kVp Beam Research Articles

Development and validation of a GEANT4 radiation transport code for CT dosimetry.

The authors have created a radiation transport code using the GEANT4 Monte Carlo toolkit to simulate pediatric patients undergoing CT examinations. The focus of this paper is to validate their simulation with real-world physical dosimetry measurements using two independent techniques. Exposure measurements were made with a standard 100-mm CT pencil ionization chamber, and absorbed doses were also measured using optically stimulated luminescent (OSL) dosimeters. Measurements were made in air with a standard 16-cm acrylic head phantom and with a standard 32-cm acrylic body phantom. Physical dose measurements determined from the ionization chamber in air for 100 and 120 kVp beam energies were used to derive photon-fluence calibration factors. Both ion chamber and OSL measurement results provide useful comparisons in the validation of the Monte Carlo simulations. It was found that simulated and measured CTDI values were within an overall average of 6% of each other.

Monte Carlo radiography simulation for assessment of absorbed radiation dose in femur bone marrow during X-ray radiography for constant mAs and AEC techniques

Background : The purpose of this study was to find the accurate absorbed dose in the femur bone marrow during the X-ray radiography for constant mAs and AEC techniques. Materials and Methods : The DOSXYZnrc was used to simulate radia$on doses in two human femurs during diagnos$c radiography. EGSnrc phantoms produced from actual CT images of human femurs were modified by adding seven micrometre layers of marrow $ssues . The X-ray machine was simulated using BEAMnrc using 30 billions par$cles for different combina$ons of energies and filters. The resultant data was used to in DOSXYSnrc simula$ons to evaluate the absorbed dose in the human femur. Results : In the head of the femur, for 2.5 mm aluminium filtered 85 kVp X-ray set at 50 mAs, the absorbed dose in the marrow was found to be 1.360 mGy, ~ 36% of the absorbed dose in the cor$cal bone. It was also found that for the constant mAs technique, the radia$on dose in the marrow over the studied energies and filter combina$on, ranges from 0.356 mGy to 2.403 mGy, with higher dose recorded for higher kVp se8ngs. However, for the AE C technique, the dose is lower for higher kVp se8ngs. For a typical se8ng, v iz. 85 kVp, 6 mAs at 48 inches SID, the bone marrow absorbed dose was found to be 0.186 mGy for the constant mAs technique and 0.0308 mGy for the AEC technique. Conclusion : It was confirmed that the radia$on dose is lower when the AEC exposure technique is used as opposed to using constant mAs technique. For the AEC technique, typical dose to the bone marrow was found to be ~ 0.05 mGy, decreasing with both kVp se8ngs and beam filtra $on. For constant mAs technique, the typical dose to bone marrow is found to be higher, ~ 0.2 mGy, decreasing with the amount of filtra$on used but increasing with the kVp se8ng.

SU‐F‐BRE‐15: Verification of Energy Dependence of MAGAT Polymer Gel at Orthovoltage Energies

Purpose:Investigation of dose response curves of methacrylic acid‐based “MAGAT” gel at different effective energies to verify an energy dependence of polymer‐gel dosimeters for orthovoltage energy x‐rays.Methods:Six small cylindrical MAGAT gel phantoms were exposed to different dose levels; one phantom was unirradiated for background subtraction. This experiment was repeated for three different effective beam energies.24 h post irradiation the spin‐spin relaxation times (T2) were measured with a 4.7 T Bruker MR scanner at 2 cm depth inside the gel. The T2 values were converted to relaxation rates (R2) and plotted against the respective dose levels corresponding to the different effective energies. The resulting dose response curves were compared for a 250 kVp beam, the 250 kVp beam filtered by 6 cm of water, and a 125 kVp beam.Results:The passage of the 250 kVp beam through water resulted in a half‐value‐layer (HVL) change from 1.05 mm Cu to 1.32 mm Cu at 6 cm depth with a change in effective energy from 81.3 keV to 89.5 keV, respectively. The dose response curves showed a shift to higher relaxation rates for the harder beam. The dose response measurements for the 125 kVp beam (HVL: 3.13 mm Al, effective energy: 33.9 keV) demonstrated even higher relaxation rates than for either of the other beams.Conclusion:The MAGAT dose response curves for three different effective energies demonstrate a complex energy dependence, with an apparent decrease in sensitivity at 89.5 keV effective energy. This energy dependence is consistent with observed discrepancies of depth dose data compared with ion‐chamber data. For future investigations of larger volumes, an energy‐dependent sensitivity function is needed to properly assess 3‐dimensional dose distributions.

