Upcoming ICRP documents: Occupational Intake of Radionuclides

The ICRP has brought series (in three parts) of draft reports “Occupational Intake of Radionuclides” for consultation. The documents are replacing the Publication 30 series and Publication 68 to provide revised dose coefficients for occupational intakes of radionuclides (OIR) by inhalation and ingestion. The revision was necessary in view of the fact that 2007 Recommendations of the ICRP (ICRP Publication 103) introduced changes to the  radiation weighting factors used in the calculation of equivalent dose to organs and tissues and also changes to the tissue weighting factors used in the calculation of effective dose. The revision adopts reference anatomical computational phantoms (that is, models of the human body based on medical imaging data), in place of the composite mathematical models that have been used for all previous calculations of organ doses.

The ICRP Publication 103 also clarified the need for separate calculation of equivalent dose to males and females and sex-averaging in the calculation of effective dose (ICRP, 2007). In the revision of dose coefficients, the opportunity has also been taken to improve calculations by updating radionuclide decay data (ICRP, 2008) and implementing more sophisticated treatments of radiation transport (ICRP, 2010) using the ICRP reference anatomical phantoms of the human body (ICRP, 2009).

The revised dose coefficients have been calculated using the Publication 100 Human Alimentary Tract Model (HATM) and a revision of the Publication 66 Human Respiratory Tract Model (HRTM) which takes account of more recent data. In addition, information has been provided on absorption to blood following inhalation and ingestion of different chemical forms of elements and their radioisotopes. Revisions have been made to many models for the systemic bio-kinetics of radionuclides absorbed to blood, making them more physiologically realistic representations of uptake and retention in organs and tissues, and of excretion.

The reports in this series provide data for the interpretation of bioassay measurements as well as giving dose coefficients, replacing Publications 54 and 78. This report provides some guidance on monitoring programmes and data interpretation (source: www.ICRP.org).

ICRP Publication 118: ICRP Statement on Tissue Reactions / Early and Late Effects of Radiation in Normal Tissues and Organs – Threshold Doses for Tissue Reactions in a Radiation Protection Context, Ann. ICRP 41(1/2), 2012

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It provides updated estimates of ‘practical’ threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the hematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. 

Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40–50 years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. 

Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events.  For these two tissues, a threshold dose of 0.5 Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgment further (source:www.ICRP.org).