CALCULATION OF COMPUTED-TOMOGRAPHY DOSE INDEX TO EFFECTIVE DOSE CONVERSION FACTORS BASED ON MEASUREMENT OF THE DOSE PROFILE ALONG THE FAN-SHAPED BEAM

The variation in computed tomography dose index (CTDI) to effective do se conversion factors between different types of CT scanner is large ( i.e. a factor of about 2 due to differences in beam shaping filters). Consequently, scanner specific conversion factors have to be applied. For some types of scanner, however, detailed information on the constr uction of beam shaping filters is not provided by the manufacturers. I t is of interest to investigate the use of measured dose profiles for the calculation of conversion factors. Based upon measured dose profil es, two appropriate photon spectra selected on the basis of measured h alf value layers, gender specific adult phantoms Adam and Eva, and the Monte Carlo neutron and photon radiation transport code (MCNP), organ and effective dose conversion factors are calculated. To validate the method, a comparison is made between results for measured and calcula ted beam profiles for a Philips Tomoscan 350. The results in terms of effective dose per slice per unit of CTDI are compared with published data. Relative difference in conversion factors per slice averaged ove r all slices used for the calculations is 13+/-4% between the two spec tra, 10.2+/-0.2% between measured and calculated beam profiles and 50/-191% between the phantoms of different gender. The relative differen ce between the averaged results for the Adam and Eva phantoms and publ ished results for a hermaphrodite phantom is on average equal to or le ss than 15+/-13%, depending on the spectrum and beam profile used, alt hough larger differences can occur for specific slices. It is conclude d that CTDI to effective dose conversion factors can be derived on the basis of measured beam profiles.