SU-E-T-599: The Variation of Hounsfield Unit and Relative Electron Density Determination as a Function of KVp and Its Effect On Dose Calculation Accuracy

Purpose: To investigate the differences in relative electron density for different energy (kVp) settings and the effect that these differences have on dose calculations. Methods: A Nuclear Associates 76-430 Mini CT QC Phantom with materials of known relative electron densities was imaged by one multi-slice (16) and one single-slice computed tomography (CT) scanner. The Hounsfield unit (HU) was recorded for each material with energies ranging from 80 to 140 kVp and a representative relative electron density (RED) curve was created. A 5 cm thick inhomogeneity was created in the treatment planning system (TPS) image at a depth of 5 cm. The inhomogeneity was assigned HU for various materials for each kVp calibration curve. The dose was then calculated with the analytical anisotropic algorithm (AAA) at points within and below the inhomogeneity and compared using the 80 kVp beam as a baseline. Results: The differences in RED values as a function of kVp showed the largest variations of 580 and 547 HU for the Aluminum and Bone materials; the smallest differences of 0.6 and 3.0 HU were observed for the air and lung inhomogeneities. The corresponding dose calculations for the different RED values assigned to the 5 cm thick slab revealedmore » the largest differences inside the aluminum and bone inhomogeneities of 2.2 to 6.4% and 4.3 to 7.0% respectively. The dose differences beyond these two inhomogeneities were between 0.4 to 1.6% for aluminum and 1.9 to 2.2 % for bone. For materials with lower HU the calculated dose differences were less than 1.0%. Conclusion: For high CT number materials the dose differences in the phantom calculation as high as 7.0% are significant. This result may indicate that implementing energy specific RED curves can increase dose calculation accuracy.« less

Investigating magnetic field dose effects in small animals: a Monte Carlo study

Purpose: In MRI-linac treatments, radiation dose distributions are affected by magnetic fields, especially at high-density/low-density interfaces. Radiobiological consequences of magnetic field dose effects are presently unknown and preclinical studies are desirable. This study investigates the optimal combination of beam energy and magnetic field strength needed for preclinical murine studies. Methods : The Monte Carlo code MCNP6 was used to simulate the effects of a magnetic field when irradiating a mouse lung phantom with a 1.0 cm × 1.0 cm photon beam. Magnetic field dose effects were examined using various beam energies (225 kVp, 662 keV [Cs-137], and 1.25MeV [Co-60]) and magnetic field strengths (0.75 T, 1.5 T, and 3 T). The resulting dose distributions were compared to Monte Carlo results for humans with various field sizes and patient geometries using a 6MV/1.5T MRI-linac. Results: In human simulations, the addition of a 1.5 T magnetic field causes an average dose increase of 49% (range: 36% - 60%) to lung at the soft tissue-lung interface and an average dose decrease of 30% (range: 25% - 36%) at the lung-soft tissue interface. In mouse simulations, no magnetic field dose effects were seen with the 225 kVp beam. The dose increase for the Cs-137 beam was 12%, 33%, and 49% for 0.75 T, 1.5 T, and 3.0 T magnetic fields, respectively while the dose decrease was 7%, 23%, and 33%. For the Co-60 beam the dose increase was 14%, 45%, and 41%, and the dose decrease was 18%, 35%, and 35%. Conclusion : The magnetic field dose effects observed in mouse phantoms using a Co-60 beam with 1.5 T or 3 T fields or a Cs-137 beam with a 3T field fall within the range seen in humans treated with an MRI-linac. These irradiator/magnet combinations are therefore suitable for preclinical studies investigating potential biological effects of delivering radiation therapy in the presence of a magnetic field. --------------------------- Cite this article as: Rubinstein A, Guindani M, Hazle JD, Court LE. Investigating magnetic field dose effects in small animals: a Monte Carlo study. Int J Cancer Ther Oncol 2014; 2(2):020233. DOI: 10.14319/ijcto.0202.33

