Off-center Ratio Research Articles

An open-source software for calculating 1D gamma index in radiation therapy

PurposeThis study was developed to create computer software for performing the gamma index comparison between measurement and Monte Carlo (MC) simulation for the percentage depth dose (PDD) and beam off-center ratio profile (OCR). Materials and methodsThe gamma software was built in the matrix laboratory (MATLAB) software environment. The developed software was compared with ScanDosematch and Bistromath software’s gamma evaluation to assess its accuracy. A set of reference and evaluated dose distribution, which were obtained from measurement and MC simulation, was input to the software to calculate the 1D gamma index using different criteria (i.e. 3%/3 mm, 2%/3 mm, and 2%/2 mm). ResultsWe compared the two results of gamma index at 3%/3 mm, 2%/3 mm, and 2%/2 mm criteria, one calculated by the proposed software and one manually. The comparison showed high agreement between the proposed software and theoretical calculation. ConclusionsBased on the results, we concluded that our developed software has high accuracy, compared to theoretical calculation. This software could serve as a non-commercial and open-source tool for researchers and students.

Characteristics of microSilicon diode detector for electron beam dosimetry.

A microSilicon™ (PTW type 60023), a new unshielded diode detector succeeding Diode E (model 60017, PTW), was characterized for electron beam dosimetry and compared with other detectors. Electron beams generated from a TrueBeam linear accelerator were measured using the microSilicon, Diode E, and microDiamond synthetic single-crystal diamond detector. Positional accuracy of microSilicon was measured by data collected in air and water. The percent depth dose (PDD), off-center ratio (OCR), dose-response linearity, dose rate dependence, and cone factors were evaluated. The PDDs were compared with data measured using a PPC40 plane-parallel ionization chamber. The maximum variations of depth of 50% and 90% of the maximum dose, and practical depth among all detectors and energies were 0.9mm. The maximum variations of the bremsstrahlung dose among all detectors and energies were within 0.3%. OCR showed good agreement within 1% for the flat and tail regions. The microSilicon detector showed a penumbra width similar to microDiamond, whereas Diode E showed the steepest penumbra shape. All detectors showed good dose-response linearity and stability against the dose rate; only Diode E demonstrated logarithmic dose rate dependency. The cone factor measured with microSilicon was within ±1% for all energies and cone sizes. We demonstrated that the characteristics of microSilicon is suitable for electron beam dosimetry. The microSilicon detector can be a good alternative for electron beam dosimetry in terms of providing an appropriate PDD curve without corrections, high spatial resolution for OCR measurements and cone factors.

Khảo sát một số đặc tính bức xạ của buồng ion hóa CC13 và diode bán dẫn razor đối với các chùm photon kích thước nhỏ trên máy gia tốc tuyến tính xạ trị Clinac IX

Purpose: Compare percent depth dose (PDD) and off-center ratio (OCR) measured by the CC13 ionization chamber and the RAZOR silicon diode in small photon beams. Method and Materials: Some dosimetric characteristics, such as PDD, OCR, penumbra and radiation field size, were considered in this study for 2x2, 3x3, and 4x4 cm2 field sizes. We used the CC13 ionization chamber and the RAZOR silicon diode to measure dose distribution with depth along the axis and off-center of the beam. From the results obtained, the team investigated the differences in radiation parameters measured by the two types of probes above. Results: There are significant differences in the radiation parameters investigated for the CC13 ionization chamber and the RAZOR silicon diode, especially the width of penumbra. For PDD curves, the difference is less than 5% from dmax to 30 cm, however the difference becomes greater in the build-up region, which reaches to 33% at the water phatom surface. The width of penumbra measured by CC13 is always larger than that of RAZOR, the ratio of the penumbra width between two detectors is 1.8 and 1.3 for energies of 6 MV and 15 MV, respectively. Conclusion: The RAZOR silicon diode has better dose response than the CC13 ionization chamber for measuring the PDD and the OCR in small photon beams.

Commissioning of total body irradiation using plastic bead bags.

