Dosimetric characterization of two radium sources for retrospective dosimetry studies.

PURPOSE: During the first part of the 20th century, (226)Ra was the most used radionuclide for brachytherapy. Retrospective accurate dosimetry, coupled with patient follow up, is important for advancing knowledge on long-term radiation effects. The purpose of this work was to dosimetrically characterize two (226)Ra sources, commonly used in Sweden during the first half of the 20th century, for retrospective dose-effect studies. METHODS: An 8 mg (226)Ra tube and a 10 mg (226)Ra needle, used at Radiumhemmet (Karolinska University Hospital, Stockholm, Sweden), from 1925 to the 1960s, were modeled in two independent Monte Carlo (MC) radiation transport codes: geant4 and mcnp5. Absorbed dose and collision kerma around the two sources were obtained, from which the TG-43 parameters were derived for the secular equilibrium state. Furthermore, results from this dosimetric formalism were compared with results from a MC simulation with a superficial mould constituted by five needles inside a glass casing, placed over a water phantom, trying to mimic a typical clinical setup. Calculated absorbed doses using the TG-43 formalism were also compared with previously reported measurements and calculations based on the Sievert integral. Finally, the dose rate at large distances from a (226)Ra point-like-source placed in the center of 1 m radius water sphere was calculated with geant4. RESULTS: TG-43 parameters [including gL(r), F(r, θ), Λ, and sK] have been uploaded in spreadsheets as additional material, and the fitting parameters of a mathematical curve that provides the dose rate between 10 and 60 cm from the source have been provided. Results from TG-43 formalism are consistent within the treatment volume with those of a MC simulation of a typical clinical scenario. Comparisons with reported measurements made with thermoluminescent dosimeters show differences up to 13% along the transverse axis of the radium needle. It has been estimated that the uncertainty associated to the absorbed dose within the treatment volume is 10%-15%, whereas uncertainty of absorbed dose to distant organs is roughly 20%-25%. CONCLUSIONS: The results provided here facilitate retrospective dosimetry studies of (226)Ra using modern treatment planning systems, which may be used to improve knowledge on long term radiation effects. It is surely important for the epidemiologic studies to be aware of the estimated uncertainty provided here before extracting their conclusions.

On the biological basis for competing macroscopic dose descriptors for kilovoltage dosimetry: cellular dosimetry for brachytherapy and diagnostic radiology.

The purpose of this work is to investigate how alternative macroscopic dose descriptors track absorbed dose to biologically relevant subcellular targets via Monte Carlo (MC) analysis of cellular models for a variety of cancerous and normal soft tissues for kilovoltage radiation. The relative mass distributions of water, light inorganic elements, and protein components of nuclear and cytoplasm compartments for various tissues are determined from a literature review. These data are used to develop representative cell models to demonstrate the range of mass elemental compositions of these subcellular structures encountered in the literature from which radiological quantities (energy absorption and attenuation coefficients; stopping powers) are computed. Using representative models of cell clusters, doses to subcellular targets are computed using MC simulation for photon sources of energies between 20 and 370 keV and are compared to bulk medium dose descriptors. It is found that cells contain significant and varying mass fractions of protein and inorganic elements, leading to variations in mass energy absorption coefficients for cytoplasm and nuclear media as large as 10% compared to water for sub-50 keV photons. Doses to subcellular structures vary by as much as 23% compared to doses to the corresponding average bulk medium or to small water cavities embedded in the bulk medium. Relationships between cellular target doses and doses to the bulk medium or to a small water cavity embedded in the bulk medium are sensitive to source energy and cell morphology, particularly for lower energy sources, e.g., low energy brachytherapy (<50 keV). Results suggest that cells in cancerous and normal soft tissues are generally not radiologically equivalent to either water or the corresponding average bulk tissue. For kilovoltage photon sources, neither dose to bulk medium nor dose to water quantitatively tracks energy imparted to biologically relevant subcellular targets for the range of cellular morphologies and tissues considered.