Cavity theory applications for kilovoltage cellular dosimetry

Relationships between macroscopic (bulk tissue) and microscopic (cellular) dose descriptors are investigated using cavity theory and Monte Carlo (MC) simulations. Small, large, and multiple intermediate cavity theory (SCT, LCT, and ICT, respectively) approaches are considered for 20 to 370 keV incident photons; ICT is a sum of SCT and LCT contributions weighted by parameter d. Considering μm-sized cavities of water in bulk tissue phantoms, different cavity theory approaches are evaluated via comparison of (where D w,m is dose-to-water-in-medium and D m,m is dose-to-medium-in-medium) with MC results. The best overall agreement is achieved with an ICT approach in which d = (1 e-βL)/(βL), where L is the mean chord length of the cavity and β is given by e-βRCSDA(R CSDA is the continuous slowing down app

SU‐E‐T‐667: On the Monte Carlo Simulation of Electron Transport in the Sub‐1 KeV Energy Range

Purpose: The validity of ‘classic’ Monte Carlo simulations of electron and positron transport at sub‐1 keV energies is investigated in the context of quantum theory.Methods: Quantum theory dictates that uncertainties on the position and energy‐momentum four vectors of radiation quanta obey Heisenberg's uncertainty relation; however, these uncertainties are neglected in ‘classical’ MC simulations of radiation transport in which position and momentum are known precisely. Using the quantum uncertainty relation and electron mean free path, the magnitude of uncertainties on electron position and momentum are calculated for different kinetic energies; a validity bound on the classical simulation of electron transport is derived. Results: In order to satisfy the Heisenberg uncertainty principle, uncertainties of 4% or greater must be assigned to position and momentum for 1 keV electrons in water; at 100 eV, these uncertainties are 15% to 22% and are even larger at lower energies. In gaseous media such as air, these uncertainties are much smaller (less than 1% for electrons with energy 20 eV or greater). Conclusions: The classical Monte Carlo transport treatment is incorrect for sub‐1 keV electrons in water as uncertainties on position and momentum must be large (relative to electron momentum and the mean free path) to satisfy the quantum uncertainty principle. Simulations in condensed media (e.g., water) which do not reflect the quantum nature of electrons and positrons are not faithful representations of the physical reality at these low energies, calling into question the results of MC track structure codes simulating sub‐1 keV electron transport. Further, the large differences in the scale at which quantum effects are important in gaseous and condensed media suggest that track structure measurements in gases are not necessarily representative of track structure in condensed materials on a micrometer or nanometer scale

A Monte Carlo study of macroscopic and microscopic dose descriptors for kilovoltage cellular dosimetry

This work investigates how doses to cellular targets depend on cell morphology, as well as relations between cellular doses and doses to bulk tissues and water. Multicellular models of five healthy and cancerous soft tissues are developed based on typical values of cell compartment sizes, elemental compositions and number densities found in the literature. Cells are modelled as two concentric spheres with nucleus and cytoplasm compartments. Monte Carlo simulations are used to calculate the absorbed dose to the nucleus and cytoplasm for incident photon energies of 20-370 keV, relevant for brachytherapy, diagnostic radiology, and out-of-field radiation in higher-energy external beam radiotherapy. Simulations involving cell clusters, single cells and single nuclear cavities are carried o

WE‐E‐BRD‐03: Re‐Evaluation of the Product of W/e and the Graphite to Air Stopping Power Ratio for Co‐60 Air Kerma Standards

Purpose: To reanalyze experiments which determine [formula omitted], the product of (W / e)air, the average energy deposited per coulomb of charge released in dry air, and [formula omitted], the Spencer‐Attix mass collision stopping‐power ratio for graphite to air, and to calculate an average value for this product for the BIPM [formula omitted] air kerma standard: [formula omitted]. This value could be adopted for use with [formula omitted] air kerma primary standards, along with corrections to account for variations due to cavity size. Methods and Materials: The experiments measured [formula omitted] by various methods, often involving calorimeters and ionization chambers. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated as needed for each experiment using the EGSnrc user‐codes CAVRZnrc, DOSRZnrc, and CAVITY. Stopping power ratios are evaluated using SPRRZnrc for different choices of graphite density (bulk 1.70 g/cm3 or grain 2.265 g/cm3) for the density effect correction and average excitation energy for graphite (I=78 or 87 eV). For each experiment, the corrected value of [formula omitted] is multiplied by [formula omitted], the quotient of the stopping power ratios for the BIPM chamber and the experiment in question. A least squares technique is used to compute an average value of [formula omitted]. Results: The correction factors generally decrease the value of [formula omitted] for each experiment, often outside the range of one standard deviation quoted with each experimental result. The ratio [formula omitted] varies by less than 0.1% for different choices of density correction and I‐value and hence the product [formula omitted] is also relatively insensitive to these choices. Conclusion: The preliminary analysis suggests that the accepted value of [formula omitted], 33.97 J/C ±0.15%, is 0.6% too high. This would have implications for primary [formula omitted] air kerma standards worldwide and for the value of (W / e)air which is used in low energy x‐ray standards

