Operational Quantities for External Radiation Exposure

- Deficiencies/Options -

David Thomas Bartlett, Günther Dietze, Nolan Hertel, Jean-Marc Bordy, Akira Endo, Gianfranco Gualdrini, Maurizio Pelliccioni, Peter Ambrosi, Bernd Siebert, and Ken Veinot.

MELODI October 2013

Draft discussion paper (2006) Considerations on the Operational Quantities for Monitoring External

Radiation Exposure

ICRU Committee on Fundamental Quantities and Units

David Bartlett

David Burns

Guenther Dietze

Hans Menzel

Herwig Paretzke

Stephen Seltzer

André Wambersie

Need for Operational Dose Quantities for External Exposure Situations

● The protection quantities equivalent dose in an organ or tissue and effective dose are generally not measurable.

Why do we need operational dose quantities for external radiation exposure ?

● Exposure limits are given in terms of protection quantities.

● Measurements need the calibration of instruments in terms of measurable quantities.

● Control of dose limits needs the assessment of values of the protection quantities by measurements.

Operational Dose Quantities

Exposure limits (ICRP 103) are given in terms of

● effective dose, E ● equivalent dose to the skin, Hskin

● equivalent dose to the lens of the eye, Heye lens ● equivalent dose to the hands and feet

Task Area monitoring Individual monitoring

Monitoring of Ambient dose Personal dose effective dose equivalent, H*(10) equivalent, H p(10 ) Monitoring of equivalent Directional dose Personal dose dose to the skin equivalent, H´(0.07,) equivalent, Hp(0.07) Monitoring of equivalent Directional dose Personal dose dose to the eye lens equivalent, H´(3,) equivalent, Hp(3)

Quantities for area monitoring, H*(d) and H´(d)

● Primary standards for ambient and directional dose equivalent, H*(d) and H´(d), do not exist.

● Reference fields for calibration of instruments are usually realised in terms of radiation fluence rate, , (for neutrons) , air kerma rate, Ka, (for photons) , absorbed dose to tissue (electrons), and the application of fluence-(or air kerma or tissue absorbed dose) to-dose equivalent conversion coefficients.

● The monoenergetic values of conversion coefficients are fixed reference values recommended by ICRU and ICRP, and defined to have no uncertainty.

● The conversion coefficients are used in all calibration procedures.

Note that there are more sources of high-energy radiation where operational dose quantities might need to be applied [ICRU Report 39 issued in 1985]

Increasing use of medical accelerators with accelerating potentials of up to greater than 20 MV for radiotherapy with photons and electrons.

Use of high-energy proton and heavy-ion accelerators for hadrontherapy.

Radiation fields near research high-energy particle accelerators

Natural sources of high-energy radiation (at aviation altitudes and in space)

● The ICRU sphere (defined more than 30 years ago) is based on the definition of ICRU 4-element tissue-equivalent material which does not really exist, cannot be fabricated, and there are problems with computations which depend on molecule composition.

● Dose equivalent, H, is defined as absorbed dose in tissue times the radiation quality factor, Q, where Q is defined by a function Q(L), where L is the unrestricted linear energy transfer, L , of the charged particle traversing the point (or small volume) of interest , not in

tissue material at that point but in water.

L in water in keV/μm

Q

Q and RBE

Should Q be a function of L , would y or Z2/β2 be better ? [see Cucinotta, F.A., Kim, M-H, Y. and Chappell, L.J. Space Radiation Cancer Risk Projections and Uncertainties (2010) which reports a better agreement with RBE of a function based on Z2/β2 ; Sato, T., Endo, A. and Niita, K. Comparison of the mean quality factors for astronauts calculated using the Q-functions proposed by ICRP, ICRU, and NASA.Adv. Space Res. 52 79-85 (2013); comments in ICRP Publication 123, Assessment of Radiation Exposure of Astronauts in Space] [This is important for the operational quantities now, but in the future may be a consideration by ICRP in the use of wR and Q.]

