Beam Central Axis Research Articles

Estimation of peripheral dose from Co60 beam in water phantom measured in Secondary Standard Dosimetry Laboratory, Pakistan

Peripheral or scatter dose harms neighbouring normal tissues during administration of dose to cancerous tissues, therefore, knowledge of peripheral dose is an important consideration in radiotherapy. In present study, absorbed dose measurements in a water phantom were performed for three field sizes, 7×7cm2, 10×10cm2 and 15×15cm2. For each field size, dose was measured at six depths below the front surface of the water phantom; 2.5-15cm with an interval of 2.5cm. Measurements were made at equal transverse distances along the horizontal axis, from 1cm to 6cm, on both sides of the central beam axis and normalized with central axis dose of each field. All measurements were made at the source to surface distance of 100cm. Variation of peripheral dose with lateral distance was analysed and an appropriate parametric equation for each field size and depth was constructed. The peripheral radiation dose showed a strong dependence on field size and distance from field boundary.

Low dose out-of-field radiotherapy, part 3: Qualitative and quantitative impact of scattered out-of-field radiation on MDA-MB-231 cell lines

PurposePatients who undergo external beam radiotherapy are at risk of developing second tumours due to scattered radiation outside the path of the primary beam. The aim of this study was to experimentally determine the in vitro radiobiological effects of scattered radiation in cells located outside the primary photon beam and to compare this to the effects that occur in cells inside the primary beam. The comparison was performed by assessing cell viability, DNA damage, and apoptosis. Material and methodsCells from the human breast cancer line MDA-MB-231 were inserted in a water phantom and irradiated at varying doses (1.5, 2.0, 2.5, and 3.0Gy). The cells were placed at two geometrical points: in the central beam axis and at 10cm out-of-field. The dose was constant in both geometrical points. Survival fraction, number of DNA double strand-breaks, and cleaved poly-(ADP-ribose) polymerase (PARP) levels were determined by clonogenic assay and flow cytometry. ResultsA slight, non-significant decrease of 3 to 5% in cell survival fraction was observed in cells irradiated outside the primary field. The number of PARP-positive cells and DNA double strand-breaks both increased after out-of-field irradiation. ConclusionScattered irradiation appears to induce an in vitro biological response on out-of-field cells that is stronger than the effect of primary radiation on in-field cells, independent of the bystander effect. These findings suggest that the biological response of healthy tissues outside the primary beam might be higher than previously believed.

Low dose out-of-field radiotherapy, part 1: Measurement of scattered doses

PurposeTo measure out-of-field doses in a phantom model to better quantify this radiation. Material and methodsThe individual contribution of photons and neutrons to the total out-of-field dose for 6 MV and 20 MV photons at open beam were measured in a purpose-designed water phantom. Radiation doses were measured at seven separate points (P1–P7) in the phantom with thermoluminescent detectors (TLD 100, 600, and 700) and GAFchromic™ EBT films. ResultsAt a prescribed dose of 75Gy to the isocentre, the photon dose level in the close-to-field area (P2) ranged from 2.0–2.5Gy for 6 MV and 1.5–2.0Gy for 20 MV; the total out-of-field doses at P2 and P7, respectively, were estimated to be as follows: for 6 MV: TLD 100 (<3.23% and<0.14%); radiochromic film (<2.52% and <0.03%); and for 20 MV: TLD 100 (<2.94% and <0.78%); TLD 700 (<2.02% and <0.14%); and radiochromic film (<1.73% and <0.01%). Although the dose decreased rapidly as the distance from the central beam axis increased, even distant doses could be as high as several centigrays. The neutron dose for 20 MV photons at a distance of 25cm from the isocentre was 4.0mSv/Gy. ConclusionOur results show that in the close-to-field area, the dose level could be as high as 1.5Gy assuming a prescribed dose of 75Gy to the isocentre. By contrast, the doses delivered to more distant areas from the planning target volume were much lower (centigrays). These findings show that both 6 MV and 20 MV photons could produce dosimetrically important dose levels outside of the field. The data reported here may be of value to study the potential impact of even very low doses of radiation on human tissues.

