Rigour Within Uncertainty: An Unfinished Quest

Rigour Within Uncertainty: An Rigour Within Uncertainty: An Unfinished QuestUnfinished Quest

ICRPICRP and High-LET Radiationsand High-LET Radiations

Ralph H. Thomas, Ralph H. Thomas, University of California (Retired)University of California (Retired)

Thirteenth Annual Thirteenth Annual J. Newell Stannard Lecture SeriesJ. Newell Stannard Lecture Series

Sacramento, CaliforniaSacramento, California15 April 200515 April 2005

Topics to be discussed in this lectureTopics to be discussed in this lecture

Current review of ICRP recommendationsCurrent review of ICRP recommendations External exposure and high-LET radiations External exposure and high-LET radiations

(mainly neutrons)(mainly neutrons) Modified absorbed dose quantitiesModified absorbed dose quantities Problems with the draft recommendations Problems with the draft recommendations

for 2005for 2005 Suggested solutionsSuggested solutions


Current review of ICRP Current review of ICRP recommendationsrecommendations

External exposure and high-LET radiations External exposure and high-LET radiations (mainly neutrons)(mainly neutrons)

Modified absorbed dose quantitiesModified absorbed dose quantities Problems with the draft recommendations Problems with the draft recommendations


Revision of ICRP recommendationsRevision of ICRP recommendations

Circa Circa 2000 work began on the next set 2000 work began on the next set of fundamental ICRP recommendations of fundamental ICRP recommendations intended to replace ICRP Publication intended to replace ICRP Publication 6060

2003 ICRP Publication 92, 2003 ICRP Publication 92, Relative Relative Biological Effectiveness, Quality Factor Biological Effectiveness, Quality Factor and Radiation Weighting Factorand Radiation Weighting Factor

Revision of ICRP recommendations Revision of ICRP recommendations (continued)(continued)

Autumn of 2004: Autumn of 2004: Draft for Consultation - 2005 Draft for Consultation - 2005 Recommendations of the ICRP Recommendations of the ICRP made available for made available for comment on web site; consultation period ended comment on web site; consultation period ended 31 December 200431 December 2004

Current status: “Current status: “The public consultation period is The public consultation period is now completed . . . an overwhelming response with now completed . . . an overwhelming response with detailed and very constructive proposals . . . ICRP detailed and very constructive proposals . . . ICRP intends to consult . . . the ‘foundation documents’ intends to consult . . . the ‘foundation documents’ underpinning the Recommendations . . . underpinning the Recommendations . . . Depending on the outcome of this review process, Depending on the outcome of this review process, a second, shorter round of consultation may be a second, shorter round of consultation may be held”held”


Current review of ICRP recommendationsCurrent review of ICRP recommendations External exposure and high-LET External exposure and high-LET

radiations (mainly neutrons)radiations (mainly neutrons) Modified absorbed dose quantitiesModified absorbed dose quantities Problems with the draft recommendations Problems with the draft recommendations


Importance of high-LET radiationsImportance of high-LET radiations

High-LET exposures make up 10%-20% of High-LET exposures make up 10%-20% of work force exposures (comparable with work force exposures (comparable with internal exposures)internal exposures)

Air- and cabin-crew exposures to a mixed Air- and cabin-crew exposures to a mixed radiation field, including neutrons, are among radiation field, including neutrons, are among the highest quasi-occupational exposures the highest quasi-occupational exposures

Importance of high-LET radiations Importance of high-LET radiations (continued)(continued)

The number of people exposed to high-LET The number of people exposed to high-LET radiations will almost certainly increase in the radiations will almost certainly increase in the futurefuture

The probability that exposure to high-LET The probability that exposure to high-LET radiations presents some risk at low doses is radiations presents some risk at low doses is almost certainly greater than that for low-LET almost certainly greater than that for low-LET exposuresexposures

Why high-energy and high-LET Why high-energy and high-LET make a differencemake a difference

Low-energy photons: Because only low-LET Low-energy photons: Because only low-LET charged particles are generated in tissue, the charged particles are generated in tissue, the ICRP paradigm (for both internal and external ICRP paradigm (for both internal and external exposure) is to constrain the value of the exposure) is to constrain the value of the important radiation-weighting factors (RBE, important radiation-weighting factors (RBE, Q, Q, H*(10) and wH*(10) and wRR) to the value 1) to the value 1