100

101

102

103

100

101

LET or y (keV/m)

Qualit

y F

acto

r

Q(L)

Q(y)

Radiation weighting numerical values of the central estimate of QNASA for protons, 12C, and 56Fe from 1 MeV/u to 100 GeV/u for solid cancer as a function of the LET in comparison to the values for Q(L)

and Q(y). [Sato, T., Endo, A. and Niita, K. Comparison of the mean quality factors

for astronauts calculated using the Q-functions proposed by ICRP, ICRU, and NASA. Adv. Space Res. 52 79-85 (2013).]

Radiation weighting : phantom averaged quality factor, QE,ISO,NASA, as

function of particle energy for protons (a), neutrons (b) and various other

ions (c) and isotropic exposure the adult male reference phantom. [Sato, T., Endo, A. and Niita, K. Comparison of the mean quality factors for astronauts calculated using the Q-functions proposed by ICRP, ICRU, and NASA. Adv. Space Res. 52 79-85 (2013) ]

Calculated effective dose equivalents using Q(NASA), Q (L), Q(y) for male astronauts inside the ISS at the solar minimum, classified according to the contributions from particles incident upon them. [Comparison of the mean quality factors for astronauts calculated using the Q-functions

proposed by ICRP, ICRU, and NASA, Sato, Endo, and Niita.]

Kerma approximation

All energies of the emitted secondary charged particles are fixed to be deposited in the volume element where the reaction takes place. If secondary charged particle equilibrium exists at that point, then kerma and absorbed dose have approximately the same value.

depth d

kerma

dose

Dose distribution near a surface

● Calculations of conversion coefficients for photons and neutrons are performed using the kerma approximation with the phantom in vacuo, this is not the procedure for effective dose calculations (ICRP Publication 116) which are in vacuo but follow the secondary radiations generated

● The dose equivalents deposited by external secondary particles and scattered primary are not included in the definitions (sphere in vacuo) for H*(10) and H´(d,). For H*(10) these components cannot be aligned. For photons, the energy of secondary electrons for a range equal to 10 mm is about 2 MeV, for a range of 0.07 mm about 60 to 70 keV; for neutrons the energy of protons is about 35 MeV.

0.01

0.1

1

10

100

1000

0.01 0.1 1 10 100

Photon Energy, MeV

H*(

10)/

(pS

v c

m2)

ICRU 57, kerma approx

Seltzer, kerma approx

EGS, full trans

Ferrari-Pelliccioni, full trans

Seltzer, full trans

Photon energy / MeV

H*(

10

)/

in

p

Sv c

m2

0.1 1 100 10 0.01

0.01

0.1

1

1000

100

10

kerma approximation

full transport

Calculations of conversion coefficients H*(10) / for photons performed using the ICRU sphere in vacuo

(S. Seltzer, NIST, 2011)

Effective dose

H*(10), Hp(10)

Conversion coefficients for effective dose, H*(10) and Hp(10). (K. G. Veinot and N. E. Hertel, RPD 145 (2011))

MONTE CARLO DETERMINATION OF THE CONVERSION

COEFFICIENTS Hp(3)/Ka IN A RIGHT CYLINDER PHANTOM

WITH ‘PENELOPE’ CODE. COMPARISON WITH ‘MCNP’

SIMULATIONS.

J. Daures, J. Gouriou and J. M. Bordy

RPD 2011 vol 144 no 1-4 pp 37-42

Skin AP

Photon energy (MeV)

0.01 0.10 1.00 10.00

Absorb

ed d

ose p

er

unit a

ir k

erm

a (

Gy/G

y)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Present, maleICRP 74

Photon exposure of the skin ICRP 74 kerma approximation ICRP 116 secondary charged particle follow-up

kerma approximation

full transport

Neutron energy (MeV)

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103

Pro

tection q

uantity

/ o

pera

tional quantity

0.0

0.5

1.0

1.5

2.0

E(AP) / H*(10) E(PA) / H*(10) E(LLAT) / H*(10) E(RLAT) / H*(10) E(ROT) / H*(10) E(ISO) / H*(10)

E(ICRP 116) / H*(10) for neutrons

● For higher energies, the ICRU sphere could be located in air. This requires that the distance between source and sphere needs to be defined. To achieve secondary charged particle equilibrium at the surface, this distance depends on the photon energy non-additive.

● Today, in reference photon fields used for calibration of instruments and dosimeters, secondary charged particle equilibrium is approximately realized by including tissue-equivalent material between the radiation source and the dosimeter to be calibrated.

● The depth of 10 mm is not generally adequate to assess neutron effective dose.

● At higher photon and neutron energies, 10 mm is also not adequate. Could use Hmax ?

Stay with current situation Stay with the existing situation for those particles and energy

ranges for the limited range of particle energies where the system is well established .

The definition of the operational quantities remains the same.

The ICRU sphere and the phantoms defined for calibration of personal dosimeters are not changed.

The Q(L) function remains unchanged.

The conversion coefficients are calculated with the phantoms in vacuo and using the kerma approximation for the limited range of photon and neutron energies.

But

Use sets of coefficients for H*(10) and Hp(10) for higher energies without using the kerma approximation.

.

Option I Area and individual monitoring

Stay with the existing situation for those particles and energy ranges for the limited range of particle energies where the system is well established.

The ICRU sphere phantom and the phantoms for calibration are not changed.

The Q(L) function remains unchanged.

For higher radiation energies define new sets of values of the depth d in the ICRU sphere phantom for the calculation of coefficients of H*(d) for ranges of values of photon and neutron energies to better match values of E. (In fact use Hmax).

Option IIa Area monitoring

Redefine H*(10) and H´(d,), to take account of the contributions of secondary charged particles and scattered primary particles, and calculate conversion coefficients (without using the kerma approximation )for irradiation in an infinite air medium.

Note that if an infinite air medium is not included for the calculations of coefficients ,the values will depend on the particle energy and the conditions of phantom/body exposure.

The ICRU sphere phantom and the phantoms for calibration are not changed.

The Q(L) function remains unchanged .

A new assessment of the relationships of the values of the operational and protection quantities will be needed.

Option IIIa Area monitoring

Define the operational quantities for area monitoring without using the ICRU sphere and the quality factor Q(L)

The definition of the operational quantities given by the product of

fluence/air kerma/absorbed dose x conversion coefficient

R hquantity,R or Ka hquantity,R

where the value of the fluence/air kerma of radiation R is given by the value at the point of interest.

For area monitoring the conversion coefficients are generally based on the anthropomorphic reference phantoms, on effective dose, local skin dose and dose to the lens of the eye (envelope/max functions).

If more than one type of radiation is involved, the value of the operational quantity is given by the sum over R.

Photon energy (MeV)

10-2 10-1 100 101 102 103 104

Effective d

ose p

er

fluence (

pS

v c

m2)

10-2

10-1

100

101

102

103

APPALLATRLATROTISO

Effective dose

For area monitoring and assessment of equivalent dose to the local skin or the eye lens the conversion coefficient is given by Hlocal skin/ , /Ka or /DT , or Eeye lens/Ka or /DT , respectively.

For area monitoring and assessment of effective dose the conversion coefficient is given by Emax/ or Emax/Ka for photons, respectively, where Emax is the envelope of effective dose of the various directions of radiation incidence.

Option IIb Individual monitoring Ensure that Hp(d) includes the contributions of all of the

radiation field incident on the body/phantom, including the secondary charged particles and scattered primary particles;

or

change Hp(d) to be in terms of equivalent dose to soft tissue (at the depth d) (in ICRU 4-element tissue?), using wR instead of Q(L).

Conversion coefficients for calibration are calculated for the radiation field at the point of reference, including the scattered primary plus secondary particles. Note that if these values are not calculated for an infinite air medium they will depend on the particle energy and the conditions of phantom/body exposure.

The phantoms for calibration are not changed, and the Q(L) function remains unchanged .

A new assessment of the relationships of the values of the operational and protection quantities will be needed.

Option IIIb Individual monitoring

Redefine Hp(d) to assess EAP or Emax for the radiation field incident on the body, including the contributions of secondary charged particles and scattered primary particles.

The phantoms defined for calibration of personal dosimeters are the same.

