Rigour Within Uncertainty: An Unfinished Quest

Rigour Within Uncertainty: An Unfinished Quest ICRPand High-LET Radiations Ralph H. Thomas, University of California (Retired) Thirteenth Annual J. Newell Stannard Lecture Series Sacramento, California 15 April 2005

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

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

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”

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

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

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

ICRP Publication 74 convincingly makes this point

High-LET radiations need ICRP’s 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

“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)

Analysis of mammalian cell data suggested a radiobiological basis for a Q(L)-L model Experimental curves of RBE versus LET   Mammalian tissues, various

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

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

Caveat emptor 2! ICRP Publication 92 gives the same message

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

Values of wR, q* and qE given in ICRP 92

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

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

ICRP draft recommendations for 2005 are a great disappointment!

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

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

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

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

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)

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.

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 Kellerer’s happy phrase, “rigour within uncertainty” may be achieved.

Conclusions (continued) • Finally, there is an important cosmetic aspect that must be addressed. Some have suggested that “It doesn’t 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.

Rigour Within Uncertainty: An Unfinished Quest