured >11 lp mm-1 at 13 kVp with our 200 µm pitch detector, and 8.2 lp mm-1 at 50 kVp. The energy resolution of our GMD is 5.2% at 59.6 keV, and the space-charge limited counting rate is >2 × 106 mm-2 s-1 at 3 atm for a 30 kVp beam.

Timer error and beam quality variation during "ramp-up" of a superficial X-ray therapy unit.

Superficial and orthovoltage therapy machines generally exhibit a "ramp-up region" during which the dose rate increases from zero to the steady state output. This can produce a significant dose deficit, which can be made up by adding a correction to the treatment time. This correction, which can be determined from the intercept of the dose versus time plot, takes no account of either the true duration of the ramp-up region or the beam quality variation while it takes place. A new method for investigating the ramp-up region is proposed, and the results are presented for a 100 kVp beam. Measurements of beam quality in terms of half value layer (HVL) have been made as a function of time. A change in HVL from an average of 2.9 mm aluminium in the ramp-up region to 3.1 mm for the steady state region has been observed. A significant random variation in the time between initiation of the radiation and the beginning of the ramp-up region was also observed which was traced to a time varying signal in the control system.

Broad beam attenuation of kilovoltage photon beams: effect of ion chambers.

In kilovoltage X-ray treatment, beam shaping and shielding normal tissue are accomplished by thin sheets of lead cutout, the thickness of which is selected based upon either published data or measurements. Available broad beam attenuation (BBA) data are found to be unsatisfactory and are the subject of this investigation. BBA is defined as the ratio of intensity with (I) and without (I0) attenuating medium for a large field in a phantom. BBA = I(x,t,E)/I0(x,0,E), where x is the depth of measurement, t is the thickness of attenuator, and E is the beam energy. The depth x should be zero for kilovoltage beams and dmax for megavoltage beams. Unfortunately, x is limited by the window thickness which is the core of this study. A Farmer-type cylindrical ion chamber and three parallel plate ion chambers (Capintec, PS-033; Markus; and Holt) were used to measure BBA for kilovoltage beams from a Siemens Stabilipan unit. Results indicate that attenuation is strongly dependent on the window thickness. For the 240 kVp beam, the thickness of lead for 5% and 1% transmissions are 3.1 mm, and 5.2 mm, respectively, with the Capintec chamber. The corresponding values of lead thickness for the Markus chamber are 2.3 mm and 4.0 mm; for the Holt chamber the values are 1.1 mm and 2.2 mm; and for the cylindrical chambers the values are 1.1 mm and 2.3 mm, respectively. Similar variabilities in lead thickness with ion chambers were also noted for the other kilovoltage beams. The large differences in lead thicknesses produce enormous clinical errors, especially for shielding eye and other critical structures. For small thickness of lead (< 0.1 mm), a 20-fold increase in surface dose could be observed instead of usual beam attenuation. This is due to intense low energy photoelectrons liberated from lead sheets in the contact with tissue. It is concluded that the lead thickness required to shield normal tissue varies with ion chamber. Until national or international guidelines for broad beam transmission measurements are established, the shielding materials in contact with skin should be coated with a thin (> or = 0.3 mm) low atomic number medium. In such a situation, transmission measurements will be independent of the choice of an ion chamber.