Dosimetry Parameters Research Articles

SU‐F‐T‐44: A Comparison of the Pre‐Plan, Intra‐Operative Plan, and Post‐Implant Dosimetry for a Prostate Implant Case Using Prefabricated Linear Polymer‐Encapsulated Pd‐103

Purpose:To investigate the reproducibility and limitations of Pd‐103 prostate brachytherapy using fixed length linear sources (CivaString).Methods:An LDR prostate brachytherapy case which was preplanned on MR images with prefabricated linear polymer‐encapsulated Pd‐103 sources (CivaString) was studied and compared with ultrasound based intra‐operative planning and CT based post‐implant dosimetry. We evaluated the following parameters among the three studies: prostate geometry (volume and cross sectional area), needle position and alignment deviations, and dosimetry parameters (D90).Results:The prostate volumes and axial cross sectional areas at center of prostate were measured as 41.8, 39.3 and 36.8 cc, and 14.9, 14.3, and 11.3 respectively on pre‐plan MR, inter‐op US, and post‐implant CT studies. The deviation of prostate volumes and axial cross sectional areas measured on pre‐planning MR and intra‐operative US were within 5%. 17 out of 19 pre‐planned needles were positioned within 5mm (the template grid size). One needle location was adjusted intra‐operatively and another needle was removed due to proximity to urethra. The needle pathways were not always parallel to the trans‐rectal probe due to the flexibility of CivaString. The angle of deviation was up to 10 degrees. Two pairs of needles were exchanged to better fit the length of prostate at the time of implant. This resulted in a prostate D90 of 153.8 Gy (124%) and 131.4 Gy (106.7%) for intra‐op and PID respectively.Conclusion:Preplanning is a necessary part of implants performed with prefabricated linear polymer sources. However, as is often the case, there were real‐time deviations from the pre‐plan. Intra‐op planning provides the ability conform to anatomy at the time of implant. Therefore, we propose to develop a systematic way to order extra strings of different length to provide the flexibility to perform intra‐operative planning with fixed length strands.

SU‐F‐T‐54: Determination of the AAPM TG‐43 Brachytherapy Dosimetry Parameters for A New Titanium‐Encapsulated Yb‐169 Source by Monte Carlo Calculations

Purpose:To determine the AAPM TG‐43 brachytherapy dosimetry parameters of a new titanium‐encapsulated Yb‐169 source designed to maximize the dose enhancement during gold nanoparticle‐aided radiation therapy (GNRT).Methods:An existing Monte Carlo (MC) model of the titanium‐encapsulated Yb‐169 source, which was described in the current investigators’ published MC optimization study, was modified based on the source manufacturer's detailed specifications, resulting in an accurate model of the titanium‐encapsulated Yb‐169 source that was actually manufactured. MC calculations were then performed using the MCNP5 code system and the modified source model, in order to obtain a complete set of the AAPM TG‐43 parameters for the new Yb‐169 source.Results:The MC‐calculated dose rate constant for the new titanium‐encapsulated Yb‐169 source was 1.05 ± 0.03 cGy per hr U, indicating about 10% decrease from the values reported for the conventional stainless steel‐encapsulated Yb‐169 sources. The source anisotropy and radial dose function for the new source were found similar to those reported for the conventional Yb‐169 sources.Conclusion:In this study, the AAPM TG‐43 brachytherapy dosimetry parameters of a new titanium‐encapsulated Yb‐169 source were determined by MC calculations. The current results suggested that the use of titanium, instead of stainless steel, to encapsulate the Yb‐169 core would not lead to any major change in the dosimetric characteristics of the Yb‐169 source, while it would allow more low energy photons being transmitted through the source filter thereby leading to an increased dose enhancement during GNRT. Supported by DOD/PCRP grant W81XWH‐12‐1‐0198This investigation was supported by DOD/PCRP grant W81XWH‐12‐1‐ 0198.