Energy dependence of polymer gels in the orthovoltage energy range

Purpose: Ortho-voltage energies are often used for treatment of patients’ superficial lesions, and also for small- animal irradiations. Polymer-Gel dosimeters such as MAGAT (Methacrylic acid Gel and THPC) are finding increasing use for 3-dimensional verification of radiation doses in a given treatment geometry. For mega-voltage beams, energy dependence of MAGAT has been quoted as nearly energy-independent. In the kilo-voltage range, there is hardly any literature to shade light on its energy dependence. Methods : MAGAT was used to measure depth-dose for 250 kVp beam. Comparison with ion-chamber data showed a discrepancy increasing significantly with depth. An over-response as much as 25% was observed at a depth of 6 cm. Results and Conclusion: Investigation concluded that 6 cm water in the beam resulted in a half-value-layer (HVL) change from 1.05 to 1.32 mm Cu. This amounts to an effective-energy change from 81.3 to 89.5 keV. Response measurements of MAGAT at these two energies explained the observed discrepancy in depth-dose measurements. Dose-calibration curves of MAGAT for (i) 250 kVp beam, and (ii) 250 kVp beam through 6 cm of water column are presented showing significant energy dependence. ------------------- Cite this article as : Roed Y, Tailor R, Pinksy L, Ibbott G. Energy dependence of polymer gels in the orthovoltage energy range. Int J Cancer Ther Oncol 2014; 2(2):020232. DOI : 10.14319/ijcto.0202.32

Development and validation of a measurement‐based source model for kilovoltage cone‐beam CT Monte Carlo dosimetry simulations

The purpose of this study is to adapt an equivalent source model originally developed for conventional CT Monte Carlo dose quantification to the radiation oncology context and validate its application for evaluating concomitant dose incurred by a kilovoltage (kV) cone-beam CT (CBCT) system integrated into a linear accelerator. In order to properly characterize beams from the integrated kV CBCT system, the authors have adapted a previously developed equivalent source model consisting of an equivalent spectrum module that takes into account intrinsic filtration and an equivalent filter module characterizing the added bowtie filtration. An equivalent spectrum was generated for an 80, 100, and 125 kVp beam with beam energy characterized by half-value layer measurements. An equivalent filter description was generated from bowtie profile measurements for both the full- and half-bowtie. Equivalent source models for each combination of equivalent spectrum and filter were incorporated into the Monte Carlo software package MCNPX. Monte Carlo simulations were then validated against in-phantom measurements for both the radiographic and CBCT mode of operation of the kV CBCT system. Radiographic and CBCT imaging dose was measured for a variety of protocols at various locations within a body (32 cm in diameter) and head (16 cm in diameter) CTDI phantom. The in-phantom radiographic and CBCT dose was simulated at all measurement locations and converted to absolute dose using normalization factors calculated from air scan measurements and corresponding simulations. The simulated results were compared with the physical measurements and their discrepancies were assessed quantitatively. Strong agreement was observed between in-phantom simulations and measurements. For the radiographic protocols, simulations uniformly underestimated measurements by 0.54%-5.14% (mean difference = -3.07%, SD = 1.60%). For the CBCT protocols, simulations uniformly underestimated measurements by 1.35%-5.31% (mean difference = -3.42%, SD = 1.09%). This work demonstrates the feasibility of using a measurement-based kV CBCT source model to facilitate dose calculations with Monte Carlo methods for both the radiographic and CBCT mode of operation. While this initial work validates simulations against measurements for simple geometries, future work will involve utilizing the source model to investigate kV CBCT dosimetry with more complex anthropomorphic phantoms and patient specific models.

