No abstract

Radiotherapy is an established method of cancer treatment and is widely applied to control this disease. Medical Linear Accelerators (LINAC) are widely used in modern radiotherapy due to their flexibility and high therapeutic reliability. Traditionally, national and international codes of conduct provide guidelines on dosimetry with a reference field of 10x10 cm², considering that for larger fields, dosimetric parameters are well defined and can be accurately measured. In this work, a LINAC X-ray beam of 6 MV was used to irradiate a solid water phantom, using fields of 10x10 and 5x5 cm². X-ray beam was generated in a 6MV linear accelerator (linac) model Synergy Platform from the manufacturer Elektra. Radiochromic film sheets were used to record dose profiles inside the solid water phantom. For irradiation of the phantom loaded with the film, it was positioned twice 1.0 m away from the focus of the X-ray beam for both field sizes. In the first irradiation, the phantom was exposed laterally to obtain the longitudinal dose variation profile and in the second irradiation, the phantom was irradiated frontally. The longitudinal profile of absorbed dose obtained showed the maximum dose value at 1.30 cm depth for both fields. Axial dose profiles were recorded at 1 cm depth, and showed a plateau in the axis Y for both fields. The plateau for the field of 10x10 cm² in the axis X presented a depression in the central area and that did not happen in the 5x5 cm² field.

Computed Tomography (CT) is one of the most used exams for radiological diagnosis in medicine. The increase of CT exams is a global concern due to high radiation doses. It is important to improve protocols to seek lower doses while maintaining the diagnostic image quality. The use of phantoms allows the testing of different acquisition protocols. For this, the phantom must present an absorption characteristic of the X-ray beam like the represented patient. In this study, it was used a standard head phantom made of polymethylmethacrylate (PMMA). The phantom is a cylinder with 16 cm in diameter and 15 cm in length. Different acquisition protocols were performed on a Toshiba CT scanner, Aquilion model with 64 channels. The central slice of the phantom was irradiated successively, and using a pencil ionization chamber, measurements of CT air kerma index in PMMA (Ck,PMMA,100) were performed. From these results, the CT Dose Index values weighted and volumetric (CTDIw, CTDIvol) were obtained for 10 cm scans of the central area of the head phantom, in helical mode. The scans were performed using different voltage values (80, 100 and 120 kV) and charge (mA.s). Dose values of CTDIw varied from 2.55 mGy to 7.76 mGy. The absorbed dose (CTDIvol) was 23.67 mGy with the routine protocol used in the radiological service. The pitch of the scans were adjusted and acquisition protocols were tested for each voltage and varying the charge (mA.s). Optimized protocols were approved when the noise in the central slice was less than 1%. The better protocol happens with the voltage of 120 kV and the absorbed dose was reduced in 68.31%. The other parameters tube time, thickness beam and image reconstruction were the same of the routine protocol.

Images of Positron Emission Tomography (PET) associated with Computed Tomography (CT) have important diagnostic applications, mainly for oncology. These compound tomographic devices allow the overlapping of functional images obtained from the administration of radiopharmaceuticals and anatomical images generated by X-ray beam attenuation. This work evaluated the impact of reducing the effective dose by reducing the activity injected into the patient using the ICRP 106 biokinetic model. The activity to be injected may vary according to the patient mass and the detector sensitivity. In this work was used the fixed mass of Alderson phantoms, as a standard adult, this mass is 73.5 kg for the male, and 50 kg for the female. Different values of activity to be injected were simulated, from 0.07 mCi to 0.15 mCi per kg, and with 10 mCi fixed, protocol used in some services. Thus, for the acquisition of PET scans, any reduction of the administered activity implies a proportional reduction of the effective dose in patient. The effective dose may vary up to 114% altering the injected activity between 0.07 and 0.15 mCi. The fixed value of 10 mCi is between these variations.