SU‐F‐T‐671: Effects of Collimator Material On Proton Minibeams

Purpose:To investigate the dosimetric effects of collimator material on spatially modulated proton minibeams (pMBRT). Methods: pMBRT holds promise to exhibit shallow depth normal‐tissue sparing effects similar to synchrotron based microbeams while also retaining potential for uniform dose distributions for tumor targets. TOPAS Monte Carlo simulations were performed for a 5cm thick multislit collimator with 0.3mm slits and 1mm center‐to‐center spacing for a 50.5MeV proton minibeam while varying collimator material between brass, tungsten, and iron. The collimator was placed both “flush” at the water phantom surface and at 5cm distance to investigate the effects on surface dose, peak‐to‐valley‐dose‐ratio (PVDR) and neutron contribution.Results:For flush placement, the neutron dose at the phantom surface for the tungsten collimator was approximately 20% higher than for brass and iron. This was not reflected in the overall surface dose, which was comparable for all materials due to the relatively low neutron contribution of <0.1%. When the collimator was retracted, the overall neutron contribution was essentially identical for all three collimators. Surface dose dropped by ∼40% for all collimator materials with air gap compared to being flush with the phantom surface. This surface dose reduction was at the cost of increase in valley dose for all collimator materials due to increased angular divergence of the mini‐beams at the surface and their consequent geometric penumbra at depth. When the collimator was placed at distance from the phantom surface the PVDR decreased. The peak‐to‐entrance‐dose ratio was highest for the iron collimator with 5cm air gap. Conclusion: The dosimetric difference between the collimator materials is minimal despite the relatively higher neutron contribution at the phantom surface for the tungsten collimator when placed flush. The air gap between the collimator and phantom surface strongly influences all dosimetry parameters due to the influence of scatter on the narrow spatial modulation.

SU‐F‐T‐12: Monte Carlo Dosimetry of the 60Co Bebig High Dose Rate Source for Brachytherapy

Purpose:The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG‐43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high‐dose‐rate brachytherapy. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry.Methods:The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross‐section library was used. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3.Results:The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh‐1U‐1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.Conclusion:The dose rate constant is consistent with the results of Granero et al. and Selvam and Bhola within 1%. Dose rate data are compared to GEANT4 and DORZnrc Monte Carlo code. However, the radial dose function is different by up to 10% for the points that are notably near the source on the transversal axis because of the high‐energy photons from 60Co, which causes an electronic disequilibrium at the interface between the source capsule and the liquid water for distances up to 1 cm.

SU-G-201-10: Experimental Determination of Modified TG-43 Dosimetry Parameters for the Xoft Axxent® Electronic Brachytherapy Source

Purpose: The establishment of an air kerma rate standard at NIST for the Xoft Axxent® electronic brachytherapy source (Axxent® source) motivated the establishment of a modified TG-43 dosimetry formalism. This work measures the modified dosimetry parameters for the Axxent® source in the absence of a treatment applicator for implementation in Xoft's treatment planning system. Methods: The dose-rate conversion coefficient (DRCC), radial dose function (RDF) values, and polar anisotropy (PA) were measured using TLD-100 microcubes with NIST-calibrated sources. The DRCC and RDF measurements were performed in liquid water using an annulus of Virtual Water™ designed to align the TLDs at the height of the anode at fixed radii from the source. The PA was measured at several distances from the source in a PMMA phantom. MCNP-determined absorbed dose energy dependence correction factors were used to convert from dose to TLD to dose to liquid water for the DRCC, RDF, and PA measurements. The intrinsic energy dependence correction factor from the work of Pike was used. The AKR was determined using a NIST-calibrated HDR1000 Plus well-type ionization chamber. Results: The DRCC was determined to be 8.6 (cGy/hr)/(µGy/min). The radial dose values were determined to be 1.00 (1cm), 0.60 (2cm), 0.42 (3cm), and 0.32 (4cm), with agreement ranging from (5.7% to 10.9%) from the work of Hiatt et al. 2015 and agreement from (2.8% to 6.8%) with internal MCNP simulations. Conclusion: This work presents a complete dataset of modified TG-43 dosimetry parameters for the Axxent® source in the absence of an applicator. Prior to this study a DRCC had not been measured for the Axxent® source. This data will be used for calculating dose distributions for patients receiving treatment with the Axxent® source in Xoft's breast balloon and vaginal applicators, and for intraoperative radiotherapy. Sources and partial funding for this work were provided by Xoft Inc. (a subsidiary of iCAD). This work was also supported by the Radiological Sciences T32 Training Grant through the University of Wisconsin-Madison Medical Physics department (5T32CA009206-37).

SU‐F‐T‐262: Commissioning Varian Portal Dosimetry for EPID‐Based Patient Specific QA in a Non‐Aria Environment

