[Show abstract][Hide abstract] ABSTRACT: We applied a Monte Carlo simulation in Geant4 in order to calculate the absorbed fractions for monoenergetic electrons in the energy interval between 10 keV and 2 MeV, uniformly distributed in ellipsoids made from soft tissue. For each volume, we simulated a spherical shape, four oblate and four prolate ellipsoids, and one scalene shape. For each energy and for every geometrical configuration, an analytical relationship between the absorbed fraction and a 'generalized radius' was found, and the dependence of the fit parameters from electron energy is discussed and fitted by proper parametric functions. With the proposed formulation, the absorbed fraction for electrons in the 10-2000 keV energy range can be calculated for all volumes and for every ellipsoidal shape of practical interest. This method can be directly applied to evaluation of the absorbed fraction from the radionuclide emission of monoenergetic electrons, such as Auger or conversion electrons. The average deposited energy per disintegration in the case of extended beta spectra can be evaluated through integration. Two examples of application to a pure beta emitter such as (90)Y and to (131)I, whose emission include monoenergetic and beta electrons plus gamma photons, are presented. This approach represent a generalization of our previous studies, allowing a comprehensive treatment of absorbed fractions from electron and photon sources uniformly distributed in ellipsoidal volumes of any ellipticity and volume, in the whole range of practical interest for internal dosimetry in nuclear medicine applications, as well as in radiological protection estimations of doses from an internal contamination.
Physics in Medicine and Biology 01/2011; 56(2):357-65. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Patients candidate to radioiodine treatment of autonomous functioning thyroid nodule (AFTN) are characterized by a wide range of nodule volumes with different shapes. To optimize the treatment, pretherapeutic dosimetry should account also for the dependence of deposited energy on the nodule geometry.
We developed a Monte Carlo code in Geant4 to simulate the interaction of beta and gamma radiations emitted by Na-131I into ellipsoidal volumes of soft tissue homogeneously uptaking the radionuclide, surrounded by a simplified antropomorphic phantom. We simulated 9 volumes between 0.1 and 50 cm3, each one with 8 different ellipsoidal shapes. We considered the data of 10 patients affected by AFTN, whose nodule volumes were in the range 1-40 cm3, who underwent radioiodine therapy following the traditional dosimetric approach. The patients underwent ultrasonographic (US) study, in order to determine the nodule volume, and radioiodine thyroid uptake measurements between 3 and 168 hours after radioiodine tracer dose administration.
We found an analytical relationship between the average deposited energy and the ellipsoid's semiaxes and we included it in the formula for the calculation of activity to be administered, A0. For the 10 patients studied, A0 calculated with our approach ranges from +9% to -2% with respect to the one calculated with the traditional formula.
The proposed model, accounting for the dependence of beta and gamma absorbed fractions from nodule volume and shape, can lead to a more accurate estimation of A0 during AFTN therapy. Since the measurement of nodule axes is routinely obtained from pretherapeutic US, our approach can be introduced in the clinical practice without changing the diagnostic pre-therapeutic protocol.
The quarterly journal of nuclear medicine and molecular imaging: official publication of the Italian Association of Nuclear Medicine (AIMN) [and] the International Association of Radiopharmacology (IAR), [and] Section of the Society of... 01/2011; 55(5):560-6. · 1.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The objective of this study is to develop a method to calculate the relative dose increase when a computerized tomography scan (CT) is carried out after administration of iodinated contrast medium, with respect to the same CT scan in absence of contrast medium.
A Monte Carlo simulation in GEANT4 of anthropomorphic neck and abdomen phantoms exposed to a simplified model of CT scanner was set up in order to calculate the increase of dose to thyroid, liver, spleen, kidneys, and pancreas as a function of the quantity of iodine accumulated; a series of experimental measurements of Hounsfield unit (HU) increment for known concentrations of iodinated contrast medium was carried out on a Siemens Sensation 16 CT scanner in order to obtain a relationship between the increment in HU and the relative dose increase in the organs studied. The authors applied such a method to calculate the average dose increase in three patients who underwent standard CT protocols consisting of one native scan in absence of contrast, followed by a contrast-enhanced scan in venous phase.
The authors validated their GEANT4 Monte Carlo simulation by comparing the resulting dose increases for iodine solutions in water with the ones presented in literature and with their experimental data obtained through a Roentgen therapy unit. The relative dose increases as a function of the iodine mass fraction accumulated and as a function of the Hounsfield unit increment between the contrast-enhanced scan and the native scan are presented. The data shown for the three patients exhibit an average relative dose increase between 22% for liver and 74% for kidneys; also, spleen (34%), pancreas (28%), and thyroid (48%) show a remarkable average increase.
The method developed allows a simple evaluation of the dose increase when iodinated contrast medium is used in CT scans, basing on the increment in Hounsfield units observed on the patients' organs. Since many clinical protocols employ multiple scans at different circulatory phases after administration of contrast medium, such a method can be useful to evaluate the total dose to the patient, also in view of potential clinical protocol optimizations.
Medical Physics 08/2010; 37(8):4249-56. · 3.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied through Monte Carlo simulation in Geant4 the absorbed fractions for photons, characterized by energies ranging from 10 keV to 1000 keV, which can be emitted by gamma radionuclides uniformly distributed in ellipsoidal volumes of soft tissue. The same analytical relationship between absorbed fraction and the 'generalized radius' as introduced in a previous paper was found, and the dependence of its parameters rho(0) and s on photon energy is discussed and fitted by suitably chosen parametric functions. As a consequence, the absorbed fraction for photons in the 10-1000 keV energy range can be calculated for all volumes and for every ellipsoidal shape of practical interest. Such results can be a useful complement for the dosimetry of beta- and gamma-emitting radionuclides during internal radiotherapy or gamma emitters employed in diagnostic nuclear medicine.
Physics in Medicine and Biology 09/2009; 54(20):N479-87. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed a Monte Carlo simulation in Geant4 to calculate the absorbed fractions for electrons emitted by (199)Au, (177)Lu, (131)I, (153)Sm, (186)Re and (90)Y, characterized by average energies ranging from 86 keV to 949 keV, uniformly distributed in ellipsoidal volumes of soft tissue. Code validation results with respect to reference data for doses, ranges and absorbed fractions in spheres are presented. An analytical relationship between the absorbed fraction and a 'generalized radius' is introduced in analogy with the transfer function of a first-order high-pass filter, and the dependence of its parameters rho(0) and s from the average electron energy and range is discussed. A generalization for the estimation of absorbed fractions for other radionuclides is also proposed. Such results can be useful to improve accuracy and easiness of calculation in dosimetry during internal radiotherapy.
Physics in Medicine and Biology 08/2009; 54(13):4171-80. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed a Monte Carlo simulation in Geant4 to compare the attenuation properties and the bremsstrahlung radiation yield of different types of plastic materials employed as shields for beta- radioactive sources. Code validation results against Sandia and NIST data are presented. For polypropylene (C3H6), polystyrene (C2H3), polyamide nylon-6 (C6H11ON), poly-methyl methacrylate (C5H8O2), polycarbonate (C16H6O3), polyethylene terephthalate (C10H8O4), polyvinyl chloride (C2H3Cl) and polytetrafluoroethylene (C2F4) we evaluated the mean and maximum ranges for electrons originating from 90Sr and 90Y, as well as the number and spectrum of the bremsstrahlung x-rays produced. Significant differences appear between the various materials, and the choice of the best one also depends on the physical properties requested for each specific application.
Journal of Radiological Protection 07/2009; 29(2):239-50. · 1.32 Impact Factor