Challenges in the dose calculation from urine measurements in routine internal monitoring of 131I and other radionuclides.

Abstract

The measurement of 24-hour urine samples is one of the methods of routine monitoring of intakes of radionuclides. It is briefly mentioned in relevant documents by the International Commission on Radiological Protection that for ¹³¹I the strong decrease of the excretion within the first days after an intake makes the dose calculation from urine measurements unreliable when the time pattern of the intake is unknown. This can result in a major overestimation of the committed effective dose. For quantifying the influence of the time pattern of an intake on the dose, the results of the dose calculation for an acute intake at the midpoint of a monitoring interval (standard assumption) were compared with those for a chronic intake with varying daily activity. For ¹³¹I, aerosols type F, the standard assumption of an acute intake can lead to an overestimation of the calculated dose by a factor of 140 on average as compared to a chronic intake. Among other investigated radionuclides, the strongest overestimation was found for ¹⁴C, gas/vapour type F, when measured every 180 days (factor of 330), although this method complies with current criteria from the international standard ISO 20553. It is recommended that ISO 20553 is supplemented with a criterion that describes the reliability of a monitoring method under different time patterns of an intake additional to the existing criteria. This criterion should set an upper limit for the ratio of the dose calculations under the described assumptions.