CALCULATION OF DOSE CONVERSION FACTORS FOR POSTERIOR-ANTERIOR CHEST RADIOGRAPHY OF ADULTS WITH A RELATIVELY HIGH-ENERGY X-RAY-SPECTRUM

In a survey of X-ray units as applied for thorax examinations considerable variations were observed in entrance dose among different hospitals in the Leyden region. For the median exposure conditions, i.e. 125 kVp, heavy filtering and large focus-to-skin distance (177.5 cm), absorbed dose distributions have been derived using mathematical phantoms of a standard male or female adult. For relatively high-energy X-rays, back scatter factors were calculated by Monte Carlo simulation. In addition, conversion factors were obtained, relating organ doses to air kerma, free in air. Effective dose equivalent and effective dose were calculated according to ICRP-26 and ICRP-60 recommendations, respectively. The computational procedures were compared with results reported in the literature for a similar exposure configuration but using lower-energy X-rays. Causes of relative differences ranging from - 56% to + 34% were analysed. In addition to the photon, energy spectrum and filtering, the exposure geometry appears to be a very important parameter which can be optimized for the purpose of dose reduction.