Photoneutron depth dose equivalent distributions in high-energy X-ray medical accelerators by a novel position-sensitive dosimeter

PurposeThe purpose of this study was to; (1) investigate employing a novel position-sensitive mega-size polycarbonate (MSPC) dosimeter for photoneutron (PN) depth, profile and dose equivalent distributions studies in a multilayer polyethylene phantom in a Siemens ONCOR accelerator, and (2) develop depth dose equivalent distribution matrix data at different depths and positions of the phantom for patient PN dose equivalent determination and in particular for PN secondary cancer risk estimation. MethodsPosition-sensitive MSPC dosimeters were successfully exposed at 9 different depths of the phantom in a 10×10cm2 X-ray field. The dosimeters were processed in mega-size electrochemical chambers at optimum conditions. Each MSPC dosimeter was placed at a known phantom depth for PN depth dose equivalents and profiles on transverse, longitudinal and diagonal axes and isodose equivalent distribution studies in and out of the X-ray beam. ResultsPN dose equivalent distributions at any depth showed the highest value at the beam central axis and decreases as the distance increases. PN dose equivalent at any position studied in the axes has a maximum value on the phantom surface which decreases as depth increases due to flux reduction by multi-elastic scattering interactions. ConclusionsExtensive PN dose equivalent matrix data at different depths and positions in the phantom were determined. The position-sensitive MSPC dosimeters proved to be highly efficient for PN depth, profile and isodose equivalent distribution studies. The extensive data obtained highly assists for determining PN dose equivalent of a patient undergoing high-energy X-ray therapy and for PN secondary cancer risk estimation.

Dose distribution at the Bragg peak: Dose measurements using EBT and RTQA gafchromic film set at two positions to the central beam axis.

To evaluate the impact of radiochromic film positioning relative to the central beam axis (CAX) in proton beam therapy. Secondarily, to compare the dosimetric measurements obtained by RTQA and EBT film and to compare these to the doses calculated by the treatment planning system (TPS). The EBT and RTQA dosimetric radiochromic films were immersed in a water phantom and irradiated with a proton beam. The films were placed parallel to the CAX and at a 5° angle on the horizontal plane to assess the effect of film inclination on Bragg peak profiles. Calibration was performed by irradiating small pieces of film at doses ranging from 0.0 Gy to 3.5 Gy in increments of 0.5 Gy. The TPS was used to create treatment plans for two different geometrical targets (cylindrical and cuboidal). After irradiation, all film pieces were scanned on a flatbed scanner and red channel data were extracted from the 48-bit RGB images using ImageJ, Photoshop, Origin8, and Excel software. The dose distributions from the irradiated films were compared to the dose obtained from the TPS. Bragg peak profiles were abstracted from the irradiated films and compared. The dosimetric measurements obtained by both EBT and RTQA positioned at a 5° to the CAX closely matched the dose calculated by the TPS for the cylindrical target. In contrast, dose distributions measured in the cuboidal targets were less precise. Gamma index (GI) values (3%/3 mm acceptance criteria for isodose >90% of dose) were 99.8% and 93% for EBT film placed at a 5° angle versus 47.1% and 80.8% for EBT film parallel to the beam. The dosimetric measurements in RTQA film positioned parallel to the CAX showed GI values with <27% agreement with the TPS-calculated dose. Our finding show that RTQA film can be used to accurately measure doses in the proton beam at the region of Bragg peak; however, to obtain the most accurate readings, the film should be positioned at a small angle to the CAX.

Small field depth dose profile of 6 MV photon beam in a simple air-water heterogeneity combination: A comparison between anisotropic analytical algorithm dose estimation with thermoluminescent dosimeter dose measurement.