Monte Carlo dosimetry for i 125 and P 103 d eye plaque brachytherapy with various seed models

Purpose: Dose distributions are calculated for various models of 125I and 103Pd seeds in the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). The sensitivity to seed model of dose distributions and dose distributions relative to TG-43 are investigated. Methods: Monte Carlo simulations are carried out with the EGSnrc user-code BrachyDose. Brachytherapy seeds and eye plaques are fully modeled. Simulations of one seed in the central slot of a 20 mm Modulay (gold alloy) plaque backing with and without the Silastic (silicone polymer) insert and of a 16 mm fully loaded Modulay/Silastic plaque are performed. Dose distributions are compared to those calculated under TG-43 assumptions, i.e., ignoring the effects of the plaque backing and insert and interseed attenuation. Three-dimensional dose distributions for different 125I and 103Pd seed models are compared via depth-dose curves, isodose contours, and tabulation of doses at points of interest in the eye. Results are compared to those of our recent BrachyDose study for COMS plaques containing model 6711 (125I) or 200 (103Pd) seeds [R. M. Thomson, Med. Phys. 35, 5530-5543 (2008)]. Results: Along the central axis of a plaque containing one seed, variations of less than 1% are seen in the effect of the Modulay backing alone for different seed models; for the Modulay/Silastic combination, variations are 2%. For a 16 mm plaque fully loaded with 125I (103Pd) seeds, dose decreases relative to TG-43 doses are 11%-12% (19%-20%) and 14%-15% (20%) at distances of 0.5 and 1 cm from the inner sclera along the plaque's central axis, respectively. For the same prescription dose, doses at points of interest vary by up to 8% with seed model. Doses to critical normal structures are lower for all 103Pd seed models than for 125I with the possible exception of the sclera adjacent to the plaque; scleral doses vary with seed model and are not always higher for 103Pd than for 125I. Conclusions: Dose decreases relative to doses calculated under TG-43 assumptions vary slightly with seed model (for each radionuclide). Dose distributions are sensitive to seed model; however, variations are generally no larger than the magnitudes of other systematic uncertainties in eye plaque therapy

On the Monte Carlo simulation of electron transport in the sub-1 keV energy range

Purpose: The validity of classic Monte Carlo (MC) simulations of electron and positron transport at sub-1 keV energies is investigated in the context of quantum theory. Methods: Quantum theory dictates that uncertainties on the position and energy-momentum four-vectors of radiation quanta obey Heisenberg's uncertainty relation; however, these uncertainties are neglected in classical MC simulations of radiation transport in which position and momentum are known precisely. Using the quantum uncertainty relation and electron mean free path, the magnitudes of uncertainties on electron position and momentum are calculated for different kinetic energies; a validity bound on the classical simulation of electron transport is derived. Results: In order to satisfy the Heisenberg uncertainty principle, uncertainties of 5% must be assigned to position and momentum for 1 keV electrons in water; at 100 eV, these uncertainties are 17 to 20% and are even larger at lower energies. In gaseous media such as air, these uncertainties are much smaller (less than 1% for electrons with energy 20 eV or greater). Conclusions: The classical Monte Carlo transport treatment is questionable for sub-1 keV electrons in condensed water as uncertainties on position and momentum must be large (relative to electron momentum and mean free path) to satisfy the quantum uncertainty principle. Simulations which do not account for these uncertainties are not faithful representations of the physical processes, calling into question the results of MC track structure codes simulating sub-1 keV electron transport. Further, the large difference in the scale at which quantum effects are important in gaseous and condensed media suggests that track structure measurements in gases are not necessarily representative of track structure in condensed materials on a micrometer or a nanometer scale

Re-evaluation of the product of (W/e)air and the graphite to air stopping-power ratio for 60Co air kerma standards

Experiments which determine the product of (W/e)air, the average energy deposited per coulomb of charge of one sign released by an electron coming to rest in dry air, and, the Spencer-Attix mean restricted mass collision stopping-power ratio for graphite to air, in a 60Co or 137Cs beam are reanalysed. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated using the EGSnrc Monte Carlo code system and these generally decrease the value of (W/e)air for each experiment. Stopping-power ratios are calculated for different choices of density correction and average excitation energy (I-value) for graphite. To calculate an average value (W/e)air for the BIPM air kerma standard, each experimental result is multiplied by the ratio/. While individual values of are sensitive to the I-values and density corrections assumed, this ratio varies by less than 0.1% for different choices. Hence, the product (W/e)air is relatively insensitive to these choices. The weighted mean of the updated data is (W/e)air J C -1 0.2%, suggesting that the accepted value of 33.97 J C-1 0.1% is 0.8% too high. This has implications for primary 60Co air kerma standards worldwide and potentially for the choice of graphite I-value and density correction for the calculation of the graphite stopping power, as well as the value of (W/e)air

