Radiation Effects and Safety

Michael J. Chamberlain Introduction

The International Commission on Radiological Protection (ICRP) declares that "the primary aim of radiological protection is to provide an appropriate standard of protection for man without unduly limiting the beneficial practices giving rise to radiation exposure." Nuclear medicine professionals have a duty to fellow healthcare workers, patients and their families, research volunteers, the general public and the environment to ensure the safe and responsible handling of radioactive materials used diagnostically, therapeutically and in research. Radiation protection, the observance of these safe practices, is the responsibility of every nuclear medicine professional in conjunction with the local Radiation Safety Officer (RSO), Radiation Safety Committee, government regulatory agencies and scientific advisory groups (Table 1).

Radiation Dosimetry

Types of Radiation

As seen in Chapter 1, different types of radiation can be emitted from radioactive materials. Alpha particles are basically the nuclei of helium atoms and consist of two protons and two neutrons. Beta particles are either electrons (with a negative charge) or positrons (positive electrons). Energetic photons emitted during radioactive decay from the nucleus of an atom are termed gamma rays. They are identical to X-rays except for their origin. An X-ray originates from the electron shell of the atom while a gamma ray originates from its nucleus.

Energy emitted during the radioactive decay process interacts with the matter it encounters and is the basis for its detection, therapeutic effect and any biological hazard it poses. Alpha, beta (including positron), gamma and X-ray radiations emitted from radioactive materials are of sufficiently high energy to ionize the atoms and molecules which they encounter. These different forms of energy can be in the form of particles (alpha or beta) or electromagnetic radiation (gamma or X-ray) each with different abilities to penetrate animal tissue and shielding materials. Alpha radiation will penetrate less than one mm of tissue and an external source may be shielded by a sheet of cardboard or by the surface layer of the skin. Higher energy beta particles can penetrate up to 10 mm of tissue and may be blocked by a thin layer of metal or plastic. Due to this limited penetrance, alpha and beta emitters are only hazardous to human health if ingested, injected, inhaled or deposited on the skin and are not useful labels for imaging of internal organs. Conversely, gamma and X-ray radiations have the potential to penetrate more than a metre of tissue. These more penetrating

Nuclear Medicine, edited by William D. Leslie and I. David Greenberg. ©2003 Landes Bioscience.

Table 1. Radiation protection bodies

Regulatory agencies

Advisory groups

United States: Nuclear Regulatory Commission (NRC) Canada: Canadian Nuclear Safety Commission (CNSC)

United Kingdom: British Health & Safety Executive—

Nuclear Safety Directorate Europe: Commission of the European Communities (CEC)—Euratom Treaty

International Commission on Radiological Protection (ICRP)

National Council on Radiation protection and

Measurements (NCRP) United Nations Scientific Committee on the Effects of

Atomic Radiation (UNSCEAR) Committee on the Biological Effects of Ionizing Radiations (BEIR)

emissions, while suitable labels for external imaging of radionuclide distribution within the body, require relatively thick shielding with dense materials such as lead or concrete or they will otherwise pose an external radiation hazard.

Equal absorbed doses of different types of radiation do not produce equal biological effects. The relative biological effectiveness (RBE) of the different radioactive emissions is related to their energy and their tissue penetrance. Thus the RBE of alpha radiation is high because a large amount of energy is given up over a short distance, causing a dense cluster of ionizations and potentially irreparable DNA damage. The related concept, linear energy transfer (LET), describes the energy transferred to the absorbing medium per unit length of track. This can vary from 2 keV/^m for gamma-radiation used in nuclear medicine imaging to 2000 keV/^m for heavy charged particles. Alpha radiation is therefore termed "high LET" as compared with the "low LET" gamma radiation (Fig. 1).

Units of Radiation Exposure

Nuclear medicine uses units of measure which are unfamiliar to most non-physicists (Table 2). These are used to describe the amount of radioactive material (often known as its activity), how much energy this ionizing radiation imparts to a mass of irradiated material (absorbed dose) and the biological impact of an absorbed dose that considers the quality of the radiation (dose equivalent). The fact that an older system of measures (curie, rad, rem) and the newer Système International (becquerel, gray, sievert) are both in widespread use adds to the confusion.

For Nuclear Medicine purposes the becquerel is an inconveniently small unit and the curie is inconveniently large. Activities are therefore usually stated as mega-becquerels (MBq) or milli-curies (mCi). Absorbed dose is expressed in units of rad or gray (Gy) while dose equivalent is expressed as rem or sievert (Sv). Rad is an acronym for Radiation Absorbed Dose and rem is an acronym for Roentgen Equivalent Man.

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