To establish trends of estimation error of dose calculation by anisotropic analytical algorithm (AAA) with respect to dose measured by thermoluminescent dosimeters (TLDs) in air-water heterogeneity for small field size photon. TLDs were irradiated along the central axis of the photon beam in four different solid water phantom geometries using three small field size single beams. The depth dose profiles were estimated using AAA calculation model for each field sizes. The estimated and measured depth dose profiles were compared. The over estimation (OE) within air cavity were dependent on field size (f) and distance (x) from solid water-air interface and formulated as OE = - (0.63 f + 9.40) x2+ (-2.73 f + 58.11) x + (0.06 f2 - 1.42 f + 15.67). In postcavity adjacent point and distal points from the interface have dependence on field size (f) and equations are OE = 0.42 f2 - 8.17 f + 71.63, OE = 0.84 f2 - 1.56 f + 17.57, respectively. The trend of estimation error of AAA dose calculation algorithm with respect to measured value have been formulated throughout the radiation path length along the central axis of 6 MV photon beam in air-water heterogeneity combination for small field size photon beam generated from a 6 MV linear accelerator.

Dosimetric characterization of an accessory mounted mini-beam collimator across clinically beam matched medical linear accelerators

Background and Purpose. The goal of this project was the dosimetric characterization of a mini-beam collimator across three clinically beam matched medical linear accelerators (linacs). Methods and Materials. The beam quality (%DD(10)), peak-to-valley dose ratio (PVDR), collimator factor and relative output were obtained for 6 MV mini-beam collimated fields of various sizes on three clinically beam matched Varian iX medical linear accelerators. Monte Carlo simulations of the mini-beam collimated fields were used to correlate the experimental results to the accelerators’ electron beam full width-half maximum (FWHM) incident on the Bremsstrahlung target. Results. The beam quality of the mini-beam collimated field on all three linear accelerators agreed with that of the open field beam to within ±1%. PVDR on the different linacs varied by up to ±8.1% from the mean. Similarly, the collimator factors varied from the mean by up to ±3.6%. However, changes in the mini-beam collimated field due to changes in collimator inclination with respect to the beam central axis or field size were consistent across the three linacs. The collimator factors of the linacs were found to decrease by up to 7.1% in response to changes in inclination of less than 0.1°, and have an inverse relationship to field size. Monte Carlo simulations indicated that the disagreement in collimator factor can be linked to variation in the spatial width of the electron beam incident on the Bremsstrahlung target. Conclusion. A mini-beam collimator has been dosimetrically characterized on three clinically beam matched medical linear accelerators. Discrepancies in the mini-beam collimated field characteristics were observed across accelerators. Monte Carlo simulation revealed that these differences were related to the linac electron beam FWHM.

Effect of Material and Wall Thickness Buildup Caps on the Head Scatter Factor Measurements in Irregular Fields Shielded by Cerrobend

The head scatter factor (Sc ) is important to measurements radiation beam and beam modeling of treatment planning systems used for advanced radiation therapy techniques. This study aimed to investigate the design of a miniphantom to measurement variations in collimator Sc in the presence of shielding blocks for shaping the beam using different field sizes. Copper, Brass, and Perspex buildup caps were designed and fabricated locally as material with three different thicknesses for buildup caps (miniphantoms). Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) in Shafa Kerman Hospital, Iran. The Farmer-type ion chamber FG65-P (Scanditronix, Wellhofer) was used for all measurements. To measure the Sc , miniphantom was positioned in a stand vertical to the beam central axis. The data indicate that the Sc measurements using different buildup cap materials and thicknesses in 5 × 10, 7.5 × 7.5, and two 10 × 10 cm Cerrobend shield blocks ranged 0.98 to 1.00, 1.04 to 1.05, and 1.04 to 1.06, respectively. Also, it was observed that by increasing the block shield area from 50 cm2 to both 56.25 and 100 cm2, the Sc increased in all situations. Results showed that using Brass compared to Perspex and Copper has less uncertainty due to its simple preparation and cutting which is useful to measurement of variations in collimator Sc and shaping the photon beam.