Purpose:Development of an in‐house program facilitates a workflow that allows Electronic Portal Imaging Device (EPID) patient specific quality assurance (QA) measurements to be acquired and analyzed in the Portal Dosimetry Application (Varian Medical Systems, Palo Alto, CA) using a non‐Aria Record and Verify (R&V) system (MOSAIQ, Elekta, Crawley, UK) to deliver beams in standard clinical treatment mode.Methods:Initial calibration of an in‐house software tool includes characterization of EPID dosimetry parameters by importing DICOM images of varying delivered MUs to determine linear mapping factors in order to convert image pixel values to Varian‐defined Calibrated Units (CU). Using this information, the Portal Dose Image Prediction (PDIP) algorithm was commissioned by converting images of various field sizes to output factors using the Eclipse Scripting Application Programming Interface (ESAPI) and converting a delivered configuration fluence to absolute dose units. To verify the algorithm configuration, an integrated image was acquired, exported directly from the R&V client, automatically converted to a compatible, calibrated dosimetric image, and compared to a PDIP calculated image using Varian's Portal Dosimetry Application.Results:For two C‐Series and one TrueBeam Varian linear accelerators, gamma comparisons (global 3% / 3mm) of PDIP algorithm predicted dosimetric images and images converted via the inhouse system demonstrated agreement for ≥99% of all pixels, exceeding vendor‐recommended commissioning guidelines.Conclusion:Combinations of a programmatic image conversion tool and ESAPI allow for an efficient and accurate method of patient IMRT QA incorporating a 3rd party R&V system.

Monte Carlo Evaluation of Gamma Knife Dose Profile in Real Brain Phantom

Introduction The Gamma Knife system is designed solely for non-invasive treatment of brain disorders, and it benefits from stereotactic surgical techniques. Dose calculations required in the system are performed by GammaPlan code; in this code, brain tissue is considered uniform. In the present study, we evaluated the effect of Gamma Knife system on the obtained dose through simulating a real human brain phantom. Materials and Methods In this study, a Monte Carlo simulation code (MCNPX2.7) was employed to simulate Gamma Knife system. Brain tissue equivalent Snyder phantom and combinations were considered according to International Commission on Radiological Units (ICRU)-44 report. Results To ensure accuracy of the simulations, patient’s head was modeled by a spherical water phantom. At this point, the dosimetry parameters were compared with those obtained by the Monte Carlo code EGS4 and good consistency was observed (less than 7% difference). At the next stage, the above dosimetry parameters were compared with those obtained experimentally by polystyrene phantom and EDR2 dosimetry film and improved consistency was detected (less than 0.5% difference). Finally, the Snyder phantom, as the human brain, was simulated. The Full Width at Half Maximum (FWHM) and penumbra decreased by 4.7% and 18%, respectively. Moreover, an isocenter dose reduction of 30-40%, compared to the water phantom, was noted. Conclusion The calculation of the real phantom showed that water and polystyrene could function similarly, while evaluating dosimetry parameters in the Gamma Knife system; thus, water and polystyrene are not appropriate phantom matters for this purpose.

Monte Carlo dosimetry of the IRAsource high dose rate (192)Ir brachytherapy source.

High-dose-rate (HDR) brachytherapy is a common method for cancer treatment in clinical brachytherapy. Because of the different source designs, there is a need for specific dosimetry data set for each HDR model. The purpose of this study is to obtain detailed dose rate distributions in water phantom for a first prototype HDR (192)Ir brachytherapy source model, IRAsource, and compare with the other published works. In this study, Monte Carlo N-particle (MCNP version 4C) code was used to simulate the dose rate distributions around the HDR source. A full set of dosimetry parameters reported by the American Association of Physicists in Medicine Task Group No. 43U1 was evaluated. Also, the absorbed dose rate distributions in water, were obtained in an along-away look-up table. The dose rate constant, Λ, of the IRAsource was evaluated to be equal to 1.112±0.005cGyh(-1) U(-1). The results of dosimetry parameters are presented in tabulated and graphical formats and compared with those reported from other commercially available HDR (192)Ir sources, which are in good agreement. This justifies the use of specific data sets for this new source. The results obtained in this study can be used as input data in the conventional treatment planning systems.

Simulation evaluation of NIST air-kerma rate calibration standard for electronic brachytherapy.