The goal of total body irradiation (TBI) is to deliver a dose to the whole body with uniformity within ±10%. The purpose of this study was to establish the technique of TBI using plastic bead bags. A lifting TBI bed, Model ORP-TBI-MN, was used. The space between the patient’s body and the acrylic walls of the bed was filled with polyacetal bead bags. Patients were irradiated by a 10 MV photon beam with a source to mid-plane distance of 400 cm. The monitor unit (MU) was calculated by dose-per-MU, tissue-phantom-ratio and a spoiler factor measured in solid water using an ionization chamber. The phantom-scatter correction factor, off-center ratio and the effective density of the beads were also measured. Diode detectors were used for in vivo dosimetry (IVD). The effective density of the beads was 0.90 ± 0.09. The point doses calculated in an I’mRT phantom with and without heterogeneity material showed good agreement, with measurements within 3%. An end-to-end test was performed using a RANDO phantom. The mean ± SD (range) of the differences between the calculated and IVD-measured mid-plane doses was 1.1 ± 4.8% (−5.9 to 5.0%). The differences between the IVD-measured doses and the doses calculated with Acuros XB of the Eclipse treatment planning system (TPS) were within 5%. For two patients treated with this method, the differences between the calculated and IVD-measured doses were within ±6% when excluding the chest region. We have established the technique of TBI using plastic bead bags. The TPS may be useful to roughly estimate patient dose.

Collection and analysis of photon beam data for Varian C-series linear accelerators: a potential reference beam data set.

This study aimed to collect and analyze photon beam data for the Varian C-series linear accelerators (Varian Medical Systems, Palo Alto, CA, USA). We evaluated the potential of the average data to be used as reference beam data for the radiotherapy treatment planning system commissioning verification. We collected 20 data sets for 4 and 6 MV photon beams, and 40 data sets for a 10 MV photon beam generated by the Varian C-series machines, which contained the percent depth dose (PDD), off-center ratio (OCR), and output factor (OPF) from 20 institutions. The average for each of the data types was calculated across the 20 machines. Dose differences from the average for PDD at the dose fall-off region were less than 1.0%. Relative differences from the average for the OPF data were almost within 1.0% for all energies and field sizes. For OCR data in the flat regions, the standard deviation of the dose differences from the average was within 1.0%, excluding that of the 30 × 30 mm2 field size being approximately 1.5%. For all energies and field sizes, the distance to agreement from the average in the OCR penumbra regions was less than 1.0mm. The average data except for the small field size found in this study can be used as reference beam data for verifying users' commissioning results.

Small-field dosimetry of TrueBeamTM flattened and flattening filter-free beams: A multi-institutional analysis.

PurposeDetector‐dependent interinstitutional variations of the beam data may lead to uncertainties of the delivered dose to patients. Here we evaluated the inter‐unit variability of the flattened and flattening filter‐free (FFF) beam data of multiple TrueBeam (Varian Medical Systems) linear accelerators focusing on the small‐field dosimetry.MethodsThe beam data of 6‐ and 10‐MV photon beams with and without flattening filter measured for modeling of an iPLAN treatment planning system (BrainLAB) were collected from 12 institutions — ten HD120 Multileaf Collimator (MLC) and two Millennium120 MLC. Percent‐depth dose (PDD), off‐center ratio (OCR), and detector output factors (OF det) measured with different detectors were evaluated. To investigate the detector‐associated effects, we evaluated the inter‐unit variations of the OF det before and after having applied the output correction factors provided by the International Atomic Energy Agency (IAEA) Technical Reports Series no. 483.ResultsPDD measured with a field size of 5 × 5 mm2 showed that the data measured using an ionization chamber had variations exceeding 1% from the median values. The maximum difference from median value was 2.87% for 10 MV photon beam. The maximum variations of the penumbra width for OCR with 10 × 10 mm2 field size were 0.97 mm. The OF det showed large variations exceeding 15% for a field size of 5 × 5 mm2. When the output correction factors were applied to the OF det, the variations were greatly reduced. The relative difference of almost all field output factors were within ± 5% from the median field output factors.ConclusionIn this study, the inter‐unit variability of small‐field dosimetry was evaluated for TrueBeam linear accelerators. The variations were large at a field size of 5 × 5 mm2, and most occurred in a detector‐dependent manner.

Impact of detector selections on inter-institutional variability of flattening filter-free beam data for TrueBeam™ linear accelerators.

