Changes In Beam Energy Research Articles

Real Time 3D Beam Visualization for Monitoring External Beam Radiation Therapy

The administration of radiation therapy has advanced steadily throughout its history. However, in spite of large advances in planning accuracy and motion tracking techniques, the actual administration of radiation continues to be a blind procedure. We propose that a system consisting of a flexible scintillation sheet and an array of digital cameras can be used to simultaneously visualize the patient surface and the delivered radiation beam in three dimensions and in real time. For the proof of concept system, scintillating sheets were made by mixing Gd2O2S:Tb (GOS) with silicone and casting the mixture into a thin sheet. The sheet was placed on a solid water phantom and irradiated with therapeutic photon beams from a medical linear accelerator (LINAC). The light emitted from the sheet was collected using a pair of cameras calibrated as a stereo pair. Custom designed image processing software was used to reconstruct the 3D scene and extract the beam profile. The shape and location of the extracted profile was compared to known collimator settings for validation. The system was capable of acquiring high quality images of both the phantom and the beam under various levels of ambient room lighting including those most commonly used during therapy. Images were obtained at a rate of 20 fps. The system demonstrated sub-millimeter resolution and accuracy in identifying both the shape and location of radiation beams. The intensity of the beam profile was found to be linear with dose rate and correlated to the expected surface dose as beam energy changes. A system consisting of digital cameras and a flexible scintillation sheet is capable of three dimensional visualization of external beam radiation therapy in real-time. The accuracy of the data is sufficient that further development of the system may provide the ability to verify the accuracy of treatment as it is occurring and provide a record for post-treatment analysis.

SU‐E‐T‐781: Using An Electronic Portal Imaging Device (EPID) for Correlating Linac Photon Beam Energies

Purpose:Electronic portal imaging devices (EPIDs) have proven to be useful for measuring several parameters of interest in linear accelerator (linac) quality assurance (QA). The purpose of this project was to evaluate the feasibility of using EPIDs for determining linac photon beam energies.Methods:Two non‐clinical Varian TrueBeam linacs (Varian Medical Systems, Palo Alto, CA) with 6MV and 10MV photon beams were used to perform the measurements. The linacs were equipped with an amorphous silicon based EPIDs (aSi1000) that were used for the measurements. We compared the use of flatness versus percent depth dose (PDD) for predicting changes in linac photon beam energy. PDD was measured in 1D water tank (Sun Nuclear Corporation, Melbourne FL) and the profiles were measured using 2D ion‐chamber array (IC‐Profiler, Sun Nuclear) and the EPID. Energy changes were accomplished by varying the bending magnet current (BMC). The evaluated energies conformed with the AAPM TG142 tolerance of ±1% change in PDD.Results:BMC changes correlating with a ±1% change in PDD corresponded with a change in flatness of ∼1% to 2% from baseline values on the EPID. IC Profiler flatness values had the same correlation. We observed a similar trend for the 10MV beam energy changes. Our measurements indicated a strong correlation between changes in linac photon beam energy and changes in flatness. For all machines and energies, beam energy changes produced change in the uniformity (AAPM TG‐142), varying from ∼1% to 2.5%.Conclusions:EPID image analysis of beam profiles can be used to determine linac photon beam energy changes. Flatness‐based metrics or uniformity as defined by AAPM TG‐142 were found to be more sensitive to linac photon beam energy changes than PDD.Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU‐E‐T‐775: Use of Electronic Portal Imaging Device (EPID) for Quality Assurance (QA) of Electron Beams On Varian Truebeam System

Purpose:In a previous study we have demonstrated the feasibility of using EPID to QA electron beam parameters on a single Varian TrueBeam LINAC. This study aims to provide further investigation on (1) reproducibility of using EPID to detect electron beam energy changes on multiple machines and (2) evaluation of appropriate calibration methods to compare results from different EPIDs.Methods:Ad‐hoc mode electron beam images were acquired in developer mode with XML code. Electron beam data were collected on a total of six machines from four institutions. A custom‐designed double‐wedge phantom was placed on the EPID detector. Two calibration methods ‐ Pixel Sensitivity Map (PSM) and Large Source‐to‐Imager Distance Flood Field (LSID‐FF) ‐ were used. To test the sensitivity of EPID in detecting energy drifts, Bending Magnet Current (BMC) was detuned to invoke energy changes corresponding to ∼±1.5 mm change in R50% of PDD on two machines from two institutions. Percent depth ionization (PDI) curves were then analyzed and compared with the respective baseline images using LSID‐FF calibration. For reproducibility testing, open field EPID images and images with a standard testing phantom were collected on multiple machines. Images with and without PSM correction for same energies on different machines were overlaid and compared.Results:Two pixel shifts were observed in PDI curve when energy changes exceeded the TG142 tolerance. PSM showed the potential to correct the differences in pixel response of different imagers. With PSM correction, the histogram of images differences obtained from different machines showed narrower distributions than those images without PSM correction.Conclusion:EPID is sensitive for electron energy changes and the results are reproducible on different machines. When overlaying images from different machines, PSM showed the ability to partially eliminate the intrinsic variation of various imagers.Research Funding from Varian Medical Systems Inc.Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU‐E‐T‐556: Monte Carlo Generated Dose Distributions for Orbital Irradiation Using a Single Anterior‐Posterior Electron Beam and a Hanging Lens Shield

Purpose:Various cancers of the eye are successfully treated with radiotherapy utilizing one anterior‐posterior (A/P) beam that encompasses the entire content of the orbit. In such cases, a hanging lens shield can be used to spare dose to the radiosensitive lens of the eye to prevent cataracts.Methods:This research focused on Monte Carlo characterization of dose distributions resulting from a single A‐P field to the orbit with a hanging shield in place. Monte Carlo codes were developed which calculated dose distributions for various electron radiation energies, hanging lens shield radii, shield heights above the eye, and beam spoiler configurations. Film dosimetry was used to benchmark the coding to ensure it was calculating relative dose accurately.Results:The Monte Carlo dose calculations indicated that lateral and depth dose profiles are insensitive to changes in shield height and electron beam energy. Dose deposition was sensitive to shield radius and beam spoiler composition and height above the eye.Conclusion:The use of a single A/P electron beam to treat cancers of the eye while maintaining adequate lens sparing is feasible. Shield radius should be customized to have the same radius as the patient's lens. A beam spoiler should be used if it is desired to substantially dose the eye tissues lying posterior to the lens in the shadow of the lens shield. The compromise between lens sparing and dose to diseased tissues surrounding the lens can be modulated by varying the beam spoiler thickness, spoiler material composition, and spoiler height above the eye. The sparing ratio is a metric that can be used to evaluate the compromise between lens sparing and dose to surrounding tissues. The higher the ratio, the more dose received by the tissues immediately posterior to the lens relative to the dose received by the lens.

Characterizing energy dependence and count rate performance of a dual scintillator fiber-optic detector for computed tomography.

Kilovoltage (kV) x-rays pose a significant challenge for radiation dosimetry. In the kV energy range, even small differences in material composition can result in significant variations in the absorbed energy between soft tissue and the detector. In addition, the use of electronic systems in light detection has demonstrated measurement losses at high photon fluence rates incident to the detector. This study investigated the feasibility of using a novel dual scintillator detector and whether its response to changes in beam energy from scatter and hardening is readily quantified. The detector incorporates a tissue-equivalent plastic scintillator and a gadolinium oxysulfide scintillator, which has a higher sensitivity to scatter x-rays. The detector was constructed by coupling two scintillators: (1) small cylindrical plastic scintillator, 500 μm in diameter and 2 mm in length, and (2) 100 micron sheet of gadolinium oxysulfide 500 μm in diameter, each to a 2 m long optical fiber, which acts as a light guide to transmit scintillation photons from the sensitive element to a photomultiplier tube. Count rate linearity data were obtained from a wide range of exposure rates delivered from a radiological x-ray tube by adjusting the tube current. The data were fitted to a nonparalyzable dead time model to characterize the time response. The true counting rate was related to the reference free air dose air rate measured with a 0.6 cm(3) Radcal(®) thimble chamber as described in AAPM Report No. 111. Secondary electron and photon spectra were evaluated using Monte Carlo techniques to analyze ionization quenching and photon energy-absorption characteristics from free-in-air and in phantom measurements. The depth/energy dependence of the detector was characterized using a computed tomography dose index QA phantom consisting of nested adult head and body segments. The phantom provided up to 32 cm of acrylic with a compatible 0.6 cm(3) calibrated ionization chamber to measure the reference air kerma. Each detector exhibited counting losses of 5% when irradiated at a dose rate of 26.3 mGy/s (Gadolinium) and 324.3 mGy/s (plastic). The dead time of the gadolinium oxysulfide detector was determined to be 48 ns, while the dead time of the plastic scintillating detector was unable to accurately be calculated due to poor counting statistics from low detected count rates. Noticeable depth/energy dependence was observed for the plastic scintillator for depths greater than 16 cm of acrylic that was not present for measurements using the gadolinium oxysulfide scintillator, leading us to believe that quenching may play a larger role in the depth dependence of the plastic scintillator than the incident x-ray energy spectrum. When properly corrected for dead time effects, the energy response of the gadolinium oxysulfide scintillator is consistent with the plastic scintillator. Using the integrated dual detector method was superior to each detector individually as the depth-dependent measure of dose was correctable to less than 8% between 100 and 135 kV. The dual scintillator fiber-optic detector accommodates a methodology for energy dependent corrections of the plastic scintillator, improving the overall accuracy of the dosimeter across the range of diagnostic energies.

Transmission of low-energy electrons through ultrathin layers of tin(IV) phthalocyanine oxide

The results of the investigation of the transmission of low-energy electrons through 8-nm-thick films of tin(IV) phthalocyanine oxide (SnOPc) on the surface of the oxidized silicon substrate are presented. The procedure of detecting the reflection of testing low-energy electron beam from the surface was implemented using the very low-energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode with a change in the incident electron beam energy from 0 to 25 eV. The structure of maxima in the total current spectra of SnOPc films was established and the variation in intensities of the maxima of total current spectra emitted from the deposited SnOPc film and (SiO2)n-Si substrate during an increase in the thickness of the organic coating to 8 nm was analyzed. With such an increase in the thickness of the organic coating, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from the (SiO2)n-Si substrate to the SnOPc film. Optical absorption spectra of SnOPc films were measured. The absorption spectra were compared with the spectra measured by the TCS method for the SnOPc films and the films of molecules of oxygen-free metal phthalocyanine.

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations.

Following linear accelerator commissioning, the qualified medical physicist is responsible for monitoring the machine's ongoing performance, detecting and investigating any changes in beam properties, and assessing the impact of unscheduled repairs. In support of these responsibilities, the authors developed a method of using a 2D ionization chamber array to efficiently test and validate important linear accelerator photon beam properties. A team of three physicists identified critical properties of the accelerator and developed constancy tests that were sensitive to each of the properties. The result was a 14‐field test plan. The test plan includes large and small fields at varying depths, a reduced SSD field at shallow depth for sensitivity to extra focal photon and electron components, and analysis of flatness, symmetry, dose, dose profiles, and dose ratios. Constancy tests were repeated five times over a period of six weeks and used to set upper and lower investigation levels at ±3 SDs. Deliberate variations in output, penumbra, and energy were tested to determine the suitability of the proposed method. Measurements were also performed on a similar, but distinct, machine to assess test sensitivity. The results demonstrated upper and lower investigation levels significantly smaller than the comparable TG‐142 annual recommendations, with the exception of the surrogate used for output calibration, which still fell within the TG‐142 monthly recommendation. Subtle changes in output, beam energy, and penumbra were swiftly identified for further investigation. The test set identified the distinct nature of the second accelerator. The beam properties of two photon energies can be validated in approximately 1.5 hrs using this method. The test suite can be used to evaluate the impact of minor repairs, detect changes in machine performance over time, and supplement other machine quality assurance testing.PACS numbers: 87.56bd, 87.56Fc

Evaluation of testicular dose and associated risk from common pelvis radiation therapy in Iran

This study aimed to investigate testicular dose (TD) and the associated risk of heritable disease from common pelvis radiotherapy of male patients in Iran. In this work, the relation between TD and changes in beam energy, pelvis size, source to skin distance (SSD) and beam directions (anterior or posterior) was also evaluated. The values of TDs were measured on 67 randomly selected male patients during common pelvis radiotherapy using 1.17 and 1.33 MeV, Theratron Cobalt-60 unit at SSD of 80 cm and 9 MV, Neptun 10 PC and 18 MV, GE Saturne 20 at SSD of 100 cm at Seyed-Al Shohada Hospital, Isfahan, Iran. Results showed that, the maximum TD was up to 12% of the tumor dose. Considering the risk factor for radiation-induced heritable disorders of 0.1% per Sv, an excess risk of hereditary disorders of 72 per 10,000 births was conservatively calculated. There was a significant difference in the measured TD using different treatment machines and energies (P < 0.001). The Pearson Correlation test showed that, as expected, there was a correlation between TD and patient's pelvis size (r = 0.275, P < 0.001). Using the student's t-tests, it was found that, there was not a significant difference between TD and beam direction (P = 0.231). Iranian male patients undergoing pelvic radiotherapy have the potential of receiving a TD of more than 1 Gy which might result in temporary azoospermia. The risk for induction of hereditary disorders in future generations should be considered as low but not negligible in comparison with the correspondent nominal risk.

Effect of light-ions implantation on resistivity of GaN thin film

GaN is a very well-known semiconductor because of its properties, making it useful in different types of electronic devices. In electronics applications, the resistivity of the starting materials is very important. In this study, the resistivity of GaN thin films has been investigated upon its modification with ion implantation using both n-type and p-type GaN. Ionic implantation was performed with H, He, and Ar ions. The ion beam energy (60 and 120 keV) and flux (1.0 × 1012 and 1.0 × 1015 cm−2·s−1), as well as post-implantation annealing temperature (100°C–500°C), were varied to analyze their influence on GaN resistivity. It was observed that the resistivity changed in all samples with the change in ion beam flux and energy. At room temperature, the resistivity of n-type GaN increased from 1.9 × 10−2 to 17.7 × 10−2 Ω·cm. Among all modified samples, He-ion-implanted samples show higher resistivity. During the post-annealing treatment, n-type GaN implanted with He showed more consistency compared to p-type GaN. On the contrary, p-type showed some anomalous character upon post-annealing.

SU-E-T-635: Quantitative Study On Beam Flatness Variation with Beam Energy Change

Purpose: Beam flatness check has been proposed for beam energy check for photon beams with flattering filters. In this work, beam flatness change with beam energy was investigated quantitatively using the Monte Carlo method and its significance was compared with depth dose curve change. Methods: Monte Carlo simulations for a linear accelerator with flattering filter were performed with different initial electron energies for photon beams of 6MV and 10MV. Dose calculations in a water phantom were then perform with the phase space files obtained from the simulations. The beam flatness was calculated based on the dose profile at 10 cm depth for all the beams with different initial electron energies. The percentage depth dose (PDD) curves were also analyzed. The dose at 10cm depth (D10) and the ratio of the dose at 10cm and 20cm depth (D10/D20) and their change with the beam energy were calculated and compared with the beam flatness variation. Results: It was found that the beam flatness variation with beam energy change was more significant than the change of D10 and the ratio between D10 and D20 for both 6MV and 10MV beams. Half MeV difference on the initial electron beam energy brought in at least 20% variation on the beam flatness but only half percent change on the ratio of D10 and D20. The change of D10 or D20 alone is even less significant. Conclusion: The beam energy impact on PDD is less significant than that on the beam flatness. If the PDD is used for checking the beam energy, uncertainties of the measurement could possibly disguise its change. Beam flatness changes more significantly with beam energy and therefore it can be used for monitoring the energy change for photon beams with flattering filters. However, other factors which may affect the beam flatness should be watched as well.

Measurement of changes in linear accelerator photon energy through flatness variation using an ion chamber array

To compare the use of flatness versus percent depth dose (PDD) for determining changes in photon beam energy for a megavoltage linear accelerator. Energy changes were accomplished by adjusting the bending magnet current by up to ± 15% in 5% increments away from the value used clinically. Two metrics for flatness, relative flatness in the central 80% of the field (Flat) and average maximum dose along the diagonals normalized by central axis dose (FDN), were measured using a commercially available planner ionization chamber array. PDD was measured in water at depths of 5 and 10 cm in 3 × 3 cm(2) and 10 × 10 cm(2) fields using a cylindrical chamber. PDD was more sensitive to changes in energy when the beam energy was increased than when it was decreased. For the 18-MV beam in particular, PDD was not sensitive to energy reductions below the nominal energy. The value of Flat was found to be more sensitive to decreases in energy than to increases, with little sensitivity to energy increases above the nominal energy for 18-MV beams. FDN was the only metric that was found to be sensitive to both increases and reductions of energy for both the 6- and 18-MV beams. Flatness based metrics were found to be more sensitive to energy changes than PDD, In particular, FDN was found to be the most sensitive metric to energy changes for photon beams of 6 and 18 MV. The ionization chamber array allows this metric to be conveniently measured as part of routine accelerator quality assurance.

Characterization of Tris (5-amino-1,10-phenanthroline) Ruthenium(II/III) Polymer Films Using Cyclic Voltammetry and Rutherford Backscattering Spectrometry

Platinum electrodes were chemically modified with tris(5-amino-1,10-phenanthroline) ruthenium(II) via electropolymerization. The characterization of the thin films was accomplished with cyclic voltammetry (CV) and Rutherford Backscattering Spectrometry (RBS). Data indicates a strong correlation between the peak currents from the characterization cyclic voltammograms and the number of cycles of electropoly-merization. Rutherford Backscattering Spectrometry showed the same trend, and verified that film thickness is strongly dependent on the concentration of the monomer ruthenium solution. Film thickness was determined from the change in ion beam energy as it passed through the film and was calculated to be 1.0 x 1018 atoms/cm2 – 3.4 x 1018 atoms/cm2, depending upon the number of electropolymerization cycles. The electrodes also showed differences in surface roughness, which were dependent on film thickness.

SU‐E‐T‐10: A Computational Model for the Estimation of Biological Dose in a Clinical Proton Beam

Purpose: The variability of relative biological effectiveness (RBE) of a proton beam with spatial distribution is well known. It is dependent on the proton beam characteristics, type of tissue and biological indices. A computational model based on a proton spectrum and LET is used for the estimation of the biological dose. Methods: The depth dose curve of the pristine beam is divided into two regions: the proximal region from the surface to the Bragg peak and the distal region beyond the peak. Knowing the beam energy at the surface in the proximal region, and using the published energy‐range tables, we estimated the effective proton energy at any depth proximal to the Bragg peak. Based on the energy, the LET and RBE is estimated. In the distal region, we assume that the effective beam energy changes monotonically from the value calculated at the peak to the final energy of about 3MeV at the 3% dose level on the distal edge. Applying the calculated RBE factor to the measured depth dose profile we obtain the biological dose in the pristine beam. The method can easily be applied to modulated beams. Results: Our computation model shows that it is possible to predict the biological dose for any combination of beam energy, and for SOBP size based on the available RBE data. Our results showed that the biological dose for a SOBP has finite slope rising to higher values close to distal edge of the peak. Conclusions: A computational model provides an unique tool to estimate the biological dose in any combination of beam energy and SOBP size. If the RBE vs LET data for a tissue is know our computational approach could provide the biological dose accurately in a clinical beam.

Design and simulation of a wire scanner for the CSNS linac

In this paper, we report the design and simulation of a wire scanner for the linac of the CSNS (China Spallation Neutron Source). The wire scanner is used to measure the transverse beam profile and the emittance. The effect of beam energy change upon the mechanical design of the wire scanner must be considered. The simulation results of heat on the two specified wires, tungsten and carbon, by using the finite element method software, ANSYS, are presented. In addition, the effect of wire deformation on the beam profile measurement is qualitatively analyzed, and the signal level of the wire scanner is discussed.

Dosimetric effects of rotational output variation and x‐ray target degradation on helical tomotherapy plans

In this study, two potential sources of IMRT delivery error have been identified for helical tomotherapy delivery using the HiART system (TomoTherapy, Inc., Madison, WI): Rotational output variation and target degradation. The HiArt system is known to have output variation, typically about +/- 2%, due to the absence of a dose servo system. On the HiArt system, x-ray target replacement is required approximately every 10-12 months due to target degradation. Near the end of target life, the target thins and causes a decrease in the beam energy and a softening of the beam profile at the lateral edges of the beam. The purpose of this study is to evaluate the dosimetric effects of rotational output variation and target degradation by modeling their effects and incorporating them into recalculated treatment plans for three clinical scenarios: Head and neck, partial breast, and prostate. Models were created to emulate both potential sources of error. For output variation, a model was created using a sine function to match the amplitude (+/- 2%), frequency, and phase of the measured rotational output variation data. A second model with a hypothetical variation of +/- 7% was also created to represent the largest variation that could exist without violating the allowable dose window in the delivery system. A measured beam profile near the end of target life was used to create a modified beam profile model for the target degradation. These models were then incorporated into the treatment plan by modifying the leaf opening times in the delivery sinogram. A new beam model was also created to mimic the change in beam energy seen near the end of target life. The plans were then calculated using a research version of the PLANNED ADAPTIVE treatment planning software from TomoTherapy, Inc. Three plans were evaluated in this study: Head and neck, partial breast, and prostate. The D50 of organs at risk, the D95 for planning target volumes (PTVs), and the local dose difference were used to evaluate the changes in the modified treatment plans. Dosimetric effects from rotational variation were found to be low (less than 1%) for a typical variation of +/- 2%. Even using a variation of +/- 7%, DVH values and dose distributions were altered by less than 2% for all scenarios. The dosimetric effects of target degradation were found to be slightly more significant. For a model using data taken just before target failure, dosimetric differences of 2%-4% were observed in the recalculated plans when compared to the original plans. The largest effects (up to 4.5%) were observed for PTVs that were located at deeper depths as seen in the prostate plan. Overall, the recalculated plans show that the dosimetric effects of rotational variation and target degradation are on the order of 1%-4% for helical tomotherapy on the HiART system and do not pose a risk for significant deviations from the original treatment plan.

SU‐GG‐BRC‐08: Improving Buildup Region Measurement Accuracy Using a Surface Location Method

Purpose: To determine the depth of the inflection point in the buildup region of photon beam depth‐ionization curves for 11 cylindrical ionization chambers of differing designs and investigate dependencies of the inflection point on beam energy, electron contamination, and chamber design. Method and Materials: Each ionization chamber is carefully aligned to the water surface using a precision alignment telescope with a custom, high‐precision beam scanning system. Following alignment, depth‐ionization curves are obtained including scanning the chamber beyond the air‐water interface for 6, 10, and 25 MV photon beams. Measurements are made with and without a 1 mm lead foil in the beam for a 10×10 cm2 field. The inflection point in the measured depth‐ionization data is determined by finding the maximum in its second derivative. Results: The location of each chamber‐specific inflection point is invariant to changes in beam energy, electron contamination, and chamber orientation within the 0.5 mm measurement resolution. Eight chambers, whose outer diameters are within 0.7 mm of each other, exhibit inflection points that are within 0.5 mm of one another. The location of each chamber's inflection point is most strongly impacted by its outer radius and corresponds with the shallowest depth at which the chamber is fully submersed in the water, within 0.5 mm. Conclusion: The buildup region inflection point for a given cylindrical chamber is invariant to the beam variations studied and occurs at a depth equal to the chamber outer radius, within measurement resolution, for the 11 ionization chambers studied. Using this dependence, the inflection point can be used as a robust offline check to identify the water surface location and to identify inter‐setup/inter‐user setup variations. Higher quality surface localization can ease beam modeling and improve the quality of treatment planning data.

