RADIATION PROTECTION WHEN WORKING WITH THE SOURCE OF IONIZING RADIATION IN NUCLEAR MEDICINE

Syllabus of questions and answers from seminars on radiation protection at the
Department of Nuclear Medicine University Hospital in Ostrava

What is radioactivity and ionizing radiation?

Radioactivity is a physical process in which the spontaneous transformation (decay) of atomic nuclei occurs  , while ionizing radiation is emitted.

Ionizing radiation (IR) is such radiation whose quantum has such a high energy that they are able to eject electrons from atoms and thus cause ionization in the material environment.

What types of radioactivity and ionizing radiation are most common?

Radioactivity a :

It occurs only in the heaviest uranium and transuranium nuclei. Upon decay, the particle a , which is the nucleus of helium ⁴ He ₂ , is emitted . This heavy particle , with two positive elementary charges, strongly ionizes the substance, which slows it down quickly and therefore has a very short range in the substance environment - alpha radioactivity has no significance for the field of nuclear medicine.

Radioactivity b : It is the most common type of radioactivity.

With radioactivity b ^- a particle b ^- flies out of the nucleus , which is an electron e ^- (created in the nucleus by the conversion of a neutron into a proton, an electron and a neutrino). The resulting b- radiation, with its electrical effects, ionizes the substance, thereby slowing it down and has a relatively short range in the substance environment (approx. 3-4 mm in the tissue). Note: When braking rapidly or changing the direction of radiation b in the field of atomic nuclei (especially heavy nuclei), electromagnetic braking radiation having the character of gamma radiation is generated .

When radioactivity b ⁺ kernel takes off particles b ⁺ , which is a positron e ⁺ (generated in the core during the conversion "excess" of a proton to a neutron and positron neutrino). Ionizing radiation effects b ⁺ and penetration in matter are similar to the rays b ^- but after braking occurs  annihilation of positron e ⁺ with an electron e ^- to produce two photon annihilation radiation grams high energy of 511 keV, which of the place of origin fly out in opposite directions (at an angle of 180 ° ) - used in the scintigraphic method Positron Emission Tomography (PET).

The beta radioactivity group also includes a somewhat less common electron capture , in which an "excess" proton in the nucleus "pulls" an electron from the nearest path in the envelope and combines with it to form a neutron (and a flying neutron). In this case, no corpuscular radiation is emitted from the atomic nucleus, but the electron from the higher shell immediately jumps to the missing position after the electron on the K-shell, while the characteristic X-rays are emitted.

Radiation g :

After radioactive transformation (alpha and beta), the daughter nucleus is usually formed in an excited state (schematically in the figure). When deexcitation ( "bursting") in the excited levels in atomic nuclei formed hard (high) electromagnetic radiation, which is called gamma .

Scheme of radioactive transformation (alpha or beta) of the parent nucleus A first into an excited daughter nucleus B´, which then deexcites to the nucleus B in the ground state by radiating an energy difference in the form of a gamma radiation photon

Photons of electromagnetic radiation g do not carry an electric charge, so they cause ionization not directly, but indirectly - they interact with matter either by photoeffect , Compton scattering or the formation of electron-positron pairs. This creates fast-moving electrons (at high energies and possibly positrons), which already ionize the substance directly with their electrical effects.

Radiation g is usually emitted immediately after radioactive decay. Thus, most radionuclides are mixed emitters b - g (as seen in the figure) or a - g , although pure emitters b (eg ³ H or ¹⁴ C) or alpha are also present. Pure gamma emitters do not occur in nature, but in the case of the so-called metastable excited level of the daughter nucleus, these nuclei can be separated from the mixture and artificially produced pure g emitter - an example is the radionuclide ^99m Tc, which is based on current nuclear medicine.

X-ray :

Ionizing radiation also includes X-rays (X) radiation, which is relatively hard electromagnetic radiation generated both in X-ray tubes (as braking radiation when impacted by high voltage accelerated electrons on the anode), and when electrons jump to internal levels in atomic shells (characteristic X -radiation).