This study evaluates the type of detector influencing the inter‐institutional variability in flattening filter‐free (FFF) beam‐specific parameters for TrueBeam™ linear accelerators (Varian Medical Systems,Palo Alto, CA, USA). Twenty‐four beam data sets, including the percent depth dose (PDD), off‐center ratio (OCR), and output factor (OPF) for modeling within the Eclipse (Varian Medical Systems) treatment planning system, were collected from 19 institutions. Although many institutions collected the data using CC13 (IBA Dosimetry, Schwarzenbruck, Germany) or PTW31010 semiflex (PTW Freiburg, Freiburg, Germany) ionization chambers, some institutions used diode detectors, diamond detectors, and ionization chambers with smaller cavities. The OCR data included penumbra width, full width at half maximum (FWHM), and FFF beam‐specific parameters, including unflatness and slope. The data measured by CC13/PTW31010 ionization chambers were compared with those measured by all other detectors. PDD data demonstrated the variations within ±1% at the dose fall‐off region deeper than peak depth. The penumbra widths of the OCR measured with the CC13/PTW31010 detectors were significantly larger than those measured with all other detectors (P < 0.05). Especially the EDGE detector (Sun Nuclear Corp., Melbourne, FL, USA) and the microDiamond detectors (model 60019; PTW Freiburg) demonstrated much smaller penumbra values compared to those of the CC13/PTW31010 detectors for the 30 × 30 mm2 field. There was no difference in the FWHM, unflatness, and slope parameters between the values for the CC13/PTW31010 detectors and all other detectors. OPF curves demonstrated small variations, and the relative difference from the mean value of each data point was almost within 1% for all field sizes. Although the penumbra region exhibited detector‐dependent variations, all other parameters showed tiny interunit variations regardless of the detector type.

Do the representative beam data for TrueBeam™ linear accelerators represent average data?

If the vendor's representative beam data (RBD) for TrueBeam linear accelerators are to be valid for use in clinical practice, the variations in the beam data used for beam modeling must be small. Although a few studies have reported the variation of the beam data of the TrueBeam machines, the numbers of machines analyzed in those studies were small. In this study, we investigated the variation in the beam data for 21 TrueBeam machines collected from 17 institutions with their agreement. In the exponential regions, the percent depth dose (PDD) values showed very small variation, <1% for all the photon energies analyzed. Similarly, the off‐center ratio (OCR) values also showed small variation for all energies. In the field regions, the standard deviations of the values of dose difference (DD) between the data for each machine and the study average were <1% for field sizes ≥100 × 100 mm2. The maximum distance‐to‐agreement from the average data was <0.5 mm in the penumbra regions. The output factor (OPF) values also showed very small variation (<1%) for all energies and field sizes. Both the PDD and OCR of the average study data showed good agreement with the vendor's RBD for field sizes ≥100 × 100 mm2. The OPF of the average study data also showed good agreement with the vendor's RBD for all field sizes. However, although all the institutions used ionization chambers with similar cavity volumes, the 30 × 30 mm2 field size showed large DD variations (≥2%) in OCR in the field regions. We conclude that the intermachine variability of TrueBeam linear accelerators was very small except for small field dosimetry, supporting the validity of the use of the RBD for clinical applications. The use of the vendor's RBD might greatly facilitate the quick installation of a new linear accelerator.

A New Effective-Conducting-Path-Driven Subthreshold Behavior Model for Junctionless Dual-Material Omega-Gate Nano-MOSFETs

In this paper, a generalized analytical subthreshold behavior model for junctionless omega-gate (JLΩG) MOSFETs combining dual-material gate (DMG) is built. Through the effective conducting path effects (ECPEs), we propose a new concept of off-center ratio (δ) dominating the effective scaling length to derive our model based on the effective conducting central potential (Φ eff,C ), obtained by quasi-3D potential method. It provides a more accurate expression of scaling equation for JLΩGFETs and leads to an improved subthreshold behavior prediction. Accounting for these renewed potential and current models, we utilize dual materials to achieve better immunity to short channel effects. The proposed modeling can be extensively applied to other multiple-gate (MG) nano-transistors as well.

