Abstract
The Inverse Square Law (ISL) is a mathematical rule used to adjust the KERMA and exposure to different distances of focal spot having as reference a determined point in space. Taking into account the limitations of this rule and its application, we have as main objective to verify the applicability of ISL to determine exposure on radiodiagnostic area (maximum tensions between 30kVp and 150kVp). Experimental data was collected, deterministic calculation and simulation using Monte Carlo Method (Geant4 toolkit) were applied to conventional radiology and mammography. The experimental data was collected using a calibrated ionizing chamber TNT 12000 from Fluke. The conventional X-ray equipment used was a Multix Top of Siemens, with Tungsten track and total filtration equivalent to 2.5 mm of aluminum; and the mammographic equipment was a Mammomat Inspiration from Siemens, presenting the track-add filtration combinations of Molybdenum-Molybdenum, Molybdenum-Rhodium, Tungsten-Rhodium. Both equipments have the Quality Control testes in agreement to Brazilian regulations. Based on the results it is possible conclude that the ISL presents lower performance in correct measurements on mammography spectra, i.e. the associated error (differences) achieves a value up to 77.8% and it can cause significant impact on both areas depending on the spectra energy and distance to correct.
Highlights
The Inverse Square Law (ISL) is a mathematical rule used to adjust the KERMA and exposure to different distances of focal spot having as reference a determined point in space
Taking into account the limitations of this rule and its application, we have as main objective to verify the applicability of ISL to determine exposure on radiodiagnostic area
Based on the results it is possible conclude that the ISL presents lower performance in correct measurements on mammography spectra, i.e. the associated error achieves a value up to 77.8% and it can cause significant impact on both areas depending on the spectra energy and distance to correct
Summary
Na área de proteção radiológica aplicada ao uso das radiações ionizantes na radiologia diagnóstica, tem-se como base usar dados de KERMA ou de exposição em uma determinada posição para estimar a dose em órgãos internos de sujeitos submetidos a exames de radiodiagnóstico, conforme mencionado por Kramer et al (2004) e Zankl et al (2002). Por exemplo, a grandeza dose glandular (Dg) pode ser definida como a dose absorvida pelo tecido glandular da mama (Wu et al, 1994; Gingold, 1995; Ng, 2000), considerado radiossensível, podendo essa ser determinada através da medida da exposição na entrada da pele (XESE) e os valores tabelados de dose glandular normalizadas (DgN), conforme mostra a Equação 1. Com base nos modelos apresentados acima, outros fatores podem ser considerados como a exatidão para a determinação da dose absorvida ou efetiva (dependendo do modelo), a geometria do modelo antropomórfico simulado e sua similaridade com o sujeito de pesquisa e a exatidão da medida de entrada de dados (exposição ou KERMA). Com base nos modelos acima apresentados que mostram uma relação direta entre a dose absorvida ou efetiva de interesse radiológico, o KERMA ou exposição medidos servem de entrada de dados e se relacionam diretamente com estas grandezas dosimétricas. Erros sistemáticos ou tendências de dados referentes a aplicação de um modelo aproximado devem ser evitados e/ou corrigidos sempre que possível, pois impactarão diretamente no resultado da dose estimada por um método matemático
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