Development of a dosimetric reference system for laboratory computed tomography beams

One of the factors linked to ionization radiation is its use in medical practice (diagnostic exams, treatments, and radiological protection). Computed Tomography (CT) exams are responsible for the higher doses received by the patients. Therefore, it is important that the equipment is calibrated; the dosimetry of these beams is performed using a pencil-type ionization chamber with a sensitive volume length of 10 cm. For this kind of beam, there is no primary standard, then the aim of this work was to establish a reference system for the Calibration Laboratory of Instruments (LCI) of the Instituto de Pesquisas Energéticas e Nucleares (IPEN), São Paulo, making use of a homemade extrapolation chamber for X-ray computed tomography beams. The characterization of this chamber was performed, and the results obtained were within the international recommendations, except for the energy dependence as already expected; the air kerma rates were also determined for the utilized radiation beams; therefore, it was necessary to determine several correction factors, which can be obtained experimentally and by computer simulations using the MCNP5 code (Monte Carlo). In this work, two pencil-type ionization chambers with sensitive volume lengths of 10 cm and 30 cm were characterized, and the results obtained were within the international recommended limits. All the ionization chambers were also tested at the primary standard laboratory, National Physical Laboratory (NPL), United Kingdom, and the results were compared with the results obtained at the LCI, being in agreement. Finally, it was possible to establish a reference system using an extrapolation chamber for computed tomography laboratory beams. Using this system it was possible to calibrate the standard pencil-type ionization chamber of the laboratory, and the comparison with its calibration certificate results did not offer a relevant difference, once this chamber was calibrated eleven years ago. (author)