Comparison of dosimetric properties among four commercial multi-detector computed tomography scanners

This study compared dosimetric properties among four commercial multi-detector CT (MDCT) scanners. The X-ray beam characteristics were obtained from photon intensity attenuation curves of aluminum and off-center ratio (OCR) profiles in air, which were measured with four commercial MDCT scanners. The absorbed dose for MDCT scanners was evaluated with Farmer ionization chamber measurements at the center and four peripheral points in the body- and head-type cylindrical water phantoms. Measured collected charge was converted to absorbed dose using a 60Co absorbed dose-to-water calibration factor and Monte Carlo (MC)-calculated correction factors. Four MDCT scanners were modeled to correspond with measured X-ray beam characteristics using GMctdospp (IMPS, Germany) software. Al half-value layers (Al-HVLs) with a body-bowtie filter determined from measured Al-attenuation curves ranged 7.2‒9.1mm at 120kVp and 6.1‒8.0mm at 100kVp. MC-calculated Al-HVLs and OCRs in air were in acceptable agreement within 0.5mm and 5% of measured values, respectively. The percentage difference between nominal and actual beam width was greater with decreasing collimation width. The absorbed doses for MDCT scanners at 120kVp ranged 5.1‒7.1mGy and 10.8‒17.5mGy per 100mAs at the center in the body- and head-type water phantoms, respectively. Measured doses at four peripheral points were within 5% agreement of MC-calculated values. The absorbed dose at the center in both water phantoms increased with decreasing Al-HVL for the same charge on the focus. In this study the X-ray beam characteristics and the absorbed dose were measured and compared with calculated values for four MDCT scanners.

A pencil beam dose calculation model for CyberKnife system.

CyberKnife system is initially equipped with fixed circular cones for stereotactic radiosurgery. Two dose calculation algorithms, Ray-Tracing and Monte Carlo, are available in the supplied treatment planning system. A multileaf collimator system was recently introduced in the latest generation of system, capable of arbitrarily shaped treatment field. The purpose of this study is to develop a model based dose calculation algorithm to better handle the lateral scatter in an irregularly shaped small field for the CyberKnife system. A pencil beam dose calculation algorithm widely used in linac based treatment planning system was modified. The kernel parameters and intensity profile were systematically determined by fitting to the commissioning data. The model was tuned using only a subset of measured data (4 out of 12 cones) and applied to all fixed circular cones for evaluation. The root mean square (RMS) of the difference between the measured and calculated tissue-phantom-ratios (TPRs) and off-center-ratio (OCR) was compared. Three cone size correction techniques were developed to better fit the OCRs at the penumbra region, which are further evaluated by the output factors (OFs). The pencil beam model was further validated against measurement data on the variable dodecagon-shaped Iris collimators and a half-beam blocked field. Comparison with Ray-Tracing and Monte Carlo methods was also performed on a lung SBRT case. The RMS between the measured and calculated TPRs is 0.7% averaged for all cones, with the descending region at 0.5%. The RMSs of OCR at infield and outfield regions are both at 0.5%. The distance to agreement (DTA) at the OCR penumbra region is 0.2 mm. All three cone size correction models achieve the same improvement in OCR agreement, with the effective source shift model (SSM) preferred, due to their ability to predict more accurately the OF variations with the source to axis distance (SAD). In noncircular field validation, the pencil beam calculated results agreed well with the film measurement of both Iris collimators and the half-beam blocked field, fared much better than the Ray-Tracing calculation. The authors have developed a pencil beam dose calculation model for the CyberKnife system. The dose calculation accuracy is better than the standard linac based system because the model parameters were specifically tuned to the CyberKnife system and geometry correction factors. The model handles better the lateral scatter and has the potential to be used for the irregularly shaped fields. Comprehensive validations on MLC equipped system are necessary for its clinical implementation. It is reasonably fast enough to be used during plan optimization.

SAD vs SSD methods for cyberknife ocr measurements

Commissioning of Cyberknife (CK) system at Radiooncologia department Júlio Teixeira S.A started in February 2016. The MultiPlan TPS requires OCR (off-center-ratio) processed data at a fixed SAD (source-to-axis distance) of 800 mm. Accuray physics essencial guide allows the user to measure the OCR either at fixed SSD (source-to-surface distance) or at SAD. Compare SSD and SAD methods for OCR measurements on Cyberknife commissioning acquisitions. Data was acquired on a MP3 water phantom using a SRS60018 Diode, TANDEM electrometer and measuring system MEPHYSTOmc2 from PTW. OCR measurements for fixed cones in CK were performed by making orthogonal scans at 15, 50, 100, 200, 300 mm depths. Data was processed by averaging each side of the crossplane and inplane scans (each point in the OCR curve is the average of four measured values). SSD method was performed with a constant 800 mm target-to-water surface distance being geometrically corrected for an SAD equivalent setup and the SAD method profiles were obtained by moving the robot over the Z axis and the Diode depth adjusted to maintain the 800 mm SAD. The coefficient of determination was calculated point by point for SSD and SAD curves (60, 50 and 40 mm cones). Differences observed where only significant at penumbra with values for 60 mm cone <1.6%, 50 mm cone <2.5%; and 40 mm cone <5,6%. Both methods are acceptable. Since SSD are more straightforward it was the chosen method for further data acquisition. The authors of this abstract don’t have any arrangement or affiliation with any product or services used or discussed in this paper nor any potential bias against another product or service.