For neutrons, high-energy photons, and high-For neutrons, high-energy photons, and high-LET particles, both the absorbed dose and LET LET particles, both the absorbed dose and LET (dE/dX) distributions may vary greatly with (dE/dX) distributions may vary greatly with location in the body; values of average organ location in the body; values of average organ quality factors, quality factors, QQTT, may show a correspondingly , may show a correspondingly wide variation between tissueswide variation between tissues

ICRP Publication 74 convincingly ICRP Publication 74 convincingly makes this pointmakes this point

High-LET radiations need ICRP’s High-LET radiations need ICRP’s focussed attentionfocussed attention

Before 1985Before 1985 “external” and “internal” modes of “external” and “internal” modes of exposure were treated, almost distinctly and exposure were treated, almost distinctly and separately, by two committees of ICRPseparately, by two committees of ICRP

After 1985After 1985 Committee 2, Radiation Protection Committee 2, Radiation Protection Standards, was charged with applying a unified Standards, was charged with applying a unified approach to both exposure modes; however, approach to both exposure modes; however, external pressures directed the early effort of the external pressures directed the early effort of the new committee largely towards internal new committee largely towards internal exposureexposure





““The Devil is in the Details”The Devil is in the Details”

The basis for our current quantities is some form of radiation-The basis for our current quantities is some form of radiation-weighted absorbed dose but the past 60 years shows that weighted absorbed dose but the past 60 years shows that ““the devil is in the details”the devil is in the details” circacirca 1940 absorbed dose 1940 absorbed dose 1948 RBE dose1948 RBE dose 1965 dose equivalent, H, Q (ICRP 4)1965 dose equivalent, H, Q (ICRP 4) 1973 MADE, Q(L)-L, 1973 MADE, Q(L)-L, QQ (ICRP 21) (ICRP 21) 1977, 1980 effective dose equivalent, H1977, 1980 effective dose equivalent, HEE, w, wTT (ICRP 26) (ICRP 26) 1980 dose equivalent indexes (ICRU 33)1980 dose equivalent indexes (ICRU 33) 1985 ambient dose equivalent, H*(d) (ICRU 39 & 42) 1985 ambient dose equivalent, H*(d) (ICRU 39 & 42) 1991 effective dose, E, w1991 effective dose, E, wRR (ICRP 60) (ICRP 60)

Analysis of mammalian cell data Analysis of mammalian cell data suggested a radiobiological basis suggested a radiobiological basis

for a Q(L)-L modelfor a Q(L)-L model

Experimental curves Experimental curves of RBE versus LETof RBE versus LET

Mammalian tissues, Mammalian tissues, variousvarious

ICRP Publication 21 (1971) ICRP Publication 21 (1971) recommended that a smooth recommended that a smooth Q(L)-L model needed to be Q(L)-L model needed to be

established “established “as a common basis for as a common basis for dose equivalent calculationdose equivalent calculation” and ” and ICRP 60 recommended a revised ICRP 60 recommended a revised

modelmodel

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

Caveat emptor 2!Caveat emptor 2! ICRP Publication 92 gives the same messageICRP Publication 92 gives the same message

Effective dose equivalent versus Effective dose equivalent versus effective doseeffective dose

At first sight HAt first sight HEE and E appear to be identical and both and E appear to be identical and both defined bydefined by

where where T is the irradiated tissue or organT is the irradiated tissue or organ

wwTT is the tissue-weighting factor for T is the tissue-weighting factor for T

HHTT is the equivalent dose for T is the equivalent dose for T However, different methods of radiation weighting produce However, different methods of radiation weighting produce

significant differences, which have been discussed in the significant differences, which have been discussed in the scientific literature, most recently in ICRP Publication 92 scientific literature, most recently in ICRP Publication 92

TT

THw



(mainly neutrons)(mainly neutrons) Modified absorbed dose quantitiesModified absorbed dose quantities Problems with the draft Problems with the draft

recommendations for 2005recommendations for 2005 Suggested solutionsSuggested solutions

Values of wValues of wRR, q* and q, q* and qEE given in given in

ICRP 92ICRP 92

Draft 2005 recommended values of Draft 2005 recommended values of wwRR for neutrons for neutrons

Values of qValues of qEE calculated for a human phantom and calculated for a human phantom and using the Q(L)-L relationship recommended ICRP using the Q(L)-L relationship recommended ICRP Publication 60Publication 60

qqEE is the human body averaged mean quality factor is the human body averaged mean quality factor