Dosimetry for the model S700 50 kV electronic brachytherapy (eBT) source (Xoft, Inc., a subsidiary of iCAD, San Jose, CA) was simulated using Monte Carlo (MC) methods by Rivard et al. ["Calculated and measured brachytherapy dosimetry parameters in water for the Xoft Axxent x-ray source: An electronic brachytherapy source," Med. Phys. 33, 4020-4032 (2006)] and recently by Hiatt et al. ["A revised dosimetric characterization of the model S700 electronic brachytherapy source containing an anode-centering plastic insert and other components not included in the 2006 model," Med. Phys. 42, 2764-2776 (2015)] with improved geometric characterization. While these studies examined the dose distribution in water, there have not previously been reports of the eBT source calibration methods beyond that recently reported by Seltzer et al. ["New national air-kerma standard for low-energy electronic brachytherapy sources," J. Res. Natl. Inst. Stand. Technol. 119, 554-574 (2014)]. Therefore, the motivation for the current study was to provide an independent determination of air-kerma rate at 50 cm in air K̇air(d=50 cm) using MC methods for the model S700 eBT source. Using CAD information provided by the vendor and disassembled sources, an MC model was created for the S700 eBT source. Simulations were run using the mcnp6 radiation transport code for the NIST Lamperti air ionization chamber according to specifications by Boutillon et al. ["Comparison of exposure standards in the 10-50 kV x-ray region," Metrologia 5, 1-11 (1969)], in air without the Lamperti chamber, and in vacuum without the Lamperti chamber. K̇air(d=50 cm) was determined using the *F4 tally with NIST values for the mass energy-absorption coefficients for air. Photon spectra were evaluated over 2 π azimuthal sampling for polar angles of 0° ≤ θ ≤ 180° every 1°. Volume averaging was averted through tight radial binning. Photon energy spectra were determined over all polar angles in both air and vacuum using the F4 tally with 0.1 keV resolution. A total of 10(11) simulated histories were run for the Lamperti chamber geometry (statistical uncertainty of 0.14%), with 10(10) histories for the in-air and in-vacuum simulations (statistical uncertainty of 0.04%). The total standard uncertainty in the calculated air-kerma rate determination amounted to 6.8%. MC simulations determined the air-kerma rate at 50 cm from the source with the modeled Lamperti chamber to be (1.850 ± 0.126) × 10(-4) Gy/s, which was within the range of K̇air(d=50 cm) values (1.67-2.11) × 10(-4) Gy/s measured by NIST. The ratio of the photon spectra in air and in vacuum were in good agreement above 13 keV, and for θ < 150° where the influence of the Kovar sleeve and the Ag epoxy components caused increased scatter in air. Below 13 keV, the ratio of the photon spectra in air to vacuum exhibited a decrease that was attributed to increased attenuation of the photons in air. Across most of the energy range on the source transverse plane, there was good agreement between the authors' simulated spectra and that measured by NIST. Discrepancies were observed above 40 keV where the NIST spectrum had a steeper fall-off towards 50 keV. Through MC simulations of radiation transport, this study provided an independent validation of the measured air-kerma rate at 50 cm in air at NIST for the model S700 eBT source, with mean results in agreement within 3.3%. This difference was smaller than the range (i.e., 23%) of the measured values.

Highly conformal CT based surface mould brachytherapy for non-melanoma skin cancers of earlobe and nose.

PurposeBrachytherapy (BT), due to rapid dose fall off and minor set-up errors, should be superior to external beam radiotherapy (EBRT) for treatment of lesions in difficult locations like nose and earlobe. Evidences in this field are scarce. We describe computed tomography (CT) based surface mould BT for non-melanoma skin cancers (NMSC), and compare its conformity, dose coverage, and tissue sparing ability to EBRT.Material and methodsWe describe procedure of preparation of surface mould applicator and dosimetry parameters of BT plans, which were implemented in 10 individuals with NMSC of nose and earlobe. We evaluated dose coverage by minimal dose to 90% of planning target volume (PTV) (D90), volumes of PTV receiving 90-150% of prescribed dose (PD) (VPTV90-150), conformal index for 90 and 100% of PD (COIN90, COIN100), dose homogeneity index (DHI), dose nonuniformity ratio (DNR), exposure of organs. Prospectively, we created CT-based photons and electrons plans. We compared conformity (COIN90, COIN100), dose coverage of PTV (D90, VPTV90, VPTV100), volumes of body receiving 10-90% of PD (V10-V90) of EBRT and BT plans.ResultsWe obtained mean BT-DHI = 0.76, BT-DNR = 0.23, EBRT-DHI = 1.26. We observed no significant differences in VPTV90 and D90 between BT and EBRT. Mean BT-VPTV100 (89.4%) was higher than EBRT-VPTV100 (71.2%). Both COIN90 (BT-COIN90 = 0.46 vs. EBRT-COIN90 = 0.21) and COIN100 (BT-COIN100 = 0.52 vs. EBRT-COIN100 = 0.26) were superior for BT plans. We observed more exposure of normal tissues for small doses in BT plans (V10, V20), for high doses in EBRT plans (V70, V90).ConclusionsComputed tmography-based surface mould brachytherapy for superficial lesions on irregular surfaces is a highly conformal method with good homogeneity. Brachytherapy is superior to EBRT in those locations in terms of conformity and normal tissue sparing ability in high doses.

Episcleral eye plaque dosimetry comparison for the Eye Physics EP917 using Plaque Simulator and Monte Carlo simulation.