Study of 6 MV Photon Beam Dose Profiles and Investigation of Jaw Motion Effects on the Beam Dose Profiles and the Dose Delivered in a Water Phantom

This study investigated the medical linear accelerator jaw motion effects on the delivered dose. The focus was to examinate the photon beam dose profiles as a function of the jaw motion and depending on the depth in the water phantom along the central beam axis and the irradiation field sizes. The measurements of dose profiles were performed for a 6 MV photon beam in a water phantom, the field sizes of 3×3 cm², 4×4 cm², 6×6 cm², 8×8 cm², 10×10 cm², 12×12 cm², 15×15 cm², 20×20 cm², 25×25 cm², 30×30 cm², and 35×35 cm² at depth of 1.5 cm, 5 cm, 10 cm, 20 cm, and 30 cm on the central beam axis, and source-to-surface distance (SSD) of 100 cm for Varian Clinac 2100 linear accelerator. The purpose of this study was to investigate the jaw motion effects on the dose profiles and subsequently on the delivered dose depending on the field size and depth in the water phantom and also the beam dose profiles’ symmetry about the central beam axis. We proceeded to calculate two parameters: the normalized width and the difference of the width between the left and right width of dose profiles as a function of the field size and depth along the central beam axis. We have shown that the motion of the jaws to define the irradiation field size have impacts on the beam dose profiles and its symmetry. the spread of the width difference was in a wave motion along the central beam axis and within a range of off-axis distance from -2 mm to 2 mm, which it should be considered in the treatment of tumors and spare the healthy cells.

Influence of standard RF coil materials on surface and buildup dose from a 6 MV photon beam in magnetic field.

Magnetic resonance guided teletherapy systems aspire to image the patient concurrently with the radiation delivery. Thus, the radiofrequency (RF) coils used for magnetic resonance imaging, placed on or close to patient skin and in close proximity to the treatment volume, would be irradiated leading to modifications of radiation dose to the skin and in the buildup region. The purpose of this work is to measure and assess these dose modifications due to standard off-the-shelf RF coil materials. A typical surface coil was approximated as layered sheets of polycarbonate, copper tape, and Teflon to emulate the base, conductor, and cover, respectively. A separate investigation used additional coil materials, such as copper pipe, plastic coil housing, a typical coil padding material, and a thin copper conductor. The materials were placed in the path of a 6 MV photon beam at various distances from polystyrene phantoms in which the surface and buildup doses were measured. The experiments were performed on a clinical Varian linac with no magnetic field and with a 0.21 T electromagnet producing a magnetic field parallel to the beam central axis. The authors repeated similar experiments in the presence of a 0.22 T magnetic field oriented perpendicular to the beam central axis using an earlier linac-MR prototype, with a biplanar permanent magnet. The radiation detectors used for the measurements were two different parallel plate ion chambers and GAFChromic films. A typical open beam surface dose of 20% (relative to open beam Dmax) was increased to 63% by the coil padding material and to >74% by all other materials when placed in direct contact with the phantom, irrespective of magnetic field presence or orientation. Without a magnetic field, the surface dose decreased as the test materials were moved away from the phantom surface toward the radiation source, reaching between 30% and 40% at 10 cm gap between the phantom and the test materials. In the presence of the transverse magnetic field, the surface dose reduction was more rapid reaching a dose level of 30%-40% with only 3-4 cm gap. In the presence of the parallel magnetic field, as expected, the surface dose did not decrease considerably as the gap between the phantom surface and test materials was increased; the surface dose remained >60% at 10 cm gap for all tested materials except for the thin copper conductor. As expected, placing coil materials in direct contact with the phantom surface increases the surface dose considerably. The surface dose is reduced by creating a gap between the coil materials and phantom surface. This dose reduction happens more rapidly in the presence of a transverse magnetic field. However, the surface dose stays relatively large irrespective of the gap in the presence of a parallel magnetic field. Thus, the standard, off-the-shelf RF coils should be used with caution in integrated linac-MR systems, especially those using a parallel magnetic field orientation in which case the RF coils will probably need to be reconfigured to create open ports for the radiation beam.

Proton computed tomography using a 1D silicon diode array.

