[ RadSafe ] Epidemiological Magic: Dose Lagging and Threshold Disappearance

[Scott, Bobby]

Epidemiological Magic: Dose Lagging and Threshold Disappearance

 

Paper Discussed:  E. Cardis et al. Risk of cancer after low doses of
ionizing radiation-retrospective cohort study in 15 countries. BMJ
Online First bmj.com (Accepted 23 May 2005)

 

The above paper reports results of a multinational retrospective cohort
study of cancer mortality among 407,391 nuclear industry workers
individually monitored for external radiation with a total follow-up of
5.2 million person years.  Data for nuclear industry workers in 15
countries were used.  Dose estimates to the colon and bone marrow were
used for analysis of mortality from all cancers excluding leukemia and
from leukemia, respectively, for persons exposed mainly to high-energy
photon radiation.  Organ doses estimates were derived by dividing
recorded doses by a secret (i.e., unspecified) bias factor.  A portion
of the doses obtained were then thrown away (called dose lagging).  For
leukemia, 2 years of dose accumulation were thrown away.  For other
cancers, 10 years of dose accumulation were thrown away.  Note that for
a person working for 10 years or less, and for cancers other than
leukemia, all of there dose was apparently thrown away.  With the
indicated dose lagging, the dose thresholds for excess cancers that
would have been expected after chronic, low-dose-rate, low-LET
irradiation [Tubiana et al. 2005, see below] disappeared and LNT
magically appeared again as the champion.   Additional magic was
performed via dose lagging:  low-dose-rate, low-LET irradiation was
found to be more carcinogenic than essentially instantaneous exposure
from atomic bombs in Japan. Dividing excess relative risk (RR) by any
corresponding small dose estimate (after throwing away a large part of
the actual dose) will of course give you a high value for the risk
coefficient (excess RR per Sv).

 

Reported a priori absorbed dose estimates ranged from < 5 mSv (mGy) to >
500 mSv (mGy). However, no information was provided on RR as a function
of the dose groups used.  Such information is useful relative to
evaluating shapes of dose-response relationships (e.g. significant
departure from LNT).

    

Based on the faulty study design (related to methods of analysis), the
researchers concluded the following: "These estimates, from the largest
study of nuclear workers ever conducted, are higher than, but
statistically compatible with, the risk estimates used for current
radiation protection standards.  The results suggest that there is a
small excess risk of cancer, even at the low doses and dose rates
typically received by nuclear workers in this study."  These results
cannot be trusted!

 

Cited Reference: Tubiana M. et al.  Dose-effect Relationships and
Estimation of the Carcinogenic Effects of Low Doses of Ionizing
Radiation.  Academy of Sciences - National Academy of  Medicine Report,
Paris, France, March 30, 2005.

 

Radsafe Digest Readers:  Any comments related to dose lagging and
dividing doses by a bias factor in epidemiological studies?

 

Bobby R. Scott

Lovelace Respiratory Research Institute

Albuquerque, NM USA