This work is a comparative study of the dosimetry calculated by Plaque Simulator, a treatment planning system for eye plaque brachytherapy, to the dosimetry calculated using Monte Carlo simulation for an Eye Physics model EP917 eye plaque. Monte Carlo (MC) simulation using MCNPX 2.7 was used to calculate the central axis dose in water for an EP917 eye plaque fully loaded with 17 IsoAid Advantage 125I seeds. In addition, the dosimetry parameters Λ, gL(r), and F(r,θ) were calculated for the IsoAid Advantage model IAI‐125 125I seed and benchmarked against published data. Bebig Plaque Simulator (PS) v5.74 was used to calculate the central axis dose based on the AAPM Updated Task Group 43 (TG‐43U1) dose formalism. The calculated central axis dose from MC and PS was then compared. When the MC dosimetry parameters for the IsoAid Advantage 125I seed were compared with the consensus values, Λ agreed with the consensus value to within 2.3%. However, much larger differences were found between MC calculated gL(r) and F(r,θ) and the consensus values. The differences between MC‐calculated dosimetry parameters are much smaller when compared with recently published data. The differences between the calculated central axis absolute dose from MC and PS ranged from 5% to 10% for distances between 1 and 12 mm from the outer scleral surface. When the dosimetry parameters for the 125I seed from this study were used in PS, the calculated absolute central axis dose differences were reduced by 2.3% from depths of 4 to 12 mm from the outer scleral surface. We conclude that PS adequately models the central dose profile of this plaque using its defaults for the IsoAid model IAI‐125 at distances of 1 to 7 mm from the outer scleral surface. However, improved dose accuracy can be obtained by using updated dosimetry parameters for the IsoAid model IAI‐125 125I seed.PACS number: 87.55.K‐

Testing the Assessment of New Radiation Oncology Technology and Treatments framework using the evaluation of post-prostatectomy radiotherapy techniques.

We tested the ability of the Assessment of New Radiation Oncology Technology and Treatments framework to determine the clinical efficacy and safety of intensity-modulated radiation therapy (IMRT) compared with 3-dimensional radiation therapy (3DCRT) for post-prostatectomy radiation therapy (PPRT) to support its timely health economic evaluation. Treatment plans produced using FROGG guidelines provided dosimetry parameters for both techniques at 64 Gy and 70 Gy and were also used to model early and late outcome probabilities. Clinical parameters were derived from early toxicity and quality of life patient data, systematic literature review and expert opinion. Dosimetry parameters were correlated with the measures of clinical efficacy and safety. Data from two patient cohorts (29 and 27 respectively) were collected within the project timeframe, providing evidence for acute toxicity and quality of life, and dosimetric comparisons. Relative rates of tumour control probability (TCP) and normal tissue control probability (NTCP) modelling were readily derived from the planning exercise and demonstrated advantages in uncomplicated TCP for IMRT over 3DCRT, predominantly due to normal tissue sparing. The safety of IMRT delivery was demonstrated with TCP uncompromised by IMRT protocol violations, which achieved rectal sparing only by reducing minimum target dose and coverage. Sources of desk-top and patient-based evidence were successfully used to demonstrate potential improved clinical efficacy and safety of applying dose escalation using IMRT instead of 3DCRT in PPRT.

Monte Carlo Dosimetry of the 60Co BEBIG High Dose Rate for Brachytherapy.

IntroductionThe use of high-dose-rate brachytherapy is currently a widespread practice worldwide. The most common isotope source is 192Ir, but 60Co is also becoming available for HDR. One of main advantages of 60Co compared to 192Ir is the economic and practical benefit because of its longer half-live, which is 5.27 years. Recently, Eckert & Ziegler BEBIG, Germany, introduced a new afterloading brachytherapy machine (MultiSource®); it has the option to use either the 60Co or 192Ir HDR source. The source for the Monte Carlo calculations is the new 60Co source (model Co0.A86), which is referred to as the new BEBIG 60Co HDR source and is a modified version of the 60Co source (model GK60M21), which is also from BEBIG.Objective and MethodsThe purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG-43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high-dose-rate brachytherapy to investigate the required treatment-planning parameters. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength S k, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The spectrum to simulate 60Co was composed of two photon energies, 1.17 and 1.33 MeV. Only the gamma part of the spectrum was used; the contribution of the electrons to the dose is negligible because of the full absorption by the stainless-steel wall around the metallic 60Co. The XCOM photon cross-section library was used in subsequent simulations, and the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. The cut-off energy was 10 keV for electrons and photons. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3. The liquid water density was 0.998 g/cm3, and photon histories of up to 1010 were used to obtain the results with a standard deviation of less than 0.5% (k = 1). The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh-1U-1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.