SU-F-T-376: The Efficiency of Calculating Photonuclear Reaction On High-Energy Photon Therapy by Monte Carlo Method

Purpose: Secondary-neutrons having harmful influences to a human body are generated by photonuclear reaction on high-energy photon therapy. Their characteristics are not known in detail since the calculation to evaluate them takes very long time. PHITS(Particle and Heavy Ion Transport code System) Monte Carlo code since versions 2.80 has the new parameter “pnimul” raising the probability of occurring photonuclear reaction forcibly to make the efficiency of calculation. We investigated the optimum value of “pnimul” on high-energy photon therapy. Methods: The geometry of accelerator head based on the specification of a Varian Clinac 21EX was used for PHITS ver. 2.80. The phantom (30 cm * 30 cm * 30 cm) filled the composition defined by ICRU(International Commission on Radiation Units) was placed at source-surface distance 100 cm. We calculated the neutron energy spectra in the surface of ICRU phantom with “pnimal” setting 1, 10, 100, 1000, 10000 and compared the total calculation time and the behavior of photon using PDD(Percentage Depth Dose) and OCR(Off-Center Ratio). Next, the cutoff energy of photon, electron and positron were investigated for the calculation efficiency with 4, 5, 6 and 7 MeV. Results: The calculation total time until the errors of neutron fluence become within 1% decreased as increasing “pnimul”. PDD and OCR showed no differences by the parameter. The calculation time setting the cutoff energy like 4, 5, 6 and 7 MeV decreased as increasing the cutoff energy. However, the errors of photon become within 1% did not decrease by the cutoff energy. Conclusion: The optimum values of “pnimul” and the cutoff energy were investigated on high-energy photon therapy. It is suggest that using the optimum “pnimul” makes the calculation efficiency. The study of the cutoff energy need more investigation.

Evaluation of the incident directional dependence of radiochromic film by use of Monte Carlo simulation and measurement.

In high-precision radiotherapy, absolute and relative doses are evaluated for patient-specific intensity-modulated radiation therapy (IMRT) quality assurance (QA). In our institution, we use GAFCHROMIC EBT3 (EBT3) for relative dose evaluation in IMRT QA. We usually use two directional film configurations, which are in the axial and sagittal planes. The QA in our institution shows some differences between the gamma pass rates in the axial and sagittal directions. The purpose of this study was to evaluate the incident directional dependence of EBT3 by using the percent depth dose (PDD) and the off-center ratio (OCR) between EBT3 films positioned perpendicular to the beam axis and along the beam axis. Furthermore, we compared the PDD in EBT3 films positioned perpendicular to the beam axis and the PDD by using an ionization chamber. In addition, PDDs in water phantoms with and without EBT3 films were calculated by Monte Carlo simulation. The results showed that the PDD in EBT3 films positioned perpendicular to the beam axis increased with the depth from the phantom surface. Monte Carlo simulation showed the same trend as did the film measurements. The OCR results were slightly different at dose levels below 20%. The OCR in EBT3 films positioned along the beam axis was higher than that perpendicular to the beam axis. Thus, we conclude that EBT3 film has incident directional dependence. In IMRT QA, the gamma analysis results may be affected by the incident directional dependence of EBT3 film.

