Impact of PET Radionuclide Decay Schemes

The short half-lives of clinical PET radionuclides limit the internal radiation dose to patients and the external radiation dose to persons who come in contact with the patient some time after the PET scan. However, they confer no particular benefit on PET staff who must contend with high dose rates from patients and many patients to be scanned each day. The various aspects of "exposure to radiation" need to be described in specific dosimetric terms.

The radiation absorbed dose, D, is the energy deposited per unit mass of an absorbing material, including biological tissue. In SI units, absorbed dose is expressed in grays (Gy): 1 Gy is 1 joule per kilogram. Absorbed doses from natural background radiation are of the order of 2 mGy per year, absorbed doses in medicine typically range up to a few tens of mGy from diagnostic procedures and tens of Gy to tissues targeted in therapeutic applications. Two derived dose quantities are invoked to regulate the exposure of persons at work and the public at large [1]: equivalent dose, H, and effective dose, E, both of which are expressed in sieverts (Sv). Equivalent dose is absorbed dose weighted for the type of radiation and averaged over the whole organ or tissue (except in the case of skin). Fortunately, for simplicity in most medical applications, the radiation weighting factor for electrons, positrons, X- and gamma rays is one and therefore the equivalent dose in Sv is numerically equal to the absorbed dose in Gy. Effective dose is also a mathematical construct: a weighted sum of the equivalent doses to the individual organs and tissues of the body. The tissue weighting factors take account of the relative susceptibility of different tissues to radiation damage. Effective dose represents the long-term risk of harm from low-level exposure, essentially the risk of radiogenic cancer.

Most countries have adopted the dose limits recommended by the International Commission on Radiological Protection (ICRP) as shown in Table 12.1. Medical exposures are not included in the system of recommended limits. The limit on effective dose for occupational exposure is associated with an acceptable long-term risk compared to most other occupational hazards; the limit for members of the public is considered to be acceptable because it is comparable to varia-

* Chapter reproduced from Valk PE, Bailey DL, Townsend DW, Maisey MN. Positron Emission Tomography: Basic Science and Clinical Practice. Springer-Verlag London Ltd 2003, 265-279.

Table 12.1. Dose limits recommended by the ICRP [1]

tions in natural background radiation [1]. The effective dose limits are supplemented by limits on the equivalent dose to the tissues most likely to receive a high exposure at work - the skin, eyes and the hands and feet ("extremities") - to avoid damage to skin and formation of cataracts in the lens of the eye. For the purposes of monitoring a person's exposure to an external source of radiation, the ambient dose equivalent at a depth of 10 mm in tissue, H*(10), also called the deep dose equivalent (DDE), may be taken as the effective dose from a uniform whole-body exposure. The directional dose equivalent at a depth of 0.07 mm in tissue, H'(0.07), also called the shallow dose equivalent (SDE), can be taken as the equivalent dose at the average depth - 70 |m-of the basal cell layer in skin [2].

In terms of energy deposition in tissue, PET ra-dionuclides have more in common with the radionu-clides used for therapy than those used for diagnostic imaging. The amount of energy deposited locally or at a distance from disintegrating atoms in an infinite medium is indicated by the equilibrium absorbed dose constant, A, as shown in Table 12.2 for a selection of ra-dionuclides used for diagnosis and therapy [3, 4]. Positrons, being non-penetrating charged particles, deposit their energy locally and account for most of the dose to the organs and tissues of PET patients. The annihilation photons are penetrating and account for the exposure of persons nearby. The influence of half-life on the energy available from the total decay of a source is also evident in Table 12.2.

External exposure is the most significant pathway for occupational exposure in PET facilities. The high dose rates from PET radionuclides relative to other radionu-clides used for diagnostic imaging are due to their high photon energy (511 keV) and abundance (197-200% for the PET radionuclides shown in Table 12.2 as there are two photons for each positron emitted). Other potential pathways are

(i) a skin dose from surface contamination,

(ii) a deep dose from bremsstrahlung generated in lead or other shielding material of high atomic number,

(iii) a superficial dose from positrons emitted from the surface of uncovered sources,

(iv) an immersion and inhalation dose from a release of radioactive gas into the room air.

