TU‐C‐BRA‐01: Progress in Calculations of KQ for TG‐51

The calculated values of kQ in the TG‐51 protocol are based on analytic calculations which make use of various tabulated values of measured and calculated factors such as the water to air stopping‐power ratio, Pwall, Prepl and Pcel (for details see Ch 9 of the 2009 AAPM Summer School book, Med. Phys. Publishing, Madison, WI). Since the publication of TG‐51 there have been improved calculations of almost all the factors required in TG‐51. This talk will briefly review those improved calculations which are based on Monte Carlo calculations of ratios of the dose to the cavity of the ion chamber. However, with the development of the EGSnrc Monte Carlo system (Kawrakow, Med Phys 27(2000) 499) it became possible to accurately calculate the response of ion chambers, not just the ratios of the dose to the cavity. This made it possible to accurately do ab initio Monte Carlo calculations of kQ. As computers became faster per unit cost and clever variance reduction techniques for doing ion chamber calculations in complex geometries were implemented (Wulff et al, Med Phys 35(2008)1328) these calculations became both accurate and feasible (albeit with large clusters of computers). Values of kQ have been calculated this way for a total of 33 commonly used cylindrical ion chambers (Muir and Rogers, Med Phys 37(2010)5939). Detailed estimates of the systematic uncertainty in these calculations have been made and range between 0.6% and 1.0%, depending on the assumptions made. The largest component is the uncertainty (0.5%) in the assumed constancy with beam quality of (W/e)air, which relates the energy deposited in the cavity to the charge released in the air. In a detailed comparison of the calculated kQ values to the extensive high‐ quality measurements by McEwen (Med Phys 37(2010)2179), Muir et al (submitted, 2011) found the mean percentage differences between the calculations and the experiments are 0.08(0.17), 0.07(0.32) and 0.23(0.31) in 6, 10 and 25 MV beams respectively (bracketed values are rms deviations). These discrepancies are well within the stated uncertainties of the measurements (about 0.3% to 0.4%) and the calculations (about 0.3% to 0.4% ignoring W/e uncertainties and assuming correlated uncertainties in photon cross section). These comparisons can be used to set an upper limit of 0.4% on the variation of (W/e)air with beam quality between 60Co and 25 MV beams (95% confidence). More importantly, the close agreement with experiment gives confidence in the accuracy of the Monte Carlo calculated values of kQ with a 68% confidence uncertainty of between 0.4% and 0.5%. Learning Objectives 1. Understand the basis of calculated kQ values in the original TG‐51 protocol 2. Become aware of the improvements made in many of the required correction factors in the last decade 3. Understand how ab initio calculations of kQ are done 4. Understand the uncertainties involved in ab initio calculations of kQ

WE‐SAMS‐AUD‐03: The Physics of the TG‐51 Dosimetry Protocol

The report of Task Group 51 on a “Protocol for Clinical Reference Dosimetry of High‐Energy Photon and Electron Beams” was published nearly 8 years ago (Medical Physics 26 (1999) 1847 – 1870) and according to the RPC has been adopted by about 80% of all clinics. This talk will review how the various factors in TG‐51 were calculated, including a derivation of the relevant equations and a discussion of the sources of physical data used. The rationale for the choice of beam quality specifiers will be given and the role of the lead foil explained. The talk will conclude with a review of some of the published experimental data which confirms the calculated kQ values used in TG‐51 and review some of the post‐TG51‐publication research which has an impact on the physics in the protocol. The protocol should continue to be used as written. Educational Objectives: 1. To review the basic physics and sources of data underlying TG‐51. 2. To derive the equations used to calculate kQ and k′R50. 3. To review some experimental verifications of TG‐51 quantities, especially kQ. 4. To discuss the implications of more recent dosimetry research and its effects on future dosimetry protocols

Accuracy of the Burns equation for stopping-power ratio as a function of depth and R50

The accuracy of the Burns et al. equation [Med. Phys. 23, 489-501 (1996)] for the Spencer-Attix water to air stopping-power ratio as a function of depth in a water phantom and electron beam quality in terms of R50 is investigated by comparison to the original data on which this fit was based. It is shown that using this equation provides dose estimates on the central axis in a clinical electron beam that are accurate to within 1% of dose maximum for all 24 clinical beams investigated except very close to the surface in swept beams. In contrast, the error in the dose as a percentage of the local dose is much higher for values of the depth/R50 greater than 1.2

Comment on 'Monte Carlo simulation on a gold nanoparticle irradiated by electron beams'

A recent paper by Chow et al (Phys. Med. Biol 57 3323-31) quantifies the dose due to secondary electrons created by gold nanoparticles when irradiated by electron beams. That paper fails to compare this dose to the overall dose from the electron beam. EGSnrc calculations are performed to show that, even for the unrealistically favourable case presented by Chow et al of a very narrow electron beam directed only at the nanoparticle, the dose outside the nanoparticle due to the secondary electrons generated by the nanoparticle is negligible compared to the dose from the primaries. Thus, it is irrelevant whether the dose from secondary particles is enhanced by the nanoparticles or no

