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| Author(s): |
Josilene Cerqueira Santos
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física (IF/SBI) |
| Defense date: | 2017-12-20 |
| Examining board members: |
Paulo Roberto Costa;
João Emilio Peixoto;
Teógenes Augusto da Silva;
Manfredo Harri Tabacniks;
Vito Roberto Vanin
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| Advisor: | Paulo Roberto Costa |
| Abstract | |
The mean glandular dose is the quantity used for dosimetry in mammography. This quantity has a strong dependence with some properties of the evaluated breast, such as its glandularity and compressed thickness. It also depends on the X-ray spectrum used for the mammographic image production, such as the target/filter combination and the tube voltage, which are related to the half value layer (HVL) of the beam. The X-ray beam characterization by means of the direct measurement of its spectrum is a complex procedure, and it is difficult to be implemented in clinical systems due to the architecture of the mammography equipment and the high photon fluence rates. These spectra provide a complete qualitative and quantitative information of the X-ray beam. The general objective of this work is to estimate mean glandular dose distributions in different depths of breast tissue-equivalent materials (bTEM) considering the X-ray spectra measured in clinical mammography devices. Radiographic techniques commonly applied for breast cancer screening were used. The behavior of the mean glandular dose normalized to the incident air kerma (DgNp), with parameters related to the breast (glandularity and compressed thickness) and to the mammographic spectra (HVL, tube voltage, target/filter combination), was evaluated. First, an experimental methodology was developed to measure X-ray spectra in clinical mammography devices. Then, the following methods for calculating the DgNp using these spectra were considered: Method I, which calculates the DgNp using the incident spectra; Method II, which uses incident and transmitted spectra by the bTEMs, and Method III, which uses incident and transmitted spectra to estimate the dose distributions in depth of the breast equivalent materials. Finally, thermoluminescent dosimeters (TLDs) were used as a comparative method to evaluate DgNp distributions. The methodology developed for measuring spectra proved to be efficient for the proper positioning and alignment of the detector and, consequently, for the measurement of direct X-ray spectra. The experimental incident spectra showed good agreement with spectra simulated in similar conditions. The results showed well-defined trends (either linear or exponential) of the distributions of these coefficients (DgNp) regarding the analyzed parameters. The DgNp values presented an exponential decay with the bTEM thickness and linear decrease with the glandularity. In addition, these coefficients increase linearly with the increase of the HVL and, consequently, with the increase of the effective energy. From the distributions of DgNp (obtained by Method III) it was possible to estimate the DgNp in the whole breast with a maximum difference of 5.2% from the values obtained using the Method II. The variation of the DgNp with the depth, obtained with TLDs, showed to be consistent with the results observed using the Method III. In conclusion, it is possible to evaluate the glandular dose in mammography examinations using X-ray spectra and the suggested methodology, with some adaptations, can be applied as an alternative procedure for dosimetry in mammography. (AU) | |
| FAPESP's process: | 13/07117-0 - EXPERIMENTAL DETERMINATION OF THE MEAN GLANDULAR DOSE DISTRIBUTION IN DIFFERENT MAMMOGRAPHIC BEAM QUALITIES |
| Grantee: | Josilene Cerqueira Santos |
| Support Opportunities: | Scholarships in Brazil - Doctorate |