X-ray spectroscopy applied to radiation shielding calculation in mammography.
The protective shielding design of a mammography facility requires the knowledge of the scattered radiation by the patient and image receptor components. The shape and intensity of secondary x-ray beams depend on the kVp applied to the x-ray tube, target/filter combination, primary x-ray field size, and scattering angle. Currently, shielding calculations for mammography facilities are performed based on scatter fraction data for Mo/Mo target/filter, even though modern mammography equipment is designed with different anode/filter combinations. In this work we present scatter fraction data evaluated based on the x-ray spectra produced by a Mo/Mo, Mo/Rh and W/Rh target/filter, for 25, 30 and 35 kV tube voltages and scattering angles between 30 and 165 degrees. Three mammography phantoms were irradiated and the scattered radiation was measured with a CdZnTe detector. The primary x-ray spectra were computed with a semiempirical model based on the air kerma and HVL measured with an ionization chamber. The results point out that the scatter fraction values are higher for W/Rh than for Mo/Mo and Mo/Rh, although the primary and scattered air kerma are lower for W/Rh than for Mo/Mo and Mo/Rh target/filter combinations. The scatter fractions computed in this work were applied in a shielding design calculation in order to evaluate shielding requirements for each of these target/filter combinations. Besides, shielding requirements have been evaluated converting the scattered air kerma from mGy/week to mSv/week adopting initially a conversion coefficient from air kerma to effective dose as 1 Sv/Gy and then a mean conversion coefficient specific for the x-ray beam considered. Results show that the thickest barrier should be provided for Mo/Mo target/filter combination. They also point out that the use of the conversion coefficient from air kerma to effective dose as 1 Sv/Gy is conservatively high in the mammography energy range and overestimate the barrier thickness.
Künzel
Roseli
R
Area de Ciências Naturais e Tecnológicas, Centro Universitário Franciscano, Santa Maria, Rio Grande do Sul, Brazil.
Levenhagen
Ronaldo Savarino
RS
Herdade
Silvio Bruni
SB
Terini
Ricardo Andrade
RA
Costa
Paulo Roberto
PR
D000465:Algorithms; D001822:Body Burden; D005374:Filtration / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008327:Mammography / Q000295:instrumentation / Q000379:methods*; D008954:Models, Biological; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control*; D012542:Scattering, Radiation; D013052:Spectrometry, X-Ray Emission / Q000379:methods*