Other types of ionizing radiation, such as neutron radiation (generated, for example, in nuclear reactors) or proton radiation (accelerators, cosmic radiation), do not occur in nuclear medicine.

What is a closed and open radioactive source ?

A sealed emitter is a radioactive emitter whose construction ensures (tested and certified) tightness and excludes the release of radioactive substances into the environment under the anticipated conditions of use and wear. In nuclear medicine, closed emitters are only various standards for the calibration of measuring instruments.

An open emitter does not meet these conditions for a closed emitter - they are mainly radioactive solutions, gases, aerosols, powders, etc. In nuclear medicine, all radioactive preparations for in vivo and in vitro examination are open emitters.

According to the severity of radiation risk, sources of ionizing radiation are further divided into 5 categories: insignificant sources (eg small closed standards for spectrometric calibration), small sources (such as stronger closed emitters and low open activities), simple sources (eg X-ray diagnostic equipment). and defectoscopy equipment), significant sources (eg closed emitters for radiotherapy, accelerators, highly active open emitters) and finally very significant sources (such as nuclear reactors or radionuclide production equipment).

How is the amount of radioactive substance and its unit defined ? What is the half-life?

The activity of a radioactive substance is defined by the average number of nuclear transformations (decays) in the source per unit time, regardless of the physical or chemical form of this substance .

The unit of activity is 1 Becquerel (1 Bq), which is 1 decay per 1 second. This unit is very low and therefore its decimal multiples are used - kilobecquerel (1 kBq = 1000 Bq), megabecquerel (1 Mbq = 1,000,000 Bq) and gigabecquerel (1 Gbq = 10 ⁹ Bq).

The time course of radioactive decay is exponential and its rate is very different for different radionuclides. The rate of radioactive decay of a pure radionuclide is expressed by the half-life (transformation) T1 / 2, which is the time during which just half of a given number of nuclei of the relevant radionuclide decays . After the next half-life, half of the remaining half decays again (so only 1/4 of the original amount remains), etc. The half-life is completely characteristic for each radionuclide - technetium ^99m Tc with a half-life of T1 / 2 = 6 is most often used in nuclear medicine. hours and ¹³¹ I with a half-life of T1 / 2 = 8 days.

How does ionizing radiation affect a living organism? What are stochastic and deterministic effects?

According to the older so-called "intervention" theory , the effect occurs when a particle or photon directly hits a molecule of a biologically active substance (eg DNA) and thus damages it. We now know that this mechanism is only of secondary importance, as the probability of such direct interventions is relatively low.

The main mechanism of the effect of ionizing radiation on the organism is explained by the so-called radical theory . It is based on the fact that each organism is composed mainly of  water in which biologically active substances are dispersed. The interaction of radiation with living tissue will therefore take place mainly on water molecules. Due to ionization, radiolysis of water will occur  , while also highly reactive free radicals H ⁺ and OH ^- are formed ^.. These free radicals then attack molecules of biologically active substances and chemically affect or destroy them. The result is a number of harmful changes, many of which may be corrected by the body's repair mechanisms, but some changes (eg in the DNA code) may be permanent or reproducible. On the effects of ionizing radiation are particularly vulnerable tissue with an intense m dividing cells, such as e.g. hematopoietic or tumor, a developing fetus (especially in the early stages of development).

If the radiation dose is not large, with the vast majority of damage to biologically active substances, the organism will successfully cope with its repair mechanisms. However, even at low doses, there is a certain probability that some damage will not be repaired and late permanent consequences of a genetic or tumor nature will occur. Because such consequences are completely accidental, individual, and unpredictable , they are called stochastic effects .

At high doses of radiation, the number of damaged molecules of biologically active substances is already so high that the organism is not able to completely repair them - some of the cells die, radiation disease arