Rigour Within Uncertainty: An Unfinished Quest

1
Rigour Within Uncertainty An Unfinished Quest

• ICRP and High-LET Radiations
• Ralph H. Thomas, University of California
  (Retired)
• Thirteenth Annual J. Newell Stannard Lecture
  Series
• Sacramento, California
• 15 April 2005

2
Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

3
Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

4
Revision of ICRP recommendations

• Circa 2000 work began on the next set of
  fundamental ICRP recommendations intended to
  replace ICRP Publication 60
• 2003 ICRP Publication 92, Relative Biological
  Effectiveness, Quality Factor and Radiation
  Weighting Factor

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Revision of ICRP recommendations (continued)

• Autumn of 2004 Draft for Consultation - 2005
  Recommendations of the ICRP made available for
  comment on web site consultation period ended 31
  December 2004
• Current status The public consultation period
  is now completed . . . an overwhelming response
  with detailed and very constructive proposals . .
  . ICRP intends to consult . . . the foundation
  documents underpinning the Recommendations . . .
  Depending on the outcome of this review process,
  a second, shorter round of consultation may be
  held

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Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

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Importance of high-LET radiations

• High-LET exposures make up 10-20 of work force
  exposures (comparable with internal exposures)
• Air- and cabin-crew exposures to a mixed
  radiation field, including neutrons, are among
  the highest quasi-occupational exposures

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Importance of high-LET radiations (continued)

• The number of people exposed to high-LET
  radiations will almost certainly increase in the
  future
• The probability that exposure to high-LET
  radiations presents some risk at low doses is
  almost certainly greater than that for low-LET
  exposures

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Why high-energy and high-LET make a difference

• Low-energy photons Because only low-LET charged
  particles are generated in tissue, the ICRP
  paradigm (for both internal and external
  exposure) is to constrain the value of the
  important radiation-weighting factors (RBE, ?Q,
  H(10) and wR) to the value 1
• For neutrons, high-energy photons, and high-LET
  particles, both the absorbed dose and LET (dE/dX)
  distributions may vary greatly with location in
  the body values of average organ quality
  factors, ?QT, may show a correspondingly wide
  variation between tissues

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ICRP Publication 74 convincingly makes this point
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High-LET radiations need ICRPs focussed
attention

• Before 1985 external and internal modes of
  exposure were treated, almost distinctly and
  separately, by two committees of ICRP
• After 1985 Committee 2, Radiation Protection
  Standards, was charged with applying a unified
  approach to both exposure modes however,
  external pressures directed the early effort of
  the new committee largely towards internal
  exposure

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Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

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The Devil is in the Details

• The basis for our current quantities is some form
  of radiation-weighted absorbed dose but the past
  60 years shows that the devil is in the details
• circa 1940 absorbed dose
• 1948 RBE dose
• 1965 dose equivalent, H, Q (ICRP 4)
• 1973 MADE, Q(L)-L, ?Q (ICRP 21)
• 1977, 1980 effective dose equivalent, HE, wT
  (ICRP 26)
• 1980 dose equivalent indexes (ICRU 33)
• 1985 ambient dose equivalent, H(d) (ICRU 39
  42)
• 1991 effective dose, E, wR (ICRP 60)

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Analysis of mammalian cell data suggested a
radiobiological basis for a Q(L)-L model

• Experimental curves of RBE versus LET
• ? ? Mammalian tissues, various

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ICRP Publication 21 (1971) recommended that a
smooth Q(L)-L model needed to be established as
a common basis for dose equivalent calculation
and ICRP 60 recommended a revised model
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Caveat emptor!

• Neutron physics makes the extrapolation of
  neutron RBEs to humans uncertain (e.g. Dietze and
  Siebert Rad. Res. 140, 132-133 1994)

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Caveat emptor 2!

• ICRP Publication 92 gives the same message

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Effective dose equivalent versus effective dose

• At first sight HE and E appear to be identical
  and both defined by
• where
• T is the irradiated tissue or organ
• wT is the tissue-weighting factor for T
• HT is the equivalent dose for T
• However, different methods of radiation weighting
  produce significant differences, which have been
  discussed in the scientific literature, most
  recently in ICRP Publication 92

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Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

21
Values of wR, q and qE given in ICRP 92
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Draft 2005 recommended values of wR for neutrons

• Values of qE calculated for a human phantom and
  using the Q(L)-L relationship recommended ICRP
  Publication 60
• qE is the human body averaged mean quality factor
• Values of qE 2 for neutron energies below 1 keV
  were accepted and wR was defined to be equal to
  qE wR qE in this energy region

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Draft 2005 recommended values of wR for neutrons
(continued)