TU‐E‐116‐01: Clinical Implementation for Advanced Brachytherapy Dose Calculation Algorithms Beyond the TG‐43 Formalism

With the recent introduction of heterogeneity correction algorithms for brachytherapy, the AAPM community is still unclear on how to commission and implement these into clinical practice. The recently‐published AAPM TG‐186 report discusses important issues for clinical implementation of these algorithms. In this practical medical physics course, specific examples on how to perform the commissioning process are presented, as well as descriptions of the clinical impact from recent literature reporting comparisons of TG‐43 and heterogeneity‐based dosimetry. A proposed commissioning flowchart will be discussed, guiding the audience through the clinical process. Further, QA tests specific to these new heterogeneity correction algorithms for brachytherapy will be explained. Potential changes in brachytherapy dose prescriptions will be discussed, with pitfalls identified to minimize likelihood for errors. Learning Objectives: 1. Identify key clinical applications needing advanced dose calculation in brachytherapy. 2. Review TG‐186 guidelines, commission process, and dosimetry benchmarks. 3. Evaluate clinical cases using a commercially available system and compare to TG‐43 dosimetry

Investigating energy deposition within cell populations using Monte Carlo simulations

In this work, we develop multicellular models of healthy and cancerous human soft tissues, which are used to investigate energy deposition in subcellular targets, quantify the microdosimetric spread in a population of cells, and determine how these results depend on model details. Monte Carlo (MC) tissue models combining varying levels of detail on different length scales are developed: microscopically-detailed regions of interest (>1500 explicitly-modelled cells) are embedded in bulk tissue phantoms irradiated by photons (20 keV-1.25 MeV). Specific energy (z; energy imparted per unit mass) is scored in nuclei and cytoplasm compartments using the EGSnrc user-code egs-chamber; specific energy mean, , standard deviation, , and distribution, , are calculated for a variety of macroscopic doses, D. MC-calculated are compared with normal distributions having the same mean and standard deviation. For ∼mGy doses, there is considerable variation in energy deposition (microdosimetric spread) throughout a cell population: e.g. for 30 keV photons irradiating melanoma with 7.5 μm cell radius and 3 μm nuclear radius, for nuclear targets is , and the fraction of nuclei receiving no energy deposition, f z=0, is 0.31 for a dose of 10 mGy. If cobalt-60 photons are considered instead, then decreases to δz/√Z, and f z=0decreases to 0.036. These results correspond to randomly arranged cells with cell/nucleus sizes randomly sampled from a normal distribution with a standard deviation of 1 μm. If cells are arranged in a hexagonal lattice and cell/nucleus sizes are uniform throughout the population, then decreases to and for 30 keV and cobalt-60, respectively; f z=0 decreases to 0.25 and 0.000 94 for 30 keV and cobalt-60, respectively. Thus, specific energy distributions are sensitive to cell/nucleus sizes and their distributions: variations in specific energy deposited over a cell population are underestimated if targets are assumed to be uniform in size compared with more realistic variation in target size. Bulk tissue dose differs from for nuclei (cytoplasms) by up to 21%(12%) across all cell/nucleus sizes, bulk tissues, and incident photon energies, considering a 50 mGy dose level. Overall, results demonstrate the importance of microdosimetric considerations at low doses, and indicate the sensitivity of energy deposition within subcellular targets to incident photon energy, dose level, elemental compositions, and microscopic tissue model

A monte carlo investigation of lung brachytherapy treatment planning

Iodine-125 (125I) and Caesium-131 (131Cs) brachytherapy have been used in conjunction with sublobar resection to reduce the local recurrence of stage I non-small cell lung cancer compared with resection alone. Treatment planning for this procedure is typically performed using only a seed activity nomogram or look-up table to determine seed strand spacing for the implanted mesh. Since the post-implant seed geometry is difficult to predict, the nomogram is calculated using the TG-43 formalism for seeds in a planar geometry. In this work, the EGSnrc user-code BrachyDose is used to recalculate nomograms using a variety of tissue models for 125I and 131Cs seeds. Calculated prescription doses are compared to those calculated using TG-43. Additionally, patient CT and contour data are used to generate virtual implants to study the effects that post-implant deformation and patient-specific tissue heterogeneity have on perturbing nomogram-derived dose distributions. Differences of up to 25% in calculated prescription dose are found between TG-43 and Monte Carlo calculations with the TG-43 formalism underestimating prescription doses in general. Differences between the TG-43 formalism and Monte Carlo calculated prescription doses are greater for 125I than for 131Cs seeds. Dose distributions are found to change significantly based on implant deformation and tissues surrounding implants for patient-specific virtual implants. Results suggest that accounting for seed grid deformation and the effects of non-water media, at least approximately, are likely required to reliably predict dose distributions in lung brachytherapy patients