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

Effective point of measurement of thimble ion chambers in megavoltage photon beams

Purpose: Determine the effective point of measurement (EPOM) of 12 thimble ion chambers, including miniature chambers and three models widely used for clinical reference dosimetry. The EPOM is the point at which the measured dose would arise in the measurement medium in the absence of the probe: For cylindrical chambers, it is shifted upstream relative to the central axis of the chamber. Although current dosimetry protocols prescribe a blanket upstream EPOM shift of 0.6r, with r as the chamber cavity radius, it has been shown in recent years that the EPOM does, in fact, depend on every detail of the chamber design and on the beam characteristics. In the wake of this finding, the authors undertake a comprehensive study of the EPOM for a series of chambers in water.Peer reviewed: YesNRC publication: Ye

Direct measurement of absorbed dose to water in HDR 192Ir brachytherapy: water calorimetry, ionization chamber, Gafchromic film, and TG-43

Purpose: Gafchromic film and ionometric calibration procedures for HDR 192Ir brachytherapy sources in terms of dose rate to water are presented and the experimental results are compared to the TG-43 protocol as well as with the absolute dose measurement results from a water calorimetry-based primary standard. Methods: EBT-1 Gafchromic films, an A1SL Exradin miniature Shonka thimble type chamber, and an SI HDR 1000 Plus well-type chamber (Standard Imaging, Inc., Middleton, WI) with an ADCL traceable Sk calibration coefficient (following the AAPM TG-43 protocol) were used. The Farmer chamber and Gafchromic film measurements were performed directly in water. All results were compared to direct and absolute absorbed dose to water measurements from a 4\u2009\ub0C stagnant water calorimeter. Results: Based on water calorimetry, the authors measured the dose rate to water to be 361\ub17\u2002\u3bcGy/(h\u2009U) at a 55 mm source-to-detector separation. The dose rate normalized to air-kerma strength for all the techniques agree with the water calorimetry results to within 0.83%. The overall 1-sigma uncertainty on water calorimetry, ionization chamber, Gafchromic film, and TG-43 dose rate measurement amounts to 1.90%, 1.44%, 1.78%, and 2.50%, respectively. Conclusions: This work allows us to build a more realistic uncertainty estimate for absorbed dose to water determination using the TG-43 protocol. Furthermore, it provides the framework necessary for a shift from indirect HDR 192Ir brachytherapy dosimetry to a more accurate, direct, and absolute measurement of absorbed dose to water.But : un film gafchromique et des proc\ue9dures d\u2019\ue9talonnage ionom\ue9trique pour des sources de curieth\ue9rapie HDD 192Ir par le d\ue9bit de dose absorb\ue9e dans l\u2019eau sont pr\ue9sent\ue9s et les r\ue9sultats exp\ue9rimentaux au protocole TG-43 de m\ueame qu\u2019aux r\ue9sultats de mesure de dose absolue provenant d\u2019un \ue9talon primaire ax\ue9 sur la calorim\ue9trie \ue0 eau sont compar\ue9s. M\ue9thodologie : on a utilis\ue9 des films gafchromiques EBT-1, une chambre-d\ue9 Shonka miniature Exradin A1SL et une chambre d\u2019ionisation \ue0 puits SI HDR 1000 Plus (Standard Imaging, Inc., Middleton, WI) avec un coefficient de dissym\ue9trie (Sk) d\u2019\ue9talonnage tra\ue7able de l\u2019ADCL (suivant le protocole TG-43 de l\u2019AAPM). Des mesures de la chambre de Farmer et du film gafchromique ont \ue9t\ue9 effectu\ue9es directement dans l\u2019eau. Tous les r\ue9sultats ont \ue9t\ue9 compar\ue9s \ue0 des mesures de dose absorb\ue9e dans l\u2019eau directes et absolues \ue0 partir d\u2019un calorim\ue8tre \ue0 eau stagnante \ue0 4 \ub0C. R\ue9sultats : gr\ue2ce \ue0 la calorim\ue9trie \ue0 eau, les auteurs rapportent un d\ue9bit de dose absorb\ue9e dans l\u2019eau de 361 \ub1 7 \u3bcGy/(h U) \ue0 une distance de 55 mm entre la source et le d\ue9tecteur. Le d\ue9bit de dose absorb\ue9e normalis\ue9 \ue0 la force kerma de l\u2019air pour toutes les techniques concorde avec les r\ue9sultats de la calorim\ue9trie \ue0 eau dans les limites de 0,83 %. L\u2019incertitude 1-sigma globale pour la calorim\ue9trie \ue0 eau, la chambre d\u2019ionisation, le film gafchromique et la mesure du d\ue9bit de dose absorb\ue9e du protocole TG-43 correspond \ue0 1,90, \ue0 1,44, \ue0 1,78 et \ue0 2,50 %, respectivement. Conclusions : ces travaux nous permettent d\u2019\ue9tablir une estimation d\u2019incertitude r\ue9aliste pour la d\ue9termination de la dose absorb\ue9e dans l\u2019eau au moyen du protocole TG-43. De plus, ils fournissent le cadre n\ue9cessaire pour un passage de la dosim\ue9trie curieth\ue9rapique HDD 192Ir indirecte vers une mesure plus pr\ue9cise, directe et absolue de la dose absorb\ue9e dans l\u2019eau.Peer reviewed: YesNRC publication: Ye

On the conversion of dose to bone to dose to water in radiotherapy treatment planning systems

Background and purpose: Conversion factors between dose to medium (Dm,m) and dose to water (Dw,w) provided by treatment planning systems that model the patient as water with variable electron density are currently based on stopping power ratios. In the current paper it will be illustrated that this conversion method is not correct. Materials and methods: Monte Carlo calculations were performed in a phantom consisting of a 2 cm bone layer surrounded by water. Dw,w was obtained by modelling the bone layer as water with the electron density of bone. Conversion factors between Dw,w and Dm,m were obtained and compared to stopping power ratios and ratios of mass-energy absorption coefficients in regions of electronic equilibrium and interfaces. Calculations were performed for 6 MV and 20 MV photon beams. Results: In the region of electronic equilibrium the stopping power ratio of water to bone (1.11) largely overestimates the conversion obtained using the Monte Carlo calculations (1.06). In that region the MC dose conversion corresponds to the ratio of mass energy absorption coefficients. Near the water to bone interface, the MC ratio cannot be determined from stopping powers or mass energy absorption coefficients. Conclusion: Stopping power ratios cannot be used for conversion from Dm,m to Dw,w provided by treatment planning systems that model the patient as water with variable electron density, either in regions of electronic equilibrium or near interfaces. In regions of electronic equilibrium mass energy absorption coefficient ratios should be used. Conversions at interfaces require detailed MC calculations. Keywords: Dose to water, Monte Carlo, Dosimetry, TPS compariso