Future of Diagnostic Medicine

It is estimated that 10 million people receive diagnostic, therapeutic or interventional medical radiation procedures every day. The number of occupationally exposed workers is much higher in medicine than in any other professional field.

Now for the first time in history, several countries are experiencing population doses from medical uses of radiation that exceed those from natural background radiation and exposure from other artificial sources. Thus, there is a strong need to protect patients and medical staff from accidental and unnecessary exposure.

Medical experts from about 90 countries and 17 international organizations are gathering during 3-7 December, 2012 in Bonn, Germany, at the IAEA's “International Conference on Radiation Protection in Medicine - Setting the Scene for the Next Decade” to discuss the pressing issue of overexposure to ionizing radiation, the threat posed to patients and health workers, and ways to handle and reverse the problem. The conference is intended to come out with a detailed plan of action for the reduction of medical radiation exposure (source: www.iaea.org).

Dose limit for the eyes reduced

The prevailing belief in radiation protection fraternity has been that human radiation-related cataract occurs only after relatively high doses and the ICRP guidelines on minimal doses for cataract induction in humans are given in the table for single exposure and protracted exposure scenarios.

Table: ICRP Guidelines on Minimal Lens Doses for Cataract Induction

End point	Brief exposure (Sv)	Protracted exposure (Sv)	Annual dose (Sv)
Detectable opacities	0.5 to 2	5	>0.1
Visual impairment	5	>8	>0.15

Epidemiological studies among Chernobyl clean-up workers, Atom - bomb survivors in Japan, astronauts, residents of contaminated buildings, radiological technicians and recent surveys of staff in interventional rooms indicate that there is an increased incidence of lens opacities at doses below 1 Gy.

The IAEA studies on radiation induced cataract among cardiologists and support staff in cardiac catheterization laboratories, published in “Radiation Research” received wider attention since it pointed towards possibility of opacities in the lens of the eyes below the currently specified threshold by International Commission of Radiological Protection (ICRP). However, there are issues such as difficulty in accurate dose estimation in eyes of medical staff as hardly any data is available that can be used to correlate with lens opacities. Only rough estimations based on work load and typical factors used in the procedures performed by staff could be made. In contrast, there is much better dosimetry in A-Bomb survivors and much longer follow up period.

Based on the overwhelming data, the ICRP released a statement in 2011 recommending a change in the threshold dose for the eye lens and dose limits for eye for occupationally exposed persons.

According to this statement, the threshold in absorbed dose for the lens of the eye is now considered to be 0.5 Gy. Further, for occupational exposure in planned exposure situations the Commission now recommends an equivalent dose limit for the lens of the eye of 20 mSv in a year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv.

The Commission continues to recommend that optimisation of protection be applied in all exposure situations and for all categories of exposure. With the recent evidence, the Commission further emphasises that protection should be optimised not only for whole body exposures, but also for exposures to specific tissues, particularly the lens of the eye, and to the heart and the cerebrovascular system.

The implementation of the limit amongst the occupational workers in nuclear and radiological facilities is operationally difficult in view of the fact that there is hardly any reliable and recorded dosimetric data available. To begin with, however, some rough assessment of the exposure to eyes can still be made using whole-body exposure data.  

New ICRP Publication: ICRP-119, 2012

Compendium of Dose Coefficients based on ICRP Publication 60, ICRP Publication 119 Ann ICRP 41(s), 2012, K. Eckerman, J. Harrison, H-G. Menzel, C.H. Clement

This report is a compilation of dose coefficients for intakes of radionuclides by workers and members of the public, and conversion coefficients for use in occupational radiological protection against external radiation from Publications 68, 72, and 74. It serves as a comprehensive reference for dose coefficients based on the primary radiation protection guidance given in the Publication 60 recommendations. The coefficients tabulated in this publication will be superseded in due course by values based on the Publication 103 recommendations. This publication is available for free download at www.ICRP.org.

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).

Latest ICRP Publication: ICRP Publication - 115, Lung Cancer risk from radon exposures

Lung Cancer Risk from Radon and Progeny and Statement on Radon. ICRP Publication 115, Ann. ICRP 40(1), ICRP Publication 115, Ann. ICRP 40(1), 2010.

The document reviews recent epidemiological studies of the association between lung cancer and exposure to radon and its decay products. Particular emphasis is given to pooled case-control studies of residential exposures and to cohorts of underground miners exposed to relatively low levels of radon. The residential and miner epidemiological studies provide consistent estimates of lung cancer risk with statistically significant associations observed at average annual concentrations of about 200 Bq m-3 and cumulative occupational levels of about 50 WLM, respectively.

Based on recent results from combined analyses of epidemiological studies of miners, a lifetime excess absolute risk of 5 × 10-4 per WLM (14 × 10-5 per mJ h m-3) should now be used as the nominal probability coefficient for radon and radon progeny induced lung cancer, replacing the previous ICRP Publication 65 value of 2.8 × 10-4 per WLM (8 × 10-5 per mJ h m-3).

Unlike the recommendations of ICRP Publication 65, it is now concluded that radon and its progeny should be treated in the same way as other radionuclides within the ICRP system of protection; that is, doses from radon and radon progeny should be calculated using ICRP biokinetic and dosimetric models.

ICRP will provide dose coefficients per unit exposure to radon and radon progeny for different reference conditions of domestic and occupational exposure, with specified equilibrium factors and aerosol characteristics (source:www.ICRP.org).