Proton radiography (PR) and proton computed tomography (PCT) can be used to measure proton stopping power directly. However, practical and cost effective proton imaging detectors are not widely available. In this study, the authors investigated the feasibility of proton imaging using a silicon diode array. A one-dimensional silicon diode detector array (1DSDA) was aligned with the central axis (CAX) of the proton beam. Polymethyl methacrylate (PMMA) slabs were used to find the correspondence between the water equivalent thickness (WET) and 1DSDA channel number. Two-dimensional proton radiographs were obtained by translation and rotation of a phantom relative to CAX while the proton nozzle and 1DSDA were kept stationary. A PCT image of one slice of the phantom was reconstructed using filtered backprojection. PR and PCT images of the PMMA cube were successfully acquired using the 1DSDA. The WET of the phantom was measured using PR data. The resolution and maximum error in WET measurement are 2.0 and 1.5 mm, respectively. Structures down to 2.0 mm in size could be resolved completely. Reconstruction of a PCT image showed very good agreement with simulation. Limitations in spatial resolution are attributed to limited spatial sampling, beam collimation, and proton scatter. The results demonstrate the feasibility of using silicon diode arrays for proton imaging. Such a device can potentially offer fast image acquisition and high spatial and energy resolution for PR and PCT.

Poster – 12: Radiological assessment of the secondary barrier shielding for IMRT treatments delivered through patient inhomogeneities

Purpose:To assess the impact of radiation treatment delivery through patient inhomogeneities on the secondary barrier shielding requirements in IMRT treatments using Monte Carlo Simulations.Materials and Methods:Scatter factors were calculated at a distance of 1m from the center of a virtual phantom in Geant4.10.01. Phantom (30×30×30 cm3) was inserted with lung (30×30×8 cm3), stainless steel (5×5×5 cm3) and aluminum (5×5×5 cm3) to represent the inhomogeneities. Scatter factor was defined according to the NCRP‐151 recommendations and was calculated for angles of 3° to 120° with respect to the beam's central axis. A virtual radiation source, with energy sampled from a histogram representing 6 MV FFF beam, was used for irradiation with a field size of 15×15 cm2 and SSD of 100 cm.Results:Irradiation through the inhomogeneity affects the patient scattered dose. For high Z material inhomogeneities the scattered dose is reduced due to significant attenuation of the primary radiation. On the other hand if the inhomogeneity is a low Z material such as lung the scattered dose is higher by a maximum of 26%. The average increase in scatter factors for the lung phantom was 17% for angles between 3° and 63° compared to the homogeneous water phantom.Conclusions:In IMRT type treatments delivered through low density patient inhomogeneities (lung) the scattered dose increases significantly. Considering a large proportion of patients receiving radiation therapy for lung cancers the increase in the scattered dose should be incorporated in the shielding calculations for the secondary barriers.

Evaluation of Multiple-Sampling Function used with a Microtek flatbed scanner for Radiation Dosimetry Calibration of EBT2 Film

The radiochromic EBT2 film is a widely used quality assurance device for radiation therapy. This study evaluated the film calibration performance of the multiple-sampling function, a function of the ScanWizard Pro scanning software provided by the manufacturer, when used with Microtek 9800XL plus (9800XL+) flatbed scanner. By using the PDD method, each one of the eight EBT2 films, four delivered by 290 monitor unit (MU) and four by 88 MU via 6-MV photon beams, was tightly sandwiched in a 303-cm3 water equivalent polystyrene phantom prior to irradiation. Before and after irradiation, all films were scanned using the Microtek 9800XL+ scanner with five different modes of the multiple-sampling function, which could generate the image with the averaged result of multiple-sampling. The net optical densities (netOD) on the beam central axis of film were assigned to corresponding depth doses for calibration. For each sampling mode with either delivered MU, the depth-dose uncertainty of a single film from repeated scans and that of a single scan of the four films were analyzed. Finally, the calibration error and the combined calibration uncertainty between film determined depth-doses and delivered depth-doses were calculated and evaluated for each sampling mode. All standard deviations and the calibration error were demonstrated to be unrelated to the number of sampling lines. The calibration error of the 2-line and 16-line mode was within 3cGy and better than that of the other modes. The combined uncertainty of the 2-line mode was the lowest, which was generally less than 6cGy except for the delivered dose around 100cGy. The evaluation described herein revealed that the EBT2 film calibrated with the 2-line mode has relatively lower error, scanning time and combined uncertianty. Therefore, it is recommended for routine EBT2 film calibration and verification of treatment plans.