Values of qValues of qEE = 2 for neutron energies below 1 keV = 2 for neutron energies below 1 keV were accepted and wwere accepted and wRR was defined to be equal to was defined to be equal to qqEE: w: wRR = q = qEE in this energy region in this energy region

T

TTEE DwHq

Draft 2005 recommended values of Draft 2005 recommended values of wwRR for neutrons (continued) for neutrons (continued)

The calculated value of qThe calculated value of qEE = 13 for neutron energies in the 1-MeV = 13 for neutron energies in the 1-MeV range was not accepted and wrange was not accepted and wRR was set at ~ 21 (based on RBE was set at ~ 21 (based on RBE values for values for smallsmall animals); w animals); wRR q qEE and a fudge factor equation was and a fudge factor equation was adopted for the energy region between 1 keV and 1MeVadopted for the energy region between 1 keV and 1MeVwwRR ═ 1.6q ═ 1.6qEE -1 -1

No changes from the ICRP 60 values above 1 MeV were No changes from the ICRP 60 values above 1 MeV were recommendedrecommended

The following empirical functions for wThe following empirical functions for wRR are also given are also given

EEn n 1 MeV 1 MeV

EEn n 1 MeV 1 MeV

The recommended value of wThe recommended value of wRR for high-energy protons is 2 for high-energy protons is 2

]6/)(lnexp[2.185.2 2nR Ew

]6/)2(lnexp[0.170.5 2nR Ew

ICRP draft recommendations for ICRP draft recommendations for 2005 are a great disappointment!2005 are a great disappointment!

Logical miscues in the evaluation Logical miscues in the evaluation of wof wRR for neutrons in the draft for neutrons in the draft

recommendationsrecommendations

The Q(L)-L relationship recommended in ICRP The Q(L)-L relationship recommended in ICRP Publication 60, now used to calculate some Publication 60, now used to calculate some values of wvalues of wRR was “discredited” by ICRP in was “discredited” by ICRP in

Publication 60 (paragraph A9)Publication 60 (paragraph A9)

Values of qValues of qEE = 2 for neutron energies below 1 = 2 for neutron energies below 1

keV were accepted consequently and wkeV were accepted consequently and wRR was was

defined to be equal to qdefined to be equal to qEE

Logical miscues in the evaluation Logical miscues in the evaluation of wof wRR for neutrons in the draft for neutrons in the draft

recommendationsrecommendations (continued) (continued)

The calculated value of qThe calculated value of qEE = 13 for neutrons = 13 for neutrons energies in the 1 MeV was not accepted and wenergies in the 1 MeV was not accepted and wRR was set at ~ 21 (based on RBE values for was set at ~ 21 (based on RBE values for smallsmall animals)animals)

If, after radiobiological review, the values wIf, after radiobiological review, the values wRR below 1 keV are acceptable but at 1 MeV below 1 keV are acceptable but at 1 MeV unacceptable then it must be concluded that the unacceptable then it must be concluded that the recommended Q(L)-L relationship and the value recommended Q(L)-L relationship and the value of wof wRR at 1 MeV are inconsistent at 1 MeV are inconsistent





Goals for an “ideal” system of Goals for an “ideal” system of dosimetry for radiological protectiondosimetry for radiological protection Universal: applies to all radiations, whatever their Universal: applies to all radiations, whatever their

energyenergy Integrated: independent of the origin of the radiation Integrated: independent of the origin of the radiation

(either outside or inside the human body)(either outside or inside the human body) Unambiguous: standards are set in determinable Unambiguous: standards are set in determinable

quantities (no distinction between “protection” and quantities (no distinction between “protection” and “operational quantities”)“operational quantities”)

Rigorous: logically and mathematically coherent and Rigorous: logically and mathematically coherent and consistent with mathematical logic and physical laws consistent with mathematical logic and physical laws

Stable: avoiding frequent changes in names and Stable: avoiding frequent changes in names and symbols of dosimetric concepts symbols of dosimetric concepts

Suggestions for a remedySuggestions for a remedy

Abandon the dual concept of protection (limiting) and Abandon the dual concept of protection (limiting) and operational quantitiesoperational quantities

Define only protection quantities and leave it to the Define only protection quantities and leave it to the ingenuity of dosimetrists to deduce the means of ingenuity of dosimetrists to deduce the means of measurement thus effectively abandoning the dual measurement thus effectively abandoning the dual concept of protection and operational quantitiesconcept of protection and operational quantities

Review the experimental and theoretical basis for the Review the experimental and theoretical basis for the recommendations of RBE for humans, paying particular recommendations of RBE for humans, paying particular attention to the experimental irradiation conditions for attention to the experimental irradiation conditions for small samples (animals)small samples (animals)

Redefine the function Q(L)-L on the basis of this reviewRedefine the function Q(L)-L on the basis of this review

Suggestions for a remedy Suggestions for a remedy (continued)(continued)

The form of the new Q(L)-L function should be similar to The form of the new Q(L)-L function should be similar to that of the current ICRP definition of ICRP 60 but that of the current ICRP definition of ICRP 60 but mathematically more tractable, avoiding breaks and mathematically more tractable, avoiding breaks and cuspscusps

Revert to the quantity of effective dose equivalentRevert to the quantity of effective dose equivalent The new function Q(L)-L must generate values of qThe new function Q(L)-L must generate values of qEE for for

neutrons that are consistent with the needs of ICRP and neutrons that are consistent with the needs of ICRP and the laws of physics. The 2005 draft suggests that the the laws of physics. The 2005 draft suggests that the constraints appear to beconstraints appear to be• qqEE = 2 for low energy neutrons (seems to be correct to the = 2 for low energy neutrons (seems to be correct to the

physicists and satisfies the radiobiologists and ICRP) physicists and satisfies the radiobiologists and ICRP) • qqEE = 20 for 1 MeV neutrons (satisfies ICRP but perhaps the = 20 for 1 MeV neutrons (satisfies ICRP but perhaps the

choice needs revision or a better justification than given hitherto choice needs revision or a better justification than given hitherto by ICRP) by ICRP)

Suggestions for a remedySuggestions for a remedy (continued)(continued)

At high energies (hundred MeV region) select At high energies (hundred MeV region) select the specific of qthe specific of qEE for neutrons to be compatible for neutrons to be compatible

with the selected value for high-energy protons. with the selected value for high-energy protons. Most physicists agree that wMost physicists agree that wRR (q (qEE) for high-) for high-

energy protons and neutrons should approach energy protons and neutrons should approach the same value. This is a matter of energy the same value. This is a matter of energy deposition (deposition (i.ei.e., physics) and should therefore be ., physics) and should therefore be acceptable to the radiobiologists. A value of acceptable to the radiobiologists. A value of qqEE 2 would be about right in the mid-100 MeV 2 would be about right in the mid-100 MeV

region.region.

ConclusionsConclusions

A major problem with both the A major problem with both the ICRP 92ICRP 92 and the ICRP and the ICRP Draft for ConsultationDraft for Consultation proposals is that they proposals is that they appearappear to fix to fix the values of the values of wwRR to neutrons to conform to a to neutrons to conform to a preconceived notion that preconceived notion that wwRR for “fission neutrons” for “fission neutrons” mustmust take a value of about 20. The radiobiological arguments take a value of about 20. The radiobiological arguments for this are not well explained by ICRP but rather are for this are not well explained by ICRP but rather are buttressed by administrative and legal concerns.buttressed by administrative and legal concerns.

Consequently there is a danger that science might be Consequently there is a danger that science might be relegated to political disputes. ICRP would be better relegated to political disputes. ICRP would be better served by focussing on the relevant science that can be served by focussing on the relevant science that can be brought to bear and ensuring that it is the best that it brought to bear and ensuring that it is the best that it possibly can be so that, in Kellerer’s happy phrase, possibly can be so that, in Kellerer’s happy phrase, “rigour within uncertainty” may be achieved.“rigour within uncertainty” may be achieved.

Conclusions (continued)Conclusions (continued)

Finally, there is an important cosmetic aspect Finally, there is an important cosmetic aspect that must be addressed. Some have suggested that must be addressed. Some have suggested that “that “It doesn’t seem wise to give the impression It doesn’t seem wise to give the impression that we are keeping two sets of books.”that we are keeping two sets of books.” Frankly, Frankly, the approach of the draft for consultation has the the approach of the draft for consultation has the appearance of “appearance of “cooking the books”cooking the books” and my and my guess is that ICRP will draw immediate adverse guess is that ICRP will draw immediate adverse criticism if it moves in this direction.criticism if it moves in this direction.

Happily there is a rather simple remedy to these Happily there is a rather simple remedy to these concerns in the unlikely event that ICRP can be concerns in the unlikely event that ICRP can be persuaded to take itpersuaded to take it..

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Rigour Within Uncertainty: An Unfinished Quest