Lung cancer risk of airborne particles for Italian population

Airborne particles, including both ultrafine and supermicrometric particles, contain various carcinogens. Exposure and risk-assessment studies regularly use particle mass concentration as dosimetry parameter, therefore neglecting the potential impact of ultrafine particles due to their negligible mass compared to supermicrometric particles. The main purpose of this study was the characterization of lung cancer risk due to exposure to polycyclic aromatic hydrocarbons and some heavy metals associated with particle inhalation by Italian non-smoking people. A risk-assessment scheme, modified from an existing risk model, was applied to estimate the cancer risk contribution from both ultrafine and supermicrometric particles. Exposure assessment was carried out on the basis of particle number distributions measured in 25 smoke-free microenvironments in Italy. The predicted lung cancer risk was then compared to the cancer incidence rate in Italy to assess the number of lung cancer cases attributed to airborne particle inhalation, which represents one of the main causes of lung cancer, apart from smoking. Ultrafine particles are associated with a much higher risk than supermicrometric particles, and the modified risk-assessment scheme provided a more accurate estimate than the conventional scheme. Great attention has to be paid to indoor microenvironments and, in particular, to cooking and eating times, which represent the major contributors to lung cancer incidence in the Italian population. The modified risk assessment scheme can serve as a tool for assessing environmental quality, as well as setting up exposure standards for particulate matter.

Determination of surface dose rate of indigenous 32P patch brachytherapy source by experimental and Monte Carlo methods

Isotope production and Application Division of Bhabha Atomic Research Center developed 32P patch sources for treatment of superficial tumors. Surface dose rate of a newly developed 32P patch source of nominal diameter 25mm was measured experimentally using standard extrapolation ionization chamber and Gafchromic EBT film. Monte Carlo model of the 32P patch source along with the extrapolation chamber was also developed to estimate the surface dose rates from these sources. The surface dose rates to tissue (cGy/min) measured using extrapolation chamber and radiochromic films are 82.03±4.18 (k=2) and 79.13±2.53 (k=2) respectively. The two values of the surface dose rates measured using the two independent experimental methods are in good agreement to each other within a variation of 3.5%. The surface dose rate to tissue (cGy/min) estimated using the MCNP Monte Carlo code works out to be 77.78±1.16 (k=2). The maximum deviation between the surface dose rates to tissue obtained by Monte Carlo and the extrapolation chamber method is 5.2% whereas the difference between the surface dose rates obtained by radiochromic film measurement and the Monte Carlo simulation is 1.7%. The three values of the surface dose rates of the 32P patch source obtained by three independent methods are in good agreement to one another within the uncertainties associated with their measurements and calculation. This work has demonstrated that MCNP based electron transport simulations are accurate enough for determining the dosimetry parameters of the indigenously developed 32P patch sources for contact brachytherapy applications.

SU‐C‐303‐05: Photosensitizer Determination for PDT Using Interstitial and Surface Measurements of Fluorescence

Purpose:Photosensitizer concentration during photodynamic therapy (PDT) is an important parameter for accurate dosimetry. Fluorescence signal can be used as a measure of photosensitizer concentration. Two methods of data acquisition were compared to an ex vivo study both for in vivo and phantom models.Methods:Fluorescence signal of commonly used photosensitizer benzoporphyrin derivative monoacid ring A (BPD) was obtained in phantoms and mouse tumors using an excitation light of 405 nm. Interstitial fluorescence signal was obtained using a side‐cut fiber inserted into the tumor tissue of interest. Using a previously developed multi‐fiber probe, tumor surface fluorescence measurements were also collected. Signals were calibrated according to optical phantoms with known sensitizer fluorescence. Optical properties for each sample were determined and the influence of different absorption and scattering properties on the fluorescence signals was investigated. Using single value decomposition of the spectra, the sensitizer concentration was determined using the two different measurement geometries. An ex vivo analysis was also performed for tumor samples to determine the sensitizer concentration.Results:The two fluorescence signals obtained from the surface multi‐fiber probe and the interstitial measurements were compared and were corresponding for both phantoms and mouse models. The values obtained were comparable to the ex vivo measurements as well. Despite the difference in geometry, the surface probe measurements can still be used as a metric for determining the presence of sensitizer in small volume tumors.Conclusion:The multi‐fiber contact probe can be used as a tool to measure fluorescence at the surface of the treatment area for PDT and predict sensitizer concentration throughout the tumor. This is advantageous in that the measurement does not damage any tissue. Future work will include investigating the dependence of these results on intratumor sensitizer distribution.