SU‐F‐T‐461: Dosimetric Evaluation of Indigenous Farmer Type Chamber FAR65‐ GB for Reference Dosimetry of FFF MV Photon Beam

Purpose:Indigenous Farmer type chamber FAR 65 GB is a reference class 0.6 cc ion chamber. It can be used for dosimetric evaluation of photon and high energy electron beams. We studied dosimetric characteristics of the chamber for 6MV and 10MV Flattening filter free FFF photon beams available on trueBEAM STx Linac.Methods:The study was carried out on trueBEAM STx Linac having 6 and 10 MV FFF photon beam with maximum dose rate 1400 and 2400 MU per min respectively. The dosimetric device to be evaluated is Rosalina Instruments FAR 65‐GB Ion Chamber with active volume 0.65 cc, total active length 23.1cm, inner diameter of cylinder 6.2mm, wall thickness 0.4mm, inner electrode diameter 1mm. Inner and outer electrodes are made from Aluminium 2.7 gm per cc and graphite 1.82 gm per cc respectively. The ion chamber was placed along central axis of beam at 10cm depth and irradiated for 10cm × 10cm field size at SAD of 100 cm in plastic phantom. We studied Precision, Dose Linearity, Dose Rate dependence, directional dependence, Recombination effect. Recombination effect was determined using standard two‐voltage method.Results:1. Measurements were reproducible std deviation of 0.0105 and type A uncertainty 0.003265 under same set of reference conditions 2. Chamber exhibit dose linearity over a wider dose range. 3. Chamber shows dose rate independence for all available dose rate range. 4. Response of chamber with the angle of incidence of radiation is constant. 5. Recombination correction factors were 1.01848 and 1.02537 for dose rate 1400 and 2400 MU per min resp.Conclusion:Our study reveals that the chamber is prone to saturation effect at dose rate of 2400 MU per min. FAR 65‐GB can be used for reference dosimetry of FFF MV photon beam with proper calculation of recombination effect.

SU‐F‐T‐300: Impact of Electron Density Modeling of ArcCHECK Cylindricaldiode Array On 3DVH Patient Specific QA Software Tool Analysis

Purpose:3DVH software is a patient specific quality assurance tool which estimates the 3D dose to the patient specific geometry with the help of Planned Dose Perturbation algorithm. The purpose of this study is to evaluate the impact of HU value of ArcCHECK phantom entered in Eclipse TPS on 3D dose &amp; DVH QA analysis.Methods:Manufacturer of ArcCHECK phantom provides CT data set of phantom &amp; recommends considering it as a homogeneous phantom with electron density (1.19 gm/cc or 282 HU) close to PMMA. We performed this study on Eclipse TPS (V13, VMS) &amp; trueBEAM STx VMS Linac &amp; ArcCHECK phantom (SNC). Plans were generated for 6MV photon beam, 20cm×20cm field size at isocentre &amp; SPD (Source to phantom distance) of 86.7 cm to deliver 100cGy at isocentre. 3DVH software requires patients DICOM data generated by TPS &amp; plan delivered on ArcCHECK phantom. Plans were generated in TPS by assigning different HU values to phantom. We analyzed gamma index &amp; the dose profile for all plans along vertical down direction of beam's central axis for Entry, Exit &amp; Isocentre dose.Results:The global gamma passing rate (2% &amp; 2mm) for manufacturer recommended HU value 282 was 96.3%. Detector entry, Isocentre &amp; detector exit Doses were 1.9048 (1.9270), 1.00(1.0199) &amp; 0.5078(0.527) Gy for TPS (Measured) respectively.The global gamma passing rate for electron density 1.1302 gm/cc was 98.6%. Detector entry, Isocentre &amp; detector exit Doses were 1.8714 (1.8873), 1.00(0.9988) &amp; 0.5211(0.516) Gy for TPS (Measured) respectively.Conclusion:Electron density value assigned by manufacturer does not hold true for every user. Proper modeling of electron density of ArcCHECK in TPS is essential to avoid systematic error in dose calculation of patient specific QA.

