[Equivalent dose and efficient dose in risk assessment of stochastic effects in diagnostic radiology]

Purpose. The equivalent dose absorbed during a radiological examination and the resulting effective dose correlate with the probability of late stochastic effects, of which the ICRP 60 has determined the nominal coefficients normalized to 1 Sv. We have normalized the risk coefficients to 1 mSv for better simulation of working conditions. We propose a simple method for estimating the radiological stochastic risk by correctly using both the equivalent dose and the effective dose concepts.

The effective dose depends on the irradiated body volume; thus, we calculated the stochastic risk in three hypothetical radiological examinations. The equivalent dose in the volume irradiated by the main beam was assumed to be 10 mSv and homogeneous; the equivalent dose in adjacent volumes was assumed to decrease by two different dose gradients. In our models, the sum of the equivalent dose absorbed by various tissues multiplied by the different weight-tissue values gives three effective dose values. Finally, the stochastic risk is estimated by multiplying the effective dose values by the nominal risk coefficient determined by ICRP 60.

The effective dose is highest when the volume irradiated by the main beam is largest and the dose gradient in adjacent volumes is slowest. With a slow gradient, the effective dose is 10 mSv for total body examinations, 6.25 mSv for abdominopelvic examinations and 1.4 mSv for head and neck examinations. With a fast gradient, the effective dose is 10 mSv, 5.99 mSv and 1.10 mSv, respectively. The lethal tumor probability over the entire life-span is 65/10(6) for head and neck examinations, 300/10(6) for abdominopelvic examinations and 500/10(6) for total body examinations.

The risk of stochastic effects in diagnostic radiology is low, inasmuch as it is projected over the entire life-span of the subject. Nevertheless, it must not be overlooked. Our calculation method aims to explain the correct use of equivalent dose and effective dose concepts, particularly relative to that great majority of radiological examinations which involve limited body volumes. In these cases it is important to estimate correctly the dose gradient from the examined volume towards the adjacent volumes. Close collaboration between physicist and radiologist is therefore essential, as their respective specialist tasks must necessarily be integrated.