SU-F-207-01: Comparison of Beam Characteristics and Organ Dose From Four Commercial Multidetector Computed Tomography Scanners

Purpose: To compare dosimetric properties and patient organ doses from four commercial multidetector CT (MDCT) using Monte Carlo (MC) simulation based on the absorbed dose measured using a Farmer chamber and cylindrical water phantoms according to AAPM TG-111. Methods: Four commercial MDCT were modeled using the GMctdospp (IMPS, Germany) based on the EGSnrc user code. The incident photon spectrum and bowtie filter for MC simulations were determined so that calculated values of aluminum half-value layer (Al-HVL) and off-center ratio (OCR) profile in air agreed with measured values. The MC dose was calibrated from absorbed dose measurements using a Farmer chamber and cylindrical water phantoms. The dose distributions of head, chest, and abdominal scan were calculated using patient CT images and mean organ doses were evaluated from dose volume histograms. Results: The HVLs at 120 kVp of Brilliance, LightSpeed, Aquilion, and SOMATOM were 9.1, 7.5, 7.2, and 8.7 mm, respectively. The calculated Al-HVLs agreed with measurements within 0.3%. The calculated and measured OCR profiles agreed within 5%. For adult head scans, mean doses for eye lens from Brilliance, LightSpeed, Aquilion, and SOMATOM were 21.7, 38.5, 47.2 and 28.4 mGy, respectively. For chest scans, mean doses for lung from Brilliance, LightSpeed, Aquilion, and SOMATOM were 21.1, 26.1, 35.3 and 24.0 mGy, respectively. For adult abdominal scans, the mean doses for liver from Brilliance, LightSpeed, Aquilion, and SOMATOM were 16.5, 21.3, 22.7, and 18.0 mGy, respectively. The absorbed doses increased with decreasing Al-HVL. The organ doses from Aquilion were two greater than those from Brilliance in head scan. Conclusion: MC dose distributions based on absorbed dose measurement in cylindrical water phantom are useful to evaluate individual patient organ doses.

New absorbed dose measurement with cylindrical water phantoms for multidetector CT

The aim of this study was to develop new dosimetry with cylindrical water phantoms for multidetector computed tomography (MDCT). The ionization measurement was performed with a Farmer ionization chamber at the center and four peripheral points in the body-type and head-type cylindrical water phantoms. The ionization was converted to the absorbed dose using a 60Co absorbed-dose-to-water calibration factor and Monte Carlo (MC) -calculated correction factors. The correction factors were calculated from MDCT (Brilliance iCT, 64-slice, Philips Electronics) modeled with GMctdospp (IMPS, Germany) software based on the EGSnrc MC code. The spectrum of incident x-ray beams and the configuration of a bowtie filter for MDCT were determined so that calculated photon intensity attenuation curves for aluminum (Al) and calculated off-center ratio (OCR) profiles in air coincided with those measured. The MC-calculated doses were calibrated by the absorbed dose measured at the center in both cylindrical water phantoms. Calculated doses were compared with measured doses at four peripheral points and the center in the phantom for various beam pitches and beam collimations. The calibration factors and the uncertainty of the absorbed dose determined using this method were also compared with those obtained by CTDIair (CT dose index in air). Calculated Al half-value layers and OCRs in air were within 0.3% and 3% agreement with the measured values, respectively. Calculated doses at four peripheral points and the centers for various beam pitches and beam collimations were within 5% and 2% agreement with measured values, respectively. The MC-calibration factors by our method were 44–50% lower than values by CTDIair due to the overbeaming effect. However, the calibration factors for CTDIair agreed within 5% with those of our method after correction for the overbeaming effect. Our method makes it possible to directly measure the absorbed dose for MDCT and is more robust and accurate than the CTDIair measurement.

Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0–15mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5cm depth occurred at 4, 9, and 15mm with 4, 6, and 9MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.

SU‐E‐I‐28: Evaluating the Organ Dose From Computed Tomography Using Monte Carlo Calculations

