Radiation protection

Ionizing radiation is used in nuclear medicine for diagnostic imaging and radionuclide therapy of benign and malignant conditions. To protect both the staff and the public, the exposure should be kept “As Low As Reasonably Achievable” (ALARA).

Entities and Units in Dosimetry for Radiation Protection

The fundamental entity in radiation dosimetry is the absorbed dose D (and dose rate dD/dt). This purely physical entity is defined as the energy E (unit joule, J) absorbed in a small (tissue) volume divided by the mass m (unit kg) of that volume. The unit of D therefore is J/kg, but it is given the special name Gray (Gy). The biological effects of radiation depend not only on the amount of energy deposited, but also on its distribution at the microscopic level, which in turn depends on patient specific biokinetics and characteristics of the radiation. Therefore, the absorbed dose is averaged over an organ or a tissue (T) and weighted by a radiation weighting factor WR to determine a parameter referred to as the equivalent dose (HT). The factor WR is equal to one for photons (X-ray and gamma) and for electrons (beta^- or beta⁺) which cover most applications in nuclear medicine. For alpha-particles a value of 20 is applied for the purposes of radiation protection. To distinguish the equivalent dose from absorbed dose, the unit of HT is sievert (Sv). To establish a single “risk parameter”, the values of H_T are finally weighted together using tissue factors W_T that reflect individual organs’ radiation sensitivity. The resulting quantity is the effective dose E (unit Sv), which is used when setting limits for whole body exposure. The precise definitions and the tables of W_R and W_T can be found in ICRP 103 and IAEA Handbook on Nuclear Medicine Physics[28,29]. The general quantities used in dosimetry are summarized in Table 1.

Effective dose is not directly measurable even in a uniform external field. For practical use in radiation protection, the following “operational” entities have been defined (details: see IAEA): Ambient dose equivalent H^*(10) (unit Sv) corresponding to equivalent dose in the depth of 10 mm in tissue (equivalent) material, and personal dose equivalent H(p)(d) (unit Sv) defined as the equivalent dose at a depth d in soft tissue below a specified point in the body. Relevant values of d are d=10 mm (used for estimating effective dose), d=0.07 mm (used for skin dose), and d=0.3 mm (for dose to the eye lens). These entities are listed here, because radiation protection equipment will normally be specified (calibrated) in terms of H^* or H(p)(d).

Rules and Regulations in Radiation Protection

The fundamental concepts of radiation protection are laid out by the ICRP in their recommendations, the most recent published in 2007[28]. Based on this, international (IAEA) and regional (EU) Basic Safety Standards (BSS) were issued[27,30]. The EU BSS-directive is currently being implemented in the national legislations of all EU-member states[27]. Despite European cooperation between authorities and attempts to harmonize, there will remain some differences between countries. Furthermore, local institutions may have their own (stronger) demands, and it is therefore recommended to always consider the local rules that may apply.

Two important issues of interest that already existed in previous directives, but may be interpreted differently in the new implementation, are: categorization of (exposed) workers and classification of workplaces. Workplaces must be divided into controlled areas with more restricted access and surveillance, and supervised areas with lower risk and greater accessibility. Workers are divided into two groups: Group A, those “liable to receive an effective dose greater than 6 mSv per year or an equivalent dose greater than 15 mSv per year for the lens of the eye or greater than 150 mSv per year for skin and extremities”, and group B, those “liable to exceed population limits”. It is possible to design nuclear medicine departments and plan the work so that it is highly unlikely that staff (including technologists, physicians, or medical physicists) would exceed the dose limits listed above. However, routine monitoring of (most) staff is necessary.

Actions for Radiation Protection

To reduce the exposure according to the ALARA principle for the patient, unnecessary and inadequate examination or treatment must be avoided. The underlying principles of the use of ionizing radiation are therefore ‘justification’ and ‘optimization’. Reduction of the exposure to staff and to the public is achieved based on the following basic actions:

1. Reduce the time spent exposed to the radiation;
2. Increase the distance to the source;
3. Use shielding to attenuate the radiation.

The importance and implementation of the corresponding actions depends on the radionuclide and level of activity administered. Note that contamination and incorporation of radioactive materials makes it impossible or difficult (item 1) to perform these actions and thus must be prevented.

Table 1. Quantities and units in radiation measurement and protection