Thermoluminescent dosimetry in clinical kilovoltage beams

AimThis study aimed at calibrating a new set of GR-200A thermoluminescent dosimeters (TLDs) in low and medium orthovoltage energy photon therapy beams and in a diagnostic beam of known beam quality, in order to determine their response and to establish if the same set of TLDs could be used across both environments for in-vivo dosimetry purposes. Methods and materialsA set of 20 TLDs was used for this study. An Oven type PCL3 was used to anneal the TLDs. The response of the TLDs was read using the Reader type LTM manufactured by Harshaw Bicron, United State of America. Vacuum tweezers were used to transfer the TLDs at the time of measurements and calibration. TLDs were kept in a subdued ultra-violet environment between the annealing and irradiation process. TLDs were placed on a 30 × 30 × 17.6 cm3 Polymethylmethacrylate (PMMA) phantom during irradiation. A calibrated Orthovoltage machine was used to deliver a known absorbed dose to the TLDs using 95 kVp (3.00 mm Al HVL), 180 kVp (1.00 mm Cu HVL), and 300 kVp (3.00 mm Cu HVL) therapy beams. A cylindrical ionization chamber (PTW 30001) and an electrometer (PTW 10008) were used to confirm the absorbed dose delivery of the orthovoltage machine at the time of measurement. Likewise, a calibrated LX40 radiotherapy Simulator was used to deliver a known diagnostic absorbed dose to the TLDs using an 80 kVp beam of 2.97 mm Al HVL. A TM77334 ionization chamber was used similarly to confirm the absorbed dose. The TLDs were also irradiated on the PMMA phantom to generate element correction coefficient and readers calibration factor necessary for calibration of the reader before actual calibration of the TLDs. The accepted variation in raw response of the individual TLDs from the average of the batch was compared and a deviation of less than ±20% was considered within tolerance. A 10% tolerance was subsequently considered suitable for the measurement of absorbed dose. ResultsOf the 20 TLDs calibrated in the 95 kVp therapy beam (3 mm Al half-value layer), 17 were within the accepted response level (i.e. ±20% deviation), 17 in the 180 kVp therapy beam (1 mm Cu half-value layer), 16 in the 300 kVp therapy beam (3 mm Cu half-value layer) and 15 in the diagnostic beam of 80 kVp (2.97 mm Al half-value layer). 16 of the 17 TLDs were within ±10% dose tolerance at 95 kVp whereas all the TLDs that were within the accepted response level at the 180 kVp and 300 kVp, were within the ±10% dose tolerance. 12 of the 15 TLDs at the diagnostic beam energy were within the ±10% dose tolerance. Three of the TLDs were therefore rejected at all energies. ConclusionThe study concludes that the same set of GR-200A TLDs could be used across both kilovoltage therapy and diagnostic fluoroscopy environments for in-vivo dosimetry purposes.

SU-D-141-02: KV OBI Spectroscopy: Measurement Vs Monte Carlo Simulation

Purpose: With IGRT there has been a dramatic increase in the use of kV based on-board imaging (OBI). Although, numerous Monte Carlo (MC) simulation studies on the OBI kV source exist, the only validation of the actual OBI photon energy spectrum has been in the form of half-value-layers. Realistic MC OBI simulation requires correct modeling of the kV beam spectrum. Here, we present for the first time, actual experimentally measured OBI spectra together with their MC simulated counterparts. Methods: The spectra produced by a commercial LINAC OBI were measured using a thermoelectric cooled CdTe detector and a multichannel analyzer. To reduce detector over saturation, the kV radiation source photon flux was reduced by the use of collimating disks placed before the CdTe surface and by setting the kV source to its minimum mAs. Preliminary measured spectra for 60 kVp, 80 kVp, 100 kVp, and 125 kVp beams for the OBI were compared to MC results based on a model of the OBI. For the 125 kV beam, a percent depth dose (PDD) curve was measured with a diamond detector, and compared to a PDD generated by MC based on the simulated spectrum. Results: The corrected measured spectra correspond roughly to the MC generated spectra, with the MC spectra generally showing more high energy photons and fewer low energy photons. The MC PDD and measured PDD agree to within 2% of absolute dose. Conclusion: We present new measurements of kV OBI spectrometry to advance kV dosimetry and more sophisticated modeling of kV imaging systems. The discrepancy between measured and simulated spectra is small, and the dosimetric effect of the discrepancy is also small.

SU-E-T-101: Is EBT3 Energy Independent for Radiation-Therapy Photon & Electron Beams?