Inverse square corrections for FACs and WAFACs

Inverse square correction factors for wide-angle free-air chambers (WAFACs) and free-air chambers (FACs) for cylindrical, conical and square-prism detectors are required for determining the on-axis air kerma from measurements or Monte Carlo calculations made with these different shaped detectors. Values of air kerma measured with these detectors use an effective volume technique related to the inverse square correction factors. This paper presents these factors in a consistent framework and the relationships between them are made clear. Using Monte Carlo simulations, the various corrections and techniques are shown to be accurate within a statistical precision of about 0.04% or better with the exception of the published correction for square prism detectors which is shown to hold only for thin detectors which have an opening angle corresponding to the NIST and NRCC WAFAC primary standards. A more accurate correction for square prism detectors is presented which properly averages 1/d2 rather than d2 where d is the distance away from the source

Inverse square corrections for FACs and WAFACs

Inverse square correction factors for wide-angle free-air chambers (WAFACs) and free-air chambers (FACs) for cylindrical, conical and square-prism detectors are required for determining the on-axis air kerma from measurements or Monte Carlo calculations made with these different shaped detectors. Values of air kerma measured with these detectors use an effective volume technique related to the inverse square correction factors. This paper presents these factors in a consistent framework and the relationships between them are made clear. Using Monte Carlo simulations, the various corrections and techniques are shown to be accurate within a statistical precision of about 0.04% or better with the exception of the published correction for square prism detectors which is shown to hold only for thin detectors which have an opening angle corresponding to the NIST and NRCC WAFAC primary standards. A more accurate correction for square prism detectors is presented which properly averages 1/d2 rather than d2 where d is the distance away from the source

An EGSnrc Monte Carlo-calculated database of TG-43 parameters

Monte Carlo methods are used to calculate a complete TG-43 dosimetry parameter data set for 27 low-energy photon emitting brachytherapy sources (18 125I and 9 103Pd). All Monte Carlo calculations are performed using the EGSnrc user-code BrachyDose. TG-43 dosimetry parameters, including dose rate constants, radial dose functions (with functional fitting parameters), and anisotropy data, are calculated with finer spatial resolution, greater range of distances, and smaller uncertainties than data currently available in the literature for many of these sources. In particular, for most of the seeds, this is the first time that anisotropy data have been tabulated at distances less than 0.5 cm from the source. These calculations employ the state-of-the-art XCOM photon cross sections, and detailed source geometries are modeled using Yegin's multigeometry package. This data set serves as a completely independent verification of the currently available dosimetry parameters calculated using other Monte Carlo codes, including MCNP and PTRAN. This report also describes the Carleton Laboratory for Radiotherapy Physics TG-43 Parameter Database, a publicly accessible web site (at http://www.physics.carleton.ca/clrp/seed_database/) through which all of the data calculated for this study can be accessed. Also available on the web site are descriptions of the methods and Monte Carlo models used in this study and comparisons of data calculated in this study with data calculated by other authors

Sci‐Fri PM: Delivery — 07: Analysis of Systematic Uncertainties in Monte Carlo Calculated Beam Quality Conversion Factors

The beam quality conversion factor, kQ, can be calculated directly using Monte Carlo simulations. In order to validate the use of these calculated values, an evaluation of the associated systematic uncertainties is required. In a Monte Carlo simulation, the relative statistical uncertainty can be reduced by increasing the number of histories at the expense of computing time. Other sources of uncertainty that must be considered originate from possible variations in photon cross‐sections, stopping powers, chamber dimensions, the choice of source use for the simulation and variation of W/e with beam energy. In this study, these systematic uncertainties are quantified with Monte Carlo calculations using different methods with the EGSnrc code system. In most cases, it is possible to assign an uncertainty on a given quantity (e.g. photon cross‐sections) based on information from the literature. The specific parameter is changed by one standard deviation and the change in kQ, ΔkQ, is calculated separately for each parameter, yielding an uncertainty in kQ from each source of uncertainty. The overall uncertainty in kQ is determined using well‐known methods. Uncertainty due to the variation of photon cross‐sections depends on whether or not the cross‐section uncertainties are correlated. If correlated the total systematic uncertainty in kQ amounts to 0.64%, or 1.0% if one assumes uncorrelated photon cross‐sections. The uncertainty in W/e (0.5%) is a major source of uncertainty which also affects all other calculations of kQ

Efficiency improvements of x-ray simulations in EGSnrc user-codes using bremsstrahlung cross-section enhancement (BCSE)

This paper presents the implementation of the bremsstrahlung cross-section enhancement (BCSE) variance-reduction technique into the EGSnrc/BEAMnrc system. BCSE makes the simulation of x-ray production from bremsstrahlung targets more efficient; it does so by artificially making the rare event of bremsstrahlung emission more abundant, which increases the number of statistically-independent photons that contribute to reducing the variance of the quantity of interest without increasing the CPU time appreciably. BCSE does not perturb the charged-particle transport in EGSnrc and it is made compatible with all other variance-reduction techniques already used in EGSnrc and BEAMnrc, including range rejection, uniform bremsstrahlung splitting, and directional bremsstrahlung splitting. When optimally combining BCSE with splitting to simulate typical situations of interest in medical physics research and in clinical practice, efficiencies can be up to five orders of magnitude larger than those obtained with analog simulations, and up to a full order of magnitude larger than those obtained with optimized splitting alone (which is the state-of-the-art of the EGSnrc/BEAMnrc system before this study was carried out). This study recommends that BCSE be combined with the existing splitting techniques for all EGSnrc/BEAMnrc simulations that involve bremsstrahlung targets, both in the kilovoltage and megavoltage range. Optimum cross-section enhancement factors for typical situations in diagnostic x-ray imaging and in radiotherapy are recommended, along with an easy algorithm for simulation optimization