SU‐E‐T‐532: Left‐Sided Breast Cancer Irradiation Using Volumatric Modulated Arc Therapy: An Evaluation of Multiple Commercial Systems

Purposes:There has been growing interest in treating breast cancer using VMAT technique. Our goal is to compare the dosimetry and treatment delivery parameters for the left‐sided breast cancer treatment using various VMAT platforms from commercially available planning systems.Methods:Five consecutive left‐sided breast cancer patients initially treated with conventional 3D‐conformal radiotherapy (3DCRT) were selected. Four VMAT plans using most popular treatment planning systems, including Eclipse (Version 11, Varian), Pinnacle (Version 9.8, Philips), Monaco (Version 2.03, Elekta) and helical Tomotherapy (V4.0, Accuray). The same structure set and same planning goals were used for all VMAT plans. The dosimetric parameters including target coverage and minimum/maximum/mean, dose‐volume endpoints for the selected normal structures: the heart, ipsilateral‐/contralateral lung and breast, were evaluated. Other dosimetric indices including heterogeneity index (HI) were evaluated. The treatment delivery parameters, such as monitor unit (MUs) and delivery time were also compared.Results:VMAT increases dose homogeneity to the treated volume and reduces the irradiated heart and left‐lung volumes. Compared to the 3DCRT technique, all VMAT plans offer better heart and left‐lung dose sparing; the mean heart doses were 4.5±1.6(Monaco), 1.2±0.4(Pinnacle), 1.3± (Eclipse) and 5.6±4.4(Tomo), the mean left‐lung doses were 5.9±1.5(Monaco), 3.7±0.7(Pinnacle), 1.4± (Eclipse) and 5.2±1.6 (Tomo), while for the 3DCRT plan, the mean heart and left‐Lung doses were 2.9±2.0, and 6.8±4.4 (Gy) respectively. The averaged contralateral‐breast and lung mean doses were higher in VMAT plans than the 3DCRT plans but were not statistically significant. Among all the VMAT plans, the Pinnacle plans often yield the lowest right‐lung/breast mean doses, and slightly better heterogeneity indices that are similar to Tomotherapy plans. Treatment delivery time of the VMAT plans (except helical Tomotherapy IMRT) is estimated to be comparable with the conventional 3DCRT.Conclusion:VMAT achieves equal or better PTV coverage and comparable OARs sparing compared to the conventional 3DCRT techniques.

TU‐G‐BRD‐06: The Imaging and Radiation Oncology Core Houston (IROC Houston) QA Center International Activities Outside North America

Purpose:To describe the extent of IROC Houston's (formerly the RPC) QA activities and audit results for radiotherapy institutions outside of North America (NA).Methods:The IROC Houston's QA program components were designed to audit the radiation dose calculation chain from the NIST traceable reference beam calibration, to inclusion of dosimetry parameters used to calculate tumor doses, to the delivery of the radiation dose. The QA program provided to international institutions includes: 1) remote TLD/OSLD audit of machine output, 2) credentialing for advanced technologies, and 3) review of patient treatment records. IROC Houston uses the same standards and acceptance criteria for all of its audits whether for North American or international sites.Results:IROC Houston's QA program has reached out to radiotherapy sites in 43 different countries since 2013 through their participation in clinical trials. In the past two years, 2,778 international megavoltage beam outputs were audited with OSLD/TLD. While the average IROC/Inst ratio is near unity for all sites monitored, there are international regions whose results are significantly different from the NA region. In the past 2 years, 477 and 87 IMRT H&amp;N phantoms were irradiated at NA and international sites, respectively. Regardless of the OSLD beam audit results, the overall pass rate (87 percent) for all international sites (no region separation) is equal to the NA sites. Of the 182 international patient charts reviewed, 10.7 percent of the dose calculation points did not meet our acceptance criterion as compared to 13.6 percent for NA sites. The lower pass rate for NA sites results from a much larger brachytherapy component which has been shown to be more error prone.Conclusion:IROC Houston has expanded its QA services worldwide and continues a long history of improving radiotherapy dose delivery in many countries.Funding received for QA audit services from the Korean GOG, DAHANCA, EORTC, ICON and CMIC Group

A generic high-dose rate (192)Ir brachytherapy source for evaluation of model-based dose calculations beyond the TG-43 formalism.