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-J-144: Stability Assessment of MVCT Imaging for Dose Calculation Purposes

Background and purpose: Using Helical Tomotherapy (HT) it is possible to acquire MVCT images of the patient prior to treatment to position the patient. Previous studies have shown that it is possible to use these images for dose calculation, given the exact Image‐value‐to‐density conversion table (IVDT). However, in a clinical setting the image quality is susceptible to changing noise levels, base‐line shifts with output changes and beam spectrum changes, which also have an influence on the IVDT. The aim of this study is to investigate the stability of these images when used for dose calculation in function of varying machine output, beam energy and component changes, and to develop and test a rigid and fast QA protocol that can be used in clinical routine for these images.Material and Methods: The effect of output and energy changes on the IVDT was measured for clinically observed output and energy changes, and the effect on the dose calculation was measured using phantom calculations. Measurements on image characteristics were also performed before and after magnetron and target changes. To be able to build a QA protocol output/energy effects were linked to image quality and noise levels. Results: Clinical variations in output and energy of the beam lead to variations in the IVDT of up to 5% and differences in dose calculation on phantom of up to 3%. Measurements show the noise level of the images can be used as test for changes in the IVDT and that changing a magnetron or target can completely alter the IVDT characteristics. A QA schedule of acquiring one IVDT every week is proposed. Conclusion: Even though studies have shown that dose calculation on tomotherapy‐MVCT images is feasible, the use of this in clinical routine is only possible when combined with a strict imaging‐QA‐protocol.

A method to improve accuracy and precision of water surface identification for photon depth dose measurementsa)

The objective of this study is to present a method to reduce the setup error inherent in clinical depth dose measurements and, in doing so, to improve entrance dosimetry measurement reliability. Ionization chamber (IC) depth dose measurements are acquired with the depth scan extended into the air above the water surface. An inflection region is obtained in each resulting percent depth ionization (PDI) curve that can be matched against other measurements or to an inflection region obtained from an analogous Monte Carlo (MC) simulation. Measurements are made with various field sizes for the 6 and 18 MV photon beams, with and without a Pb foil in the beam, to determine the sensitivity of the dose inflection region to the beam conditions. The offset between reference and test data set inflection regions is quantified using two separate methods. When comparing sets of measured data, maxima in the second derivative of ionization are compared. When comparing measured data to MC simulation, the offset that minimizes the sum of squared differences between the reference and test curves in the ionization inflection region is found. These methods can be used to quantify the offset between an initial setup (test) position and the true surface (reference) position. The ionization inflection location is found to be insensitive to changes in field size, electron contamination, and beam energy. Data from a single reference condition should be sufficient to identify the surface location. The method of determining IC offsets is general and should be applicable to any IC and other radiation sources. The measurement method could reduce the time and effort required in the initial IC setup at a water surface as setup errors can be corrected offline. Given a reliable set of reference data to compare with, this method could increase the ability of quality assurance (QA) measurements to detect discrepancies in beam output as opposed to discrepancies in IC localization. Application of the measurement method standardizes the procedure for localizing cylindrical ICs at a water surface and thereby improves the reliability of measurements taken with these devices at all depths.

SU-FF-T-01: Effective Source Distance and Virtual Source Location for MLC Based Electron Radiotherapy

Purpose: To determine the effective source distance (ESD) for electron fields used in modulated electron radiotherapy (MERT) using an existing photon multileaf collimator and to examine their behavior with changes in field size and beam energy. Methods and Materials: In order to accurately calculate the MU needed for MERT treatment the electron effective source distance should be determined. ITwo methods were used to determine the virtual source positions of electron beams at energies of 6, 9, 12, 16, and 20 MeV. For field sizes varying from 2×2 cm2 to 10×10 cm2, parallel plate ion chamber measurements in a plastic water phantom were taken, and the inverse slope method was used to determine the ESD for each field at each energy. In addition, beam profiles measured with film were used to determine the virtual source location (VSL). Results: The ESDs calculated using the inverse slope method increased with increasing field size and energy. It was observed that the ESD increase with field size was less steep at high energies than at lower energies. Comparing the ESD and VSL measurements for a 10×10 cm2 field, it was observed that the VSL was slightly larger than the ESD at both 9 and 16 MeV electron beams. Conclusions: The ESDs measured were strongly dependent on both field size and electron beam energy. Differences between ESD values may be due to differences in the electron beam scatter that is energy dependent. Discrepancies between ESD and VSL values may be due to the method used to measure ESD which more closely matches clinical conditions than the film VSL method. Determination of the effective source distance can be used for correct MU calculations for modulated electron radiotherapy.