Purpose:To evaluate organ doses from computed tomography (CT) using Monte Carlo (MC) calculations.Methods:A Philips Brilliance CT scanner (64 slice) was simulated using the GMctdospp (IMPS, Germany) based on the EGSnrc user code. The X‐ray spectra and a bowtie filter for MC simulations were determined to coincide with measurements of half‐value layer (HVL) and off‐center ratio (OCR) profile in air. The MC dose was calibrated from absorbed dose measurements using a Farmer chamber and a cylindrical water phantom. The dose distribution from CT was calculated using patient CT images and organ doses were evaluated from dose volume histograms.Results:The HVLs of Al at 80, 100, and 120 kV were 6.3, 7.7, and 8.7 mm, respectively. The calculated HVLs agreed with measurements within 0.3%. The calculated and measured OCR profiles agreed within 3%. For adult head scans (CTDIvol) =51.4 mGy), mean doses for brain stem, eye, and eye lens were 23.2, 34.2, and 37.6 mGy, respectively. For pediatric head scans (CTDIvol =35.6 mGy), mean doses for brain stem, eye, and eye lens were 19.3, 24.5, and 26.8 mGy, respectively. For adult chest scans (CTDIvol=19.0 mGy), mean doses for lung, heart, and spinal cord were 21.1, 22.0, and 15.5 mGy, respectively. For adult abdominal scans (CTDIvol=14.4 mGy), the mean doses for kidney, liver, pancreas, spleen, and spinal cord were 17.4, 16.5, 16.8, 16.8, and 13.1 mGy, respectively. For pediatric abdominal scans (CTDIvol=6.76 mGy), mean doses for kidney, liver, pancreas, spleen, and spinal cord were 8.24, 8.90, 8.17, 8.31, and 6.73 mGy, respectively. In head scan, organ doses were considerably different from CTDIvol values.Conclusion:MC dose distributions calculated by using patient CT images are useful to evaluate organ doses absorbed to individual patients.

Monte Carlo calculation of patient organ doses from computed tomography

In this study, we aimed to evaluate quantitatively the patient organ dose from computed tomography (CT) using Monte Carlo calculations. A multidetector CT unit (Aquilion 16, TOSHIBA Medical Systems) was modeled with the GMctdospp (IMPS, Germany) software based on the EGSnrc Monte Carlo code. The X-ray spectrum and the configuration of the bowtie filter for the Monte Carlo modeling were determined from the chamber measurements for the half-value layer (HVL) of aluminum and the dose profile (off-center ratio, OCR) in air. The calculated HVL and OCR were compared with measured values for body irradiation with 120 kVp. The Monte Carlo-calculated patient dose distribution was converted to the absorbed dose measured by a Farmer chamber with a (60)Co calibration factor at the center of a CT water phantom. The patient dose was evaluated from dose-volume histograms for the internal organs in the pelvis. The calculated Al HVL was in agreement within 0.3% with the measured value of 5.2 mm. The calculated dose profile in air matched the measured value within 5% in a range of 15 cm from the central axis. The mean doses for soft tissues were 23.5, 23.8, and 27.9 mGy for the prostate, rectum, and bladder, respectively, under exposure conditions of 120 kVp, 200 mA, a beam pitch of 0.938, and beam collimation of 32 mm. For bones of the femur and pelvis, the mean doses were 56.1 and 63.6 mGy, respectively. The doses for bone increased by up to 2-3 times that of soft tissue, corresponding to the ratio of their mass-energy absorption coefficients.

SU‐E‐T‐12: Clinical Implementation of the Dedicated Program for MU Calculation in Proton Beam Therapy

Purpose: The purpose of this study is to develop the dedicated in‐house program for calculation of the monitor unit (MU) of proton therapy beam and to verify the feasibility of its clinical implementation in proton beam therapy with a combination of range modulation wheel (RMW).Materials and Methods: The MU calculation for the proton beam was suggested by Dr.Sahoo in 2008 and given by the formalism using the multiplication of several factors with regard to the structure of the proton beam system. Following his theory, we implemented eight factors into the formalism for MU calculation as follows: Relative output factor (ROF), Spread‐out Bragg peak factor (SOBPF), Range shifter factor (RSF), SOBP off‐center factor (SOBPOCF), Off‐center ratio (OCR), Field size factor (FSF), Inverse square factor (ISF), and Compensator and patient scatter factor (CPSF). Our program was built by Microsoft Visual Basic language. The MU could be calculated by division of the prescribed dose with (d/MU). After building the program, the calculated doses by the formalism were compared with the measured dose. Results: The (d/MU) was calculated by the multiplication using eight factors by use of our program. Once the energy and the settings were input to the program, these factors were estimated from the measurement data. Then MU calculation was done immediately. It was simply and satisfactory fast for the user to complete calculating (d/MU). As the results of the comparison between the measured and calculated doses, the discrepancies in all referential settings were within 2%. Conclusions: The in‐house program provides the fast MU calculation for proton beams. The accuracy of the dose calculation is good in the referential settings. However, further improvement is required for the complicated condition.