In order to facilitate a smooth transition for brachytherapy dose calculations from the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) formalism to model-based dose calculation algorithms (MBDCAs), treatment planning systems (TPSs) using a MBDCA require a set of well-defined test case plans characterized by Monte Carlo (MC) methods. This also permits direct dose comparison to TG-43 reference data. Such test case plans should be made available for use in the software commissioning process performed by clinical end users. To this end, a hypothetical, generic high-dose rate (HDR) (192)Ir source and a virtual water phantom were designed, which can be imported into a TPS. A hypothetical, generic HDR (192)Ir source was designed based on commercially available sources as well as a virtual, cubic water phantom that can be imported into any TPS in DICOM format. The dose distribution of the generic (192)Ir source when placed at the center of the cubic phantom, and away from the center under altered scatter conditions, was evaluated using two commercial MBDCAs [Oncentra(®) Brachy with advanced collapsed-cone engine (ACE) and BrachyVision ACUROS™ ]. Dose comparisons were performed using state-of-the-art MC codes for radiation transport, including ALGEBRA, BrachyDose, GEANT4, MCNP5, MCNP6, and PENELOPE2008. The methodologies adhered to recommendations in the AAPM TG-229 report on high-energy brachytherapy source dosimetry. TG-43 dosimetry parameters, an along-away dose-rate table, and primary and scatter separated (PSS) data were obtained. The virtual water phantom of (201)(3) voxels (1 mm sides) was used to evaluate the calculated dose distributions. Two test case plans involving a single position of the generic HDR (192)Ir source in this phantom were prepared: (i) source centered in the phantom and (ii) source displaced 7 cm laterally from the center. Datasets were independently produced by different investigators. MC results were then compared against dose calculated using TG-43 and MBDCA methods. TG-43 and PSS datasets were generated for the generic source, the PSS data for use with the ace algorithm. The dose-rate constant values obtained from seven MC simulations, performed independently using different codes, were in excellent agreement, yielding an average of 1.1109 ± 0.0004 cGy/(h U) (k = 1, Type A uncertainty). MC calculated dose-rate distributions for the two plans were also found to be in excellent agreement, with differences within type A uncertainties. Differences between commercial MBDCA and MC results were test, position, and calculation parameter dependent. On average, however, these differences were within 1% for ACUROS and 2% for ace at clinically relevant distances. A hypothetical, generic HDR (192)Ir source was designed and implemented in two commercially available TPSs employing different MBDCAs. Reference dose distributions for this source were benchmarked and used for the evaluation of MBDCA calculations employing a virtual, cubic water phantom in the form of a CT DICOM image series. The implementation of a generic source of identical design in all TPSs using MBDCAs is an important step toward supporting univocal commissioning procedures and direct comparisons between TPSs.

A revised dosimetric characterization of the model S700 electronic brachytherapy source containing an anode-centering plastic insert and other components not included in the 2006 model

The model S700 Axxent electronic brachytherapy source by Xoft, Inc., was characterized by Rivard et al. in 2006. Since then, the source design was modified to include a new insert at the source tip. Current study objectives were to establish an accurate source model for simulation purposes, dosimetrically characterize the new source and obtain its TG-43 brachytherapy dosimetry parameters, and determine dose differences between the original simulation model and the current model S700 source design. Design information from measurements of dissected model S700 sources and from vendor-supplied CAD drawings was used to aid establishment of an updated Monte Carlo source model, which included the complex-shaped plastic source-centering insert intended to promote water flow for cooling the source anode. These data were used to create a model for subsequent radiation transport simulations in a water phantom. Compared to the 2006 simulation geometry, the influence of volume averaging close to the source was substantially reduced. A track-length estimator was used to evaluate collision kerma as a function of radial distance and polar angle for determination of TG-43 dosimetry parameters. Results for the 50 kV source were determined every 0.1 cm from 0.3 to 15 cm and every 1° from 0° to 180°. Photon spectra in water with 0.1 keV resolution were also obtained from 0.5 to 15 cm and polar angles from 0° to 165°. Simulations were run for 10(10) histories, resulting in statistical uncertainties on the transverse plane of 0.04% at r = 1 cm and 0.06% at r = 5 cm. The dose-rate distribution ratio for the model S700 source as compared to the 2006 model exceeded unity by more than 5% for roughly one quarter of the solid angle surrounding the source, i.e., θ ≥ 120°. The radial dose function diminished in a similar manner as for an (125)I seed, with values of 1.434, 0.636, 0.283, and 0.0975 at 0.5, 2, 5, and 10 cm, respectively. The radial dose function ratio between the current and the 2006 model had a minimum of 0.980 at 0.4 cm, close to the source sheath and for large distances approached 1.014. 2D anisotropy function ratios were close to unity for 50° ≤ θ ≤ 110°, but exceeded 5% for θ < 40° at close distances to the sheath and exceeded 15% for θ > 140°, even at large distances. Photon energy fluence of the updated model as compared to the 2006 model showed a decrease in output with increasing distance; this effect was pronounced at the lowest energies. A decrease in photon fluence with increase in polar angle was also observed and was attributed to the silver epoxy component. Changes in source design influenced the overall dose rate and distribution by more than 2% in several regions. This discrepancy is greater than the dose calculation acceptance criteria as recommended in the AAPM TG-56 report. The effect of the design change on the TG-43 parameters would likely not result in dose differences outside of patient applicators. Adoption of this new dataset is suggested for accurate depiction of model S700 source dose distributions.