Risk of radiation exposure – ICRP Recommendations

The International Commission of Radiological Protection (ICRP) provides recommendations on radiation protection standards. Since the ICRP-60, there has been significant progress in understanding the genetic risk associated with the induction of mutations in germ cells. The clearer understanding is that the genetic risk is much lower than the earlier estimates. In ICRP-103 (2007), the risk estimates considered only two generations rather than the all generations (theoretical equilibrium) considered in ICRP-60. The overall contribution to the detriment (total harm) from genetic effects works out to be 3-4% as compared to 18% considered in ICRP-60. Hence, the tissue weighting factor was reduced from 0.2 (ICRP-60) to 0.08 (ICRP-103).

The radiation detriment (overall harm to stochastic effects) was assessed (ICRP-60) taking in to account the cancer incidence, mortality, length of life lost if cancer occurs and the morbidity and quality of life lost due to suffering in incidences of non-fatal cancers. The ICRP-103 considered the detriment based on lethality and life impairment weighted on cancer incidence data.

The detriment values were assessed for both genders and also for working (18 to 64 years) the whole population (0 to 85years). The detriment adjusted nominal risk coefficients for cancer and hereditary effects combined have been estimated to be 5.7 and 4.2% per Sv for the whole population and working population respectively.

Based on the new risk assessments, the Tissue Weighting Factors (WT) for specific organ/tissue are the fractional harm associated with the stochastic effect (gender averaged relative detriment) were also reviewed by the ICRP in ICRP-103. The important changes in ICRP-103 are the upgrading the value of risk for breast from 0.05 (ICRP-60) to 0.12 and for gonads, the WT was reduced from 0.12 (ICRP-60) to 0.08.

Conclusion

In-spite of the some changes in the nominal risk coefficients and in WT values, the total detriment remains close to 5% per Sv. In view of this, the dose limits for occupational and public exposures remain same at an average of 20mSv/y and 1mSv/y respectively as in ICRP-60 (1991).

ICRP - Statement on Tissue Reactions

As per the ICRP communication, it has approved “Statement on Tissue Reactions” in April 21, 2011. The effects of radiation which was called as deterministic effects previously are now referred as Tissue Reactions. Based on the data various aspects of the effects with very late manifestation such as cataract of the eyes, the threshold in absorbed dose is now considered as 0.5Gy.

Similarly, for occupational exposure in planned exposure situations the Commission now recommends an equivalent dose limit for the lens of the eye of 20mSv in a year, averaged over defined periods of 5 years, with no single year exceeding 50mSv.

The Commission continues to recommend that optimization of protection be applied in all exposure situations and for all categories of exposure, including patient’s exposure during some complex interventional procedures, With the recent evidence, the Commission further emphasizes that protection should be optimized 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.