Purpose: Evaluate the new GafChromic film EBT3 for energy dependence over clinical photon and electron beams down to75kVpMethods: A single sheet of EBT3 (Lot A04041201) was cut into small pieces. Film pieces were irradiated at 6–9 known dose levels (0.4 – 12 Gy) with 9 beams: 75, 225, and 300 kVp x-rays, Cs-137, Co-60, 6 and 18 MV x-rays, and 6 and 20 MeV electrons. At each dose level, 3 pieces of film were irradiated to provide repeats. For KV x-ray beams including Cs-137, irradiations were performed in air with an appropriate build-up required for Cs-137; whereas for other beams irradiations were performed at dmax depth in water. The exposed films were read and analyzed with combination of flat-bed scanner (Epson 10000XL) and ImageJ software (red channel). Dose-consistency check was performed at 75 kVp by comparing film-response curve with exposure in-air versus that in Solid Water (depth 2.0 cm). Results: In Ortho-voltage region, energy dependence is significant. Film sensitivity (OD) decreases with decreasing beam energy (kVp). At 75 kVp, film-sensitivity (OD) is 16% lower than that for 6 MV beam at 6 Gy dose level. For Mega-voltage beams including Cs-137, film EBT3 is found to be practically energy independent within the measurement uncertainty. Dose uncertainty arising from film uniformity and film-to-film variation is about the same (5 ± 1.1%) as reported1 for EBT2. Conclusion: In Kilo-Voltage region, EBT3 shows significant energy dependence showing decrease in film-sensitivity with decreasing beam energy. However, the film is practically energy independent for Mega-voltage Photon & Electron beams including Cs-137. Film-sensitivity decrease for 75 kVp is consistent with the recently reported2 >11% reduction in OD for 50 kVp beam. In light of another recent publication3, energy dependence may arise from components < 25 KeV in the beams.

SU-D-134-04: On the (f)utility of Measuring the Lead Equivalence of Protective Garments

Purpose: Verifying claims regarding lead equivalence for lead-composite and lead-free garments is difficult. Current standards only require measurement of lead equivalence at a single beam quality. A garment may provide a high degree of protection at the specified beam quality, but underperform at others. We sought to measure the lead equivalence of several protective garments and propose a better method for quantifying protective value. Methods: We measured the penetration of primary and scattered radiation through lead sheets and three garments of nominal 0.5 mm lead equivalence, one lead and two lead-free. Penetration was measured using nominal 60, 80, 100, and 120 kVp beams. Using polynomial least-squares fits to the measured data of penetration through lead sheets, we determined the lead equivalence of the protective garments. Results: The lead garment was 0.5 mm lead equivalent across all beam qualities. While the maximum lead equivalence of the lead-free garments occurred at the manufacturer-specified beam quality, neither garment was 0.5 mm lead equivalent. The lead equivalence of the lead-free garments was a strong function of beam quality and nature of the radiation. The lead equivalence of the lead-free garments in primary beams ranged from 0.40 to 0.47 mm and in scattered beams ranged from 0.37 to 0.46 mm. We also calculated linear fits of penetration through lead as a function of HVL. We propose the Diagnostic Radiation Index of Protection (DRIP), a weighted sum of the penetration percentage across a range of beam qualities. Conclusion: The protective value of garments from both primary and scattered radiation is a strong function of beam quality. Assessment of protective value should be performed across a range of beam qualities. Methods for performing such assessment must be developed and must specify beam qualities, measurement geometry, and the appropriate weighting across the beam quality range for different applications.

WE-G-141-09: Prospective Image Planning in Radiation Therapy for Optimization of Image Quality and Reduction of Patient Dose

Purpose: X‐ray image‐guided radiotherapy (IGRT) relies on generalized, vendor‐provided acquisition parameters. These parameters are chosen via broad categorizations in patient anatomy and often do not yield optimal image contrast and patient dose. In the IGRT setting, precise prior knowledge of patient anatomy and imaging goal are available via the simulation CT scan. We developed and tested a patient‐specific image planning system (IPS) for radiotherapy and assessed its predictive capabilities and ability to reduce patient imaging dose. Methods: The IPS calculates patient‐specific attenuation of the imaging beam and integrates the detector response into the simulated image. Experimental validation was conducted using an anthropomorphic phantom over a range of mAs values for 80 and 120 kVp beam qualities. Data for the head and neck (H/N), thorax and pelvis were acquired. Clinical data was collected from patients undergoing IGRT for H/N and abdominal diseases. Retention of image contrast following imaging dose reduction was verified using mutual information as a metric. Results: The IPS was found to be capable of predicting under exposure, saturation and a contrast plateau over a wide exposure range. The IPS was successful at predicting a lack of appreciable change in contrast between the images acquired at high and low kVp settings. It was found to be capable of reducing patient imaging dose without loss of appreciable contrast. With the IPS, imaging dose reduction of at least 37 to 74 percent was achieved for the patients studied. Conclusion: Image contrast resulting from under exposure, over exposure as well as a contrast plateau can be predicted by use of a prospective image planning algorithm. Image acquisition parameters can be predicted that reduce patient dose without loss of contrast.