Impact of changes

There are different options for improving the system of operational dose quantities, but it is necessary to look at the impact of the proposed changes, and carefully consider the consequences for radiation protection practice, e.g. dosimeter design, and calibration procedures.

Need to discuss with ICRP, IAEA, NCRP, ISO,IEC, IRPA, EURADOS.

Impact of changes

Impact of proposed changes of H*(10) conversion coefficients on current instruments’ responses

Photons:

Assessment of effective dose.

Neutrons:

There are different options but need to fully consider the consequences for radiation protection practice.

Summary I

Area monitoring quantities establish the radiation protection priorities, viz assessment of hazard; designation of areas; need for individual monitoring.

Individual monitoring provides an estimate of personal exposure, can be related to organ or tissue equivalent dose and effective dose, and is entered in dose records.

Both assessments are covered by legal requirements. (See for example European Commission Report 160).

Summary II

Operational quantities (ICRU Report 39 including committee drafts, Reports 47, 51, 57, 66) assumed that in practical situations, with broad energy and direction distributions, the kerma approximation may suffice.

In current radiation protection practice, calibrations and measurements and assessments generally work (although in some circumstances the measurement quantities are incorrect with regard to the definitions).

Summary III

BUT

There are now more sources of high-energy radiations: high - energy accelerators for research; increasing use of medical accelerators with accelerating potentials of 20 to 30 MV; and hadrontherapy.

There is legislation covering natural sources of radiation, including cosmic radiation (aircraft crew are the most highly exposed occupational group).

[Note that there are no ISO reference fields above 6/7 MeV for photons and 19 MeV for neutrons.]

Summary IV •Fundamental problem that ICRU 4-element tissue cannot be fabricated, particular problem for low-energy neutron cross sections.

•Dose equivalent is defined as absorbed dose in ICRU 4-element tissue times Q(L), where L is the LET in water.

•ICRU (Q(L)) and ICRP (wR) use different radiation weighting factors, do we need to use both? New Q RBE function? Bilocality?

•Calculations of Report 57 operational quantity conversion coefficients for photons and neutrons using the kerma approximation in vacuo, and dose equivalent deposited by external secondary particles are not included in the definitions and for H*(10) this component cannot be aligned. Deficiencies and limitations are greater for H*(10), less for H‘(3) and H'(0.07), and are perhaps most easily dealt with for Hp(d).

Summary V MIGHT CONSIDER

Area monitoring quantities redefined for all particles and energies in terms of the envelope function for E, and for equivalent dose to skin and to eye lens.

Individual monitoring definitions the same as now in terms of dose equivalent, including all particles at the reference point on the body’s surface, or redefined in terms of equivalent dose.

Do we need both wR (for area monitoring) and Q for individual? (Note

future ICRP consideration of this? ICRP Publication 123 Assessment of Radiation Exposure of Astronauts in Space; perhaps also medical applications of use of E for high-energy particles. Bilocality?)

For a period, continued use of current conversion coefficients for H*(10) (and H’(0.07)) up to 2 MeV for photons, and up to 35 MeV for neutrons.

ICRU Report Committee 26: Operational Radiation Protection Quantities for

External Radiation Members:

Nolan Hertel (USA) Co-Chairman David Thomas Bartlett (UK) Co-Chairman Jean-Marc Bordy (France) Günther Dietze (Germany) Akira Endo (Japan) Gianfranco Gualdrini (Italy) (to 2013) Maurizio Pelliccioni (Italy)

Corresponding members: David Burns (BIPM) Peter Ambrosi (Germany) Bernd Siebert (Germany) Ken Veinot (USA)

ICRU Sponsors:

Hans Menzel, Steve Seltzer, Elena Fantuzzi

Do we have a choice?

• Do nothing: keep operational quantities the same, but apply special conditions at high energy

• Keep operational quantities the same, apply special conditions at high energy, write an ICRU report to explain inconsistencies, and have planned further investigations, including looking at RBE/wR/Q

• Change to a new consistent system with an ICRU report

Thank you

for your attention.

MELODI October 2013