• The calculated value of qE 13 for neutron
  energies in the 1-MeV range was not accepted and
  wR was set at 21 (based on RBE values for small
  animals) wR ? qE and a fudge factor equation was
  adopted for the energy region between 1 keV and
  1MeV
• wR - 1.6qE -1
• No changes from the ICRP 60 values above 1 MeV
  were recommended
• The following empirical functions for wR are also
  given
• En ? 1 MeV
• En ? 1 MeV
• The recommended value of wR for high-energy
  protons is 2

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ICRP draft recommendations for 2005 are a great
disappointment!
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Logical miscues in the evaluation of wR for
neutrons in the draft recommendations

• The Q(L)-L relationship recommended in ICRP
  Publication 60, now used to calculate some values
  of wR was discredited by ICRP in Publication 60
  (paragraph A9)
• Values of qE 2 for neutron energies below 1 keV
  were accepted consequently and wR was defined to
  be equal to qE

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Logical miscues in the evaluation of wR for
neutrons in the draft recommendations (continued)

• The calculated value of qE 13 for neutrons
  energies in the 1 MeV was not accepted and wR was
  set at 21 (based on RBE values for small
  animals)
• If, after radiobiological review, the values wR
  below 1 keV are acceptable but at 1 MeV
  unacceptable then it must be concluded that the
  recommended Q(L)-L relationship and the value of
  wR at 1 MeV are inconsistent

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Topics to be discussed in this lecture

• Current review of ICRP recommendations
• External exposure and high-LET radiations (mainly
  neutrons)
• Modified absorbed dose quantities
• Problems with the draft recommendations for 2005
• Suggested solutions

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Goals for an ideal system of dosimetry for
radiological protection

• Universal applies to all radiations, whatever
  their energy
• Integrated independent of the origin of the
  radiation (either outside or inside the human
  body)
• Unambiguous standards are set in determinable
  quantities (no distinction between protection
  and operational quantities)
• Rigorous logically and mathematically coherent
  and consistent with mathematical logic and
  physical laws
• Stable avoiding frequent changes in names and
  symbols of dosimetric concepts

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Suggestions for a remedy

• Abandon the dual concept of protection (limiting)
  and operational quantities
• Define only protection quantities and leave it to
  the ingenuity of dosimetrists to deduce the means
  of measurement thus effectively abandoning the
  dual concept of protection and operational
  quantities
• Review the experimental and theoretical basis for
  the recommendations of RBE for humans, paying
  particular attention to the experimental
  irradiation conditions for small samples
  (animals)
• Redefine the function Q(L)-L on the basis of this
  review

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Suggestions for a remedy (continued)

• The form of the new Q(L)-L function should be
  similar to that of the current ICRP definition of
  ICRP 60 but mathematically more tractable,
  avoiding breaks and cusps
• Revert to the quantity of effective dose
  equivalent
• The new function Q(L)-L must generate values of
  qE for neutrons that are consistent with the
  needs of ICRP and the laws of physics. The 2005
  draft suggests that the constraints appear to be
• qE 2 for low energy neutrons (seems to be
  correct to the physicists and satisfies the
  radiobiologists and ICRP)
• qE 20 for 1 MeV neutrons (satisfies ICRP but
  perhaps the choice needs revision or a better
  justification than given hitherto by ICRP)

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Suggestions for a remedy (continued)

• At high energies (hundred MeV region) select the
  specific of qE for neutrons to be compatible with
  the selected value for high-energy protons. Most
  physicists agree that wR (qE) for high-energy
  protons and neutrons should approach the same
  value. This is a matter of energy deposition
  (i.e., physics) and should therefore be
  acceptable to the radiobiologists. A value of qE
  ? 2 would be about right in the mid-100 MeV
  region.

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Conclusions

• A major problem with both the ICRP 92 and the
  ICRP Draft for Consultation proposals is that
  they appear to fix the values of wR to neutrons
  to conform to a preconceived notion that wR for
  fission neutrons must take a value of about 20.
  The radiobiological arguments for this are not
  well explained by ICRP but rather are buttressed
  by administrative and legal concerns.
• Consequently there is a danger that science might
  be relegated to political disputes. ICRP would be
  better served by focussing on the relevant
  science that can be brought to bear and ensuring
  that it is the best that it possibly can be so
  that, in Kellerers happy phrase, rigour within
  uncertainty may be achieved.

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Conclusions (continued)

• Finally, there is an important cosmetic aspect
  that must be addressed. Some have suggested that
  It doesnt seem wise to give the impression that
  we are keeping two sets of books. Frankly, the
  approach of the draft for consultation has the
  appearance of cooking the books and my guess is
  that ICRP will draw immediate adverse criticism
  if it moves in this direction.
• Happily there is a rather simple remedy to these
  concerns in the unlikely event that ICRP can be
  persuaded to take it.