Characterization of Rotating Gantry Micro-CT Configuration for theIn VivoEvaluation of Murine Trabecular Bone

The objective of this study is to determine the optimal physical parameters of a rotating gantry micro-CT system to assess in vivo changes to the trabecular bone of mice. Magnification, binning, peak kilovoltage, beam filtration, and tissue thickness are examined on a commercially available micro-CT system. The X-ray source and detector geometry provides 1.3×, 1.8×, or 3.3× magnification. Binning is examined from no binning to 2 to 4. Energy is varied from 40 to 80 kVp in 10 kVp increments and filter thickness is increased from no filtration to 1.5 mmAl in 0.5 mmAl increments. Mice are imaged at different magnifications and binning combinations to evaluate changes to image quality and microstructure estimation. Increasing magnification from 1.3× to 3.3× and lowering binning from 4 to 1 varies the spatial resolution from 2.5 to 11.8 lp/mm. Increasing the beam energy or filtration thickness decreases Hounsfield unit (HU) estimation, with a maximum rate of change being -286 HU/kVp for 80 kVp. Images for murine trabecular bone are blurred at effective pixel sizes above 60 μm. By comparing resolution, signal-to-noise ratio, and radiation dose, we find that a 3.3× magnification, binning of 2.80 kVp beam with a 0.5 mmAl filter comprises the optimal parameters to evaluate murine trabecular bone for this rotating gantry micro-CT.

Assessment of multi-directional MTF for breast tomosynthesis

A method was developed to assess the multi-directional modulation transfer function (MTF) of breast tomosynthesis imaging systems using a sphere phantom. The method was initially developed based on a simulation dataset. Projections were simulated for a uniform voxelized breast phantom with sphere inserts using a fluence modeled from a 28 kVp beam incident upon an indirect flat-panel detector. Based on cascaded systems modeling, characteristic noise and blurring were added to each projection. The projections were reconstructed using a standard filtered backprojection technique, producing a 3D volume with an isotropic voxel size of 200 µm. ROIs that completely encompassed single spheres were extracted and conical regions were prescribed along the three major axes extending from the centroids. Pixels within the cones were used to form edge spread functions (ESFs), from which the directional MTFs were calculated. Binning size and conical range were adjusted to maximize the accuracy and to minimize the noise of the MTF. A method was further devised to remove out-of-plane artifacts from the ESF in the x–y plane. Finally, the method was applied to experimentally assess the directional MTF of a prototype tomosynthesis system. Comparisons of the sphere-based MTF along the different axes and the theoretical MTF yielded good agreement. A 30° angular cone and a 20 µm sampling were found to provide an ideal trade-off between the noise and accuracy of the measurement. The removal of artifacts in ESF yielded ‘modified’ MTFs that enabled a resolution-only characterization of the in-slice resolution of tomosynthesis. Drop-off frequencies in the x- and y-directional MTFs were 1.6 cycles mm−1 and 1.5 cycles mm−1, respectively. The presented method of separating the effective resolution and artifacts from the measured ESF was found experimentally implementable and is expected to facilitate the interpretation of MTF measurements in tomosynthesis.

Image performance evaluation of a cone beam O-arm imaging system

Image performance of a cone beam O-arm system with the use of a Varian flat panel detector (PaxScan 4030CB) was evaluated. Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and tube output characteristics were measured. Image quality measurements were performed with a CATPHAN 424 phantom using a 120 kVp beam filtered with 1 mm Al and 0.1 mm Cu and three preset protocols at different exposure levels. The CPT445 Module was used to measure MTF. Water phantom images were used to calculate NPS. Then the NPS was normalized by dividing the square of the mean signal. The Photon fluence at each exposure (q) was estimated using the SpekCalc. The MTF indicates that the reconstruction filter of the O-arm is overenhancing, meaning that it yields values greater than unity. The system has a 10% MTF at 0.45 mm. The NPS shows the low frequency structured noise. The evaluation of image performance of the O-arm imaging system helps its future improvements and clinical applications.