SU‐F‐T‐265: Calibration of EBT2 Film Using the Microtek ScanMaker 9800XL Plus Flatbed Scanner with a Modified One Red‐Channel After Three‐Channel Method

Purpose:In this study, we propose a technique to accurately calibrate the film using the Microtek ScanMaker 9800XL plus flatbed scanner following the “one red‐channel after three‐channel method” [Chang et al., Med. Phys. 42, 5838–5847 (2015)], which would be modified for this application.Methods:Following the PDD method, 6‐MV photon beams of two different MUs were delivered to eight EBT2 films (four films for each MU), where each film was tightly sandwiched in a 30‐cm cubic polystyrene phantom and parallel to the beam central axis. Before and after irradiation, all films were scanned through the Microtek 9800XL plus scanner, and the pixel values for red, green, and blue channels were extracted along the beam central axis on the film and fitted to the corresponding depth doses. Before performing the “one red‐channel after three‐channel method”, the irradiated film image first needs to be modified with a template matrix, which was generated through the pre‐scanned background images. The calibrated results were compared with the traditional one red‐channel method based on the net optical density.Results:Without using the template matrix, the three channel method cannot be correctly performed, since the pixel values did not follow the rational fitting form. Through our proposed method, the differences between the fitted dose and the delivered dose were within 2%, while that of the traditional one red channel method would be higher than 3%.Conclusion:We proposed a new technique that the one‐red channel after three‐channel method can be applied to the films scanned on a Microtek 9800XL plus scanner, and in this way, the calibration accuracy was improved. This method could also be used for other brand scanner to improve the calibration accuracy.MOST No. 104‐2221‐E‐214‐014

TH‐EF‐207A‐07: An Integrated X‐Ray/bioluminescence Tomography System for Radiation Guidance and Tumor Evaluation

Purpose:CT is not able to differentiate tumors from surrounding soft tissue. This study is to develop a bioluminescence tomography (BLT) system that is integrated onto our previously developed CT guided small animal arc radiation treatment system (iSMAART) to guide radiation, monitor tumor growth and evaluate therapeutic response.Methods:The BLT system employs a CCD camera coupled with a high speed lens, and is aligned orthogonally to the x‐ray beam central axis. The two imaging modalities, CT and BLT, are physically registered through geometrical calibration. The CT anatomy provides an accurate contour of animal surface which is used to construct 3D mesh for BLT reconstruction. Bioluminescence projections are captured from multiple angles, once every 45 degree rotation. The diffusion equation based on analytical Kirchhoff approximation is adopted to model the photon propagation in tissues. A discrete cosine transform based reweighted L1‐norm regularization (DCT‐re‐L1) algorithm is used for BLT reconstruction. Experiments are conducted on a mouse orthotopic prostate tumor model (n=12) to evaluate the BLT performance, in terms of its robustness and accuracy in locating and quantifying the bioluminescent tumor cells. Iodinated contrast agent was injected intravenously to delineate the tumor in CT. The tumor location and volume obtained from CT also serve as a benchmark against BLT.Results:With our cutting edge reconstruction algorithm, BLT is able to accurately reconstruct the orthotopic prostate tumors. The tumor center of mass in BLT is within 0.5 mm radial distance of that in CT. The tumor volume in BLT is significantly correlated with that in CT (R2 = 0.81).Conclusion:The BLT can differentiate, localize and quantify tumors. Together with CT, BLT will provide precision radiation guidance and reliable treatment assessment in preclinical cancer research.

TU-AB-BRC-02: Accuracy Evaluation of GPU-Based OpenCL Carbon Monte Carlo Package (goCMC) in Biological Dose and Microdosimetry in Comparison to FLUKA Simulations

