COMPARISON OF 2 METHODS FOR ASSESSING PATIENT DOSE FROM COMPUTED-TOMOGRAPHY

Radiation exposure of the patient during routine computed tomography ( CT) examinations is known to be relatively high. In this study organ d oses were determined using two methods and these served as a basis to calculate the effective dose. Thermoluminescence dosemeters (TLDs) wer e used to measure organ doses in an anthropomorphic Rando Alderson pha ntom. In addition organ doses were obtained from measurement of the co mputed tomography dose index (CTDI) and the application of published o rgan dose conversion factors. Effective dose values obtained with the Rando phantom for CT head examinations are about 1-2 mSv. For CT exami nations of thorax and abdomen the estimation of effective doses with t he Rando phantom yielded values of 18 and 24 mSv respectively. Effecti ve doses determined from CTDI values were similar for CT head examinat ions (1-2 mSv) but were smaller for the CT thorax scan (11-15 mSv) and the CT abdomen scan (15-20 mSv). In this study effective dose values are relatively high compared with the results of other investigators w ho indicate effective doses and effective dose equivalents of 7-9 mSv for CT of the thorax and of 4-16 mSv for CT of the abdomen. Discrepanc ies between our results and those from other studies could be attribut ed to differences in the selected CT protocols and to differences in t he phantoms employed. Measurements in an anthropomorphic phantom were laborious and time-consuming. Assessment of organ doses from CTDI valu es and organ dose conversion factors will therefore be the preferable method for future dose intercomparisons at different locations in The Netherlands. It should be realized, however, that this method tends to yield up to 40% lower effective dose values compared with the assessm ent of effective dose with a Rando phantom.