P052] Influence of X-ray field position on doses in organs and tissues for chest radiography

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Purpose We studied how changing of the field position influences the absorbed doses in organs and tissues during chest radiography. Methods The ICRP voxel phantoms were adopted for modelling of chest radiography in posterior-anterior projection. The parts of phantoms that lie inside the beam and scattered radiation region located at the distance of 15 cm from the field edge were taken. The TASMIP model was used for the spectrum of X-ray tube. The following parameters were taken into account: anode voltage, filtration, voltage ripple, the focus-to-detector distance and the field size. Monte-Carlo simulation of X-ray transport within the chosen parts of phantoms was used [1] . At the standard position the center of X-ray field lied 40 cm from the top of the male phantom and 36.8 cm from the top of the female phantom. 5% to 20% shifts of the field center were considered. Results Changes in X-ray field position cause large differences in organ doses for organs that originally lied inside the beam or on the boundaries of the beam volume. After the field center shift by 20% up from the standard position the absorbed dose in adrenals of male decreases by a factor of 8.6. This effect is less than 20% for large organs and tissues such as lungs, muscle or skin. After the field center shift by 20% down the standard position the absorbed dose in female lungs decreases by 6%. Doses to organs of female are almost always larger than doses to the same organs of male. For standard field position the doses in female’s liver and lungs are 1.6 and 1.8 times higher than corresponding doses for male phantom. Conclusions Shift of X-ray field position up and down from the standard value should be considered for dose optimization during chest radiography. Differences between male and female bodies should be taken into account for more accurate patient dose assessment.