Purpose: One of the most accurate methods for radiation transport is Monte Carlo (MC) simulation. Long computation time prevents its wide applications in clinic. We have recently developed a fast MC code for carbon ion therapy called GPU-based OpenCL Carbon Monte Carlo (goCMC) and its accuracy in physical dose has been established. Since radiobiology is an indispensible aspect of carbon ion therapy, this study evaluates accuracy of goCMC in biological dose and microdosimetry by benchmarking it with FLUKA. Methods: We performed simulations of a carbon pencil beam with 150, 300 and 450 MeV/u in a homogeneous water phantom using goCMC and FLUKA. Dose and energy spectra for primary and secondary ions on the central beam axis were recorded. Repair-misrepair-fixation model was employed to calculate Relative Biological Effectiveness (RBE). Monte Carlo Damage Simulation (MCDS) tool was used to calculate microdosimetry parameters. Results: Physical dose differences on the central axis were <1.6% of the maximum value. Before the Bragg peak, differences in RBE and RBE-weighted dose were <2% and <1%. At the Bragg peak, the differences were 12.5% caused by small range discrepancy and sensitivity of RBE to beam spectra. Consequently, RBE-weighted dose difference was 11%. Beyond the peak, RBE differences were <20% and primarily caused by differences in the Helium-4 spectrum. However, the RBE-weighted dose agreed within 1% due to the low physical dose. Differences in microdosimetric quantities were small except at the Bragg peak. The simulation time per source particle with FLUKA was 0.08 sec, while goCMC was approximately 1000 times faster. Conclusion: Physical doses computed by FLUKA and goCMC were in good agreement. Although relatively large RBE differences were observed at and beyond the Bragg peak, the RBE-weighted dose differences were considered to be acceptable.

SU‐F‐T‐193: Evaluation of a GPU‐Based Fast Monte Carlo Code for Proton Therapy Biological Optimization

Purpose:Biological treatment plan optimization is of great interest for proton therapy. It requires extensive Monte Carlo (MC) simulations to compute physical dose and biological quantities. Recently, a gPMC package was developed for rapid MC dose calculations on a GPU platform. This work investigated its suitability for proton therapy biological optimization in terms of accuracy and efficiency.Methods:We performed simulations of a proton pencil beam with energies of 75, 150 and 225 MeV in a homogeneous water phantom using gPMC and FLUKA. Physical dose and energy spectra for each ion type on the central beam axis were scored. Relative Biological Effectiveness (RBE) was calculated using repair‐misrepair‐fixation model. Microdosimetry calculations were performed using Monte Carlo Damage Simulation (MCDS).Results:Ranges computed by the two codes agreed within 1 mm. Physical dose difference was less than 2.5 % at the Bragg peak. RBE‐weighted dose agreed within 5 % at the Bragg peak. Differences in microdosimetric quantities such as dose average lineal energy transfer and specific energy were &lt; 10%. The simulation time per source particle with FLUKA was 0.0018 sec, while gPMC was ∼ 600 times faster.Conclusion:Physical dose computed by FLUKA and gPMC were in a good agreement. The RBE differences along the central axis were small, and RBE‐weighted dose difference was found to be acceptable. The combined accuracy and efficiency makes gPMC suitable for proton therapy biological optimization.

SU‐F‐J‐184: Proton Computed Tomography Using 1D Silicon Diode Array

Purpose:Proton radiography and proton computed tomography (PCT) can be used to measure proton stopping power directly. However, practical and cost effective proton imaging detectors are not widely available. In this study, the authors investigated the feasibility of proton imaging using a silicon diode array.Methods:A one‐dimensional silicon‐diode detector array (1DSDA) was aligned with the central axis (CAX) of the proton beam. Polymethyl methacrylate (PMMA) slabs were used to find the correspondence between the water equivalent thickness (WET) and 1DSDA channel number. 2D proton radiographs (PR) were obtained by translation and rotation of a phantom relative to CAX while the proton nozzle and 1DSDA were kept stationary. A PCT image of one slice of the phantom was reconstructed using filtered backprojection.Results:PR and PCT images of the PMMA cube were successfully acquired using the 1DSDA. The WET of the phantom was measured using PR data with an accuracy of 4.2% or better. Structures down to 1 mm in size could be resolved. Reconstruction of a PCT image showed very good agreement with simulation. Limitations in spatial resolution are attributed to limited spatial sampling, beam collimation, and proton scatter.Conclusion:The results demonstrate the feasibility of using silicon diode arrays for proton imaging. Such a device can potentially offer fast image acquisition, high spatial and energy resolution for PR and PCT.