The starting point when planning protection against external exposure from a radioactive source is a knowledge of the dose rate from the radionuclide in question. However, it is not always a straightforward exercise to find the appropriate value from published data. Variables include the physical quantity and absorbing medium (for example, exposure, absorbed dose, kerma in air; kerma or equivalent dose in tissue), distance from the source (for example, 1 cm, 30 cm, 1 meter), source configurations (for example, point source, vial), lower bound on photon energy (for example, 10 keV, 20 keV) and, of course, units (SI or old system).

Dose rates in air were traditionally calculated using the specific gamma ray constant (m2 R mCi-1 h-1 in old units) for the exposure rate at 1 meter from the nuclide in question. The conversion factor from exposure in air (roentgens, R) to absorbed dose in tissue was close to unity and was generally ignored.1 With the introduction of SI units, the International Commission on Radiation Units and Measurements (ICRU) recommended that the specific gamma ray dose constant should be phased out and replaced by the air kerma rate constant [5].2 The conversion factor from air kerma to ambient dose equivalent is not close to unity. It takes account of scattering and attenuation in tissue and depends on the photon energy [6]. The dose rates in air and tissue at a distance of 1 meter from a 1GBq "point" source of commonly used radionuclides are given in Table 12.3 [6,7].

There is good agreement between the data in Table 12.3 for the ambient and deep dose equivalent rates from photon emissions. The rate constants can be used to check the response of survey meters, whether displayed in air kerma or ambient dose equivalent, to a reference source of known activity. Dose rates from

Table 12.1. Dose limits recommended by the ICRP [1]

Occupational

Public

Effective dose

20 mSv y-1, averaged over 5 y

1 mSv y-1

and not more than 50 mSv

in any 1 y

Equivalent dose

Lens of the eye

150 mSv y-1

15 mSv y-1

Skin

500 mSv y-1

50 mSv y-1

Hands and feet

500 mSv y-1

-

1 For an approximate conversion of exposure in roentgens to absorbed dose in rads, multiply by 0.87 for air, or 0.97 for tissue.

2 "Kerma" stands for kinetic energy released in unit mass and is expressed in the same units as absorbed dose. It is the sum of the initial kinetic energies of all the charged particles produced by photons incident on the unit mass. The kerma value may be slightly lower than the absorbed dose if some of the charged particle energy is deposited elsewhere (for example, after conversion to bremsstrahlung) [47].

Table 12.2. Energy available from the decay of nuclear medicine radionuclides

Equilibrium absorbed dose constant, Aa T i Energy from total g Gy MBq-1 h-1 2 decay of 1MBq

Non-penetrating Penetrating Total pJ An-p Ap A

Equilibrium absorbed dose constant, Aa T i Energy from total g Gy MBq-1 h-1 2 decay of 1MBq

Non-penetrating Penetrating Total pJ An-p Ap A

11C

0.227

0.588

0.815

20.3 m

397

13N

0.281

0.589

0.870

10.0 m

209

15O

0.415

0.589

1.004

2.07 m

50

18F

0.139

0.570

0.709

1.83 h

1868

90y

0.539

-

0.539

2.7 d

50295

99mTc

0.010

0.072

0.082

6.0 h

708

131I

0.109

0.219

0.328

8.05 d

91250

a derived from data in [3, 4]. The unit g is mass in grams.

a derived from data in [3, 4]. The unit g is mass in grams.

small-volume sources such as vials, syringes, or capsules containing typical "unit dosage" activities administered to a patient are illustrated in Fig. 12.1.

The superficial dose rates given for betas and electrons only do not allow for absorption in the source and walls of the container, and may substantially overestimate actual dose rates, however, they do indicate that skin and eye doses from open PET sources could be reduced significantly by interposing a barrier as thick as the maximum beta range (see Table 12.4).

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