The dosimetry of eye shields for kilovoltage X-ray beams

The objective of this work was to evaluate the dosimetry of tungsten eye shields for use with kilovoltage X-ray beam treatments. The eye shields, originally designed for megavoltage electron beams, were made of 2 mm tungsten thickness and inside diameters of 11.6 and 15.0 mm with optional aluminium caps of 0.5 and 1 mm thickness. The relative dosimetry of the eye shields were examined by measurement of transmission doses with full scatter conditions, central axis depth doses and beam profiles underneath the eye shield. The X-ray beams used in this study ranged in energy from 50 to 280 kVp. Transmission measurements were performed using an Advanced Markus ionisation chamber located at the surface of an RMI457 Solid Water phantom with a 3 cm diameter applicator flush against the phantom surface. Depth doses and profiles measurements were performed in a PTW MP3 scanning water tank with a PTW diamond detector. Results for transmission doses for the medium size eye shield increased from 1 to 22 % for 50-280 kVp while for the smaller eye shield the percentage dose increased from 3.5 to 30 % for the same energy range. There were minimal differences between using the 0.5 and 1 mm aluminium caps. Central axis depth doses measured with and without the eye shields demonstrated the 125 and 180 kVp beams had higher peak doses behind the eye shields. These results show that these tungsten eye shields are suitable for use with kilovoltage X-ray beams. However, the clinical impact needs to be considered for the higher X-ray beam energies.

Validating plastic scintillation detectors for photon dosimetry in the radiologic energy range

Photon dosimetry in the kilovolt (kV) energy range represents a major challenge for diagnostic and interventional radiology and superficial therapy. Plastic scintillation detectors (PSDs) are potentially good candidates for this task. This study proposes a simple way to obtain accurate correction factors to compensate for the response of PSDs to photon energies between 80 and 150 kVp. The performance of PSDs is also investigated to determine their potential usefulness in the diagnostic energy range. A 1-mm-diameter, 10-mm-long PSD was irradiated by a Therapax SXT 150 unit using five different beam qualities made of tube potentials ranging from 80 to 150 kVp and filtration thickness ranging from 0.8 to 0.2 mmAl + 1.0 mmCu. The light emitted by the detector was collected using an 8-m-long optical fiber and a polychromatic photodiode, which converted the scintillation photons to an electrical current. The PSD response was compared with the reference free air dose rate measured with a calibrated Farmer NE2571 ionization chamber. PSD measurements were corrected using spectra-weighted corrections, accounting for mass energy-absorption coefficient differences between the sensitive volumes of the ionization chamber and the PSD, as suggested by large cavity theory (LCT). Beam spectra were obtained from x-ray simulation software and validated experimentally using a CdTe spectrometer. Correction factors were also obtained using Monte Carlo (MC) simulations. Percent depth dose (PDD) measurements were compensated for beam hardening using the LCT correction method. These PDD measurements were compared with uncorrected PSD data, PDD measurements obtained using Gafchromic films, Monte Carlo simulations, and previous data. For each beam quality used, the authors observed an increase of the energy response with effective energy when no correction was applied to the PSD response. Using the LCT correction, the PSD response was almost energy independent, with a residual 2.1% coefficient of variation (COV) over the 80-150-kVp energy range. Monte Carlo corrections reduced the COV to 1.4% over this energy range. All PDD measurements were in good agreement with one another except for the uncorrected PSD data, in which an over-response was observed with depth (13% at 10 cm with a 100 kVp beam), showing that beam hardening had a non-negligible effect on the PSD response. A correction based on LCT compensated very well for this effect, reducing the over-response to 3%. In the diagnostic energy range, PSDs show high-energy dependence, which can be corrected using spectra-weighted mass energy-absorption coefficients, showing no considerable sign of quenching between these energies. Correction factors obtained by Monte Carlo simulations confirm that the approximations made by LCT corrections are valid. Thus, PSDs could be useful for real-time dosimetry in radiology applications.

Investigation of the dose distribution for a cone beam CT system dedicated to breast imaging

Cone-beam breast Computed Tomography (bCT) is an X-ray imaging technique for breast cancer diagnosis, in principle capable of delivering a much more homogeneous dose spatial pattern to the breast volume than conventional mammography, at dose levels comparable to two-view mammography. We present an investigation of the three-dimensional dose distribution for a cone-beam CT system dedicated to breast imaging. We employed Monte Carlo simulations for estimating the dose deposited within a breast phantom having a hemiellipsoidal shape placed on a cylinder of 3.5 cm thickness that simulates the chest wall. This phantom represents a pendulant breast in a bCT exam with the average diameter at chest wall, assumed to correspond to a 5-cm-thick compressed breast in mammography. The phantom is irradiated in a circular orbit with an X-ray cone beam selected from four different techniques: 50, 60, 70, and 80 kVp from a tube with tungsten anode, 1.8 mm Al inherent filtration and additional filtration of 0.2 mm Cu. Using the Monte Carlo code GEANT4 we simulated a system similar to the experimental apparatus available in our lab. Simulations were performed at a constant free-in-air air kerma at the isocenter (1 μGy); the corresponding total number of photon histories per scan was 288 million at 80 kVp. We found that the more energetic beams provide a more uniform dose distribution than at low energy: the 50 kVp beam presents a frequency distribution of absorbed dose values with a coefficient of variation almost double than that for the 80 kVp beam. This is confirmed by the analysis of the relative dose profiles along the radial (i.e. parallel to the “chest wall”) and longitudinal (i.e. from “chest wall” to “nipple”) directions. Maximum radial deviations are on the order of 25% for the 80 kVp beam, whereas for the 50 kVp beam variations around 43% were observed, with the lowest dose values being found along the central longitudinal axis of the phantom.

SU-E-I-105: Optimizing Bomb Squad X-Ray Systems for Incidental Human Exposures.

A current terrorist tactic involves strapping an explosive device to a live victim. Technicians wishing to x-ray the device need to use appropriate exposures to create clear images while mitigating incidental health problems for the live victim. This project involves characterizing and optimizing a popular radiographic system in the explosive investigation community, both for general use and, more specifically, for generating x-rays with a human subject in the beam path. This project seeks to minimize the dose to a victim incidentally located in the beam path, while maximizing image quality, thereby allowing appropriate image evaluation to neutralize the explosive threat. Image quality and dose optimization begins with characterization of the x-ray source. The source in this study is an XRS-3, a portable device developed by Golden Engineering, which nominally produces a 270 kVp beam. Characterization of the beam involves verifying the actual kVp, as well as determining the average energy of the beam with various amounts of filtration added to the beam path. The kVp was verified using a noninvasive technique, utilizing the measurement of the dose attenuation through lead and then modeling the energy dependent attenuation curve of the lead using specific mass attenuation coefficients and parameters suggested by Joseph (Joseph, P.M., Med. Phys. Vol. 2, July/Aug1975; 201-207) in order to calculate the kVp through linear regression. The average energy of an unfiltered beam was 52 keV, and increases through adding various combinations of Cu and brass filters to a maximum value of 121 keV with 3 mm copper + 3 mm brass filtration. The calculated kVp was 290 kV. The average energies using filtration fall within the diagnostic range. The kVp fits well within the manufacturer's specifications of 15% deviation. Optimization can be accomplished with these parameters. University of Oklahoma Health Sciences Center- Department of Radiological Sciences.

WE‐G‐BRB‐05: On the Importance of Fluorescence Within the Stem Effect of Scintillation Detectors

To quantify the nature and composition of the light produced in optical fibers under different irradiation conditions and evaluate its impact on dosimetry. Irradiation of a bare PMMA optical fiber (Mitsubishi ESKA Premier) was performed using a superficial therapy unit, an Ir-192 HDR brachytherapy source, a Co-60 external-beam unit as well as photon and electron beams from a linear accelerator. Spectra of the radiation-induced visible light in the fiber were acquired and signals were compared as a function of depth and irradiation type. Irradiation of a 75 kVp beam from the superficial therapy unit was used to isolate the fluorescence spectrum. Isolation of the Cerenkov spectrum component was obtained from irradiation of a 15 MeV electron beam at a 45 degree angle. Relative composition in fluorescence and Cerenkov of the stem effect light has been determined for all irradiations. The total stem effect spectra can be represented by a linear superposition of the fluorescence and Cerenkov spectra. The fluorescence contribution was shown to strongly differ between the superficial therapy unit (99%±1%), the Ir-192 HDR source (25%±3%) and higher energy irradiations (3%±2%). Variations within each energy regime (kV, HDR brachytherapy and MV) were small at 3% or lower. These were observed for irradiations at angle or when the fiber was near the surface. This study suggests it is better to calibrate the stem effect of a scintillation detector using the same irradiation modality. Stem effect light was shown to be composed of fluorescence and Cerenkov light in different proportions depending on the geometry of the experimental setup, nature of the irradiation, and irradiation energy. Calibrating detectors separately for fluorescence and Cerenkov may lead to better performance of the stem effect removal technique.