RESUMO
BACKGROUND: The limited bibliographic existence of research works on the use of Monte Carlo simulation to determine the energy spectra of electron beams compared to the information available regarding photon beams is a scientific task that should be resolved. AIMS: In this work, Monte Carlo simulation was performed through the PENELOPE code of the Sinergy Elekta accelerator head to obtain the spectrum of a 6 MeV electron beam and its characteristic dosimetric parameters. MATERIALS AND METHODS: The central-axis energy spectrum and the percentage depth dose curve of a 6 MeV electron beam of an Elekta Synergy linear accelerator were obtained by using Monte Carlo PENELOPE code v2014. For this, the linear accelerator head geometry, electron applicators, and water phantom were simplified. Subsequently, the interaction process between the electron beam and head components was simulated in a time of 86.4x104 s. RESULTS: From this simulation, the energy spectrum at the linear accelerator exit window and the surface of the phantom was obtained, as well as the associated percentage depth dose curves. The validation of the Monte Carlo simulation was performed by comparing the simulated and the measured percentage depth dose curves via the gamma index criterion. Measured percentage depth- dose was determined by using a Markus electron ionization chamber, type T23343. Characteristic parameters of the beam related with the PDD curves such as the maximum dose depth (R100), 90% dose depth (R90), 90% dose depth or therapeutic range (R85), half dose depth (R50), practical range (Rp), maximum range (Rmax), surface dose (Ds), normalized dose gradient (G0) and photon contamination dose (Dx) were determined. Parameters related with the energy spectrum, namely, the most probable energy of electrons at the surface (Ep,0) and electron average energy (E- 0) were also determined. CONCLUSION: It was demonstrated that PENELOPE is an attractive and accurate tool for the obtaining of dosimetric parameters of a medical linear accelerator since it can reliably reproduce important clinical data such as the energy spectrum, depth dose, and dose profile.
RESUMO
The performance of an L-alanine dosimeter with millimeter dimensions was evaluated for dosimetry in small radiation fields. Relative output factor (ROF) measurements were made for 0.5 x 0.5, 1 x 1, 3 x 3, 5 x 5, 10 x 10 cm(2) square fields and for 5-, 10-, 20-, 40-mm-diam circular fields. In beam profile (BP) measurements, only 1 x 1, 3 x 3, 5 x 5 cm2 square fields and 10-, 20-, 40-mm-diam circular fields were used. For square and circular field irradiations, Varian/Clinac 2100, and a Siemens/Mevatron 6 MV linear accelerators were used, respectively. For a batch of 800 L-alanine minidosimeters (miniALAs) the average mass was 4.3+/-0.5 (1 sigma) mg, the diameter was 1.22+/-0.07 (1 sigma) mm, and the length was 3.5+/-0.2 (l sigma) mm. A K-Band (24 GHz) electron paramagnetic resonance (EPR) spectrometer was used for recording the spectrum of irradiated and nonirradiated miniALAs. To evaluate the performance of the miniALAs, their ROF and BP results were compared with those of other types of detectors, such as an ionization chamber (PTW 0.125 cc), a miniTLD (LiF: Mg,Cu,P), and Kodak/X-Omat V radiographic film. Compared to other dosimeters, the ROF results for miniALA show differences of up to 3% for the smallest fields and 7% for the largest ones. These differences were within the miniALA experimental uncertainty (-5-6% at 1 sigma). For BP measurements, the maximum penumbra width difference observed between miniALA and film (10%-90% width) was less than 1 mm for square fields and within 1-2 mm for circular fields. These penumbra width results indicate that the spatial resolution of the miniALA is comparable to that of radiographic film and its dimensions are adequate for the field sizes used in this experiment. The K-Band EPR spectrometer provided adequate sensitivity for assessment of miniALAs with doses of the order of tens of Grays, making this dosimetry system (K-Band/miniALA) a potential candidate for use in radiosurgery dosimetry.
Assuntos
Alanina/química , Espectroscopia de Ressonância de Spin Eletrônica/métodos , Dosimetria Fotográfica/instrumentação , Dosimetria Fotográfica/métodos , Imagens de Fantasmas , Radiometria/instrumentação , Radiometria/métodosRESUMO
Source positioning close to the tumour in high dose rate (HDR) brachytherapy is not instantaneous. An increment of dose will be delivered during the movement of the source in the trajectory to its static position. This increment is the transit dose, often not taken into account in brachytherapeutic treatment planning. The transit dose depends on the prescribed dose, number of treatment fractions, velocity and activity of the source. Combining all these factors, the transit dose can be 5% higher than the prescribed absorbed dose value (Sang-Hyun and Muller-Runkel, 1994 Phys. Med. Biol. 39 1181-8, Nath et al 1995 Med. Phys. 22 209-34). However, it cannot exceed this percentage (Nath et al 1995). In this work, we use the alanine-EPR (electron paramagnetic resonance) dosimetric system using analysis of the first derivative of the signal. The transit dose was evaluated for an HDR system and is consistent with that already presented for TLD dosimeters (Bastin et al 1993 Int. J. Radiat. Oncol. Biol. Phys. 26 695-702). Also using the same dosimetric system, the radial dose function, used to evaluate the geometric dose degradation around the source, was determined and its behaviour agrees better with those obtained by Monte Carlo simulations (Nath et al 1995, Williamson and Nath 1991 Med. Phys. 18 434-48, Ballester et al 1997 Med. Phys. 24 1221-8, Ballester et al 2001 Phys. Med. Biol. 46 N79-90) than with TLD measurements (Nath et al 1990 Med. Phys. 17 1032-40).