Rafael Guerrero

Dosimetry characterization of 32P intravascular brachytherapy source wires using Monte Carlo codes PENELOPE and GEANT4.

Monte Carlo calculations using the codes PENELOPE and GEANT4 have been performed to characterize the dosimetric parameters of the new 20 mm long catheter-based 32P beta source manufactured by the Guidant Corporation. The dose distribution along the transverse axis and the two-dimensional dose rate table have been calculated. Also, the dose rate at the reference point, the radial dose function, and the anisotropy function were evaluated according to the adapted TG-60 formalism for cylindrical sources. PENELOPE and GEANT4 codes were first verified against previous results corresponding to the old 27 mm Guidant 32P beta source. The dose rate at the reference point for the unsheathed 27 mm source in water was calculated to be 0.215 +/- 0.001 cGy s(-1) mCi(-1), for PENELOPE, and 0.2312 +/- 0.0008 cGy s(-1) mCi(-1), for GEANT4. For the unsheathed 20 mm source, these values were 0.2908 +/- 0.0009 cGy s(-1) mCi(-1) and 0.311 0.001 cGy s(-1) mCi(-1), respectively. Also, a comparison with the limited data available on this new source is shown. We found non-negligible differences between the results obtained with PENELOPE and GEANT4.

Multiple scattering of 13 and 20 MeV electrons by thin foils: a Monte Carlo study with GEANT, Geant4, and PENELOPE.

PURPOSE In this work, recent results from experiments and simulations (with EGSnrc) performed by Ross et al. [Med. Phys. 35, 4121-4131 (2008)] on electron scattering by foils of different materials and thicknesses are compared to those obtained using several Monte Carlo codes. METHODS Three codes have been used: GEANT (version 3.21), Geant4 (version 9.1, patch03), and PENELOPE (version 2006). In the case of PENELOPE, mixed and fully detailed simulations have been carried out. RESULTS Transverse dose distributions in air have been obtained in order to compare with measurements. The detailed PENELOPE simulations show excellent agreement with experiment. The calculations performed with GEANT and PENELOPE (mixed) agree with experiment within 3% except for the Be foil. In the case of Geant4, the distributions are 5% narrower compared to the experimental ones, though the agreement is very good for the Be foil. Transverse dose distribution in water obtained with PENELOPE (mixed) is 4% wider than those calculated by Ross et al. using EGSnrc and is 1% narrower than the transverse dose distributions in air, as considered in the experiment. CONCLUSIONS All the codes give a reasonable agreement (within 5%) with the experimental results for all the material and thicknesses studied.

Dosimetric characterization of the 60Co BEBIG Co0.A86 high dose rate brachytherapy source using PENELOPE

(60)Co sources are being used as an alternative to (192)Ir sources in high dose rate brachytherapy treatments. In a recent document from AAPM and ESTRO, a consensus dataset for the (60)Co BEBIG (model Co0.A86) high dose rate source was prepared by using results taken from different publications due to discrepancies observed among them. The aim of the present work is to provide a new calculation of the dosimetric characteristics of that (60)Co source according to the recommendations of the AAPM and ESTRO report. Radial dose function, anisotropy function, air-kerma strength, dose rate constant and absorbed dose rate in water have been calculated and compared to the results of previous works. Simulations using the two different geometries considered by other authors have been carried out and the effect of the cable density and length has been studied.

A method to relate StarTrack(®) measurements to R50 variations in clinical linacs.

The relation between the data recorded with any device for the daily checking of the behavior of a clinical linac and the reference magnitudes to be monitored may be unknown. An experimental method relating the energy stability of the electron beam measured with StarTrack(®) to the R50 beam quality index is proposed. The bending magnet current is varied producing a change in the exit energy window and, therefore, a modification of the R50 value. For different values of this current, the output data of StarTrack(®) and the R50, obtained from depth doses measured in a water phantom are determined. A linear fit between both sets of data allows the identification of the StarTrack(®) output that provides the best way to obtain the quality index R50, for each beam nominal energy. Using these fits, an historical datum series is used to analyze the method proposed in the daily quality control. The ouput data of the StarTrack(®) and the R50 values show a good linear correlation. It is possible to establish a methodology that allows the monitoring of R50 by direct use of the daily quality control data measured with StarTrack(®). A method to monitor R50 in the daily quality control using the StarTrack(®) device has been developed. The method may be applied to similar devices in which the statistical control variable does not show a linear behavior with R50.

EP-1305: Management of the interruptions in the conventional treatment of prostate cancer upon a database analysis

Purpose/Objective: The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments requires knowledge of the position of the isocentre to sub millimetre accuracy. The deviation between the radiation and mechanical isocentres must be less than 1 mm. The use of an add-on micromultileaf collimator (μMLC) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam centre of the cone-beam CT system was included in the study. Materials and Methods: The quantification of the gantry rotational performance was done using a pointer supported by an inhouse made rigid holder attached to the gantry head. The pointer positions were measured using a digital theodolite. The displacement of the isocentre due to an add-on μMLC of 50 kg was investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocentre. Additionally, the displacement of the radiation isocentre was measured using a Winston-Lutz phantom and the electronic portal image device (EPID) system. These measurements were based on eight MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. Results: The displacement of the mechanical isocentre caused by a 50 kg heavy μMLC was found to be (-0.01±0.06, -0.10±0.03, -0.26±0.05) mm in lateral, longitudinal and vertical direction, respectively. Similarly, the displacement of the radiation isocentre was found to be (0.00±0.03, -0.08±0.06, -0.32±0.02) mm. Good agreement was found between the displacement of the two isocentres. A displacement of the kV cone-beam CT beam centre due to the attached weight of 50 kg could not be detected. By comparing the CW and CCW data the presence of the effect of backlash appears. The effect of backlash was found to be < 0.14 mm and < 0.10 mm in the lateral and vertical direction, respectively. Conclusions: General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in an isocentre displacement downward of 0.26 to 0.32 mm. We recommend that the beam centre of the kV cone-beam CT image system should be matched to the isocentre related to the weight of the μMLC. Consequently, the imperfections in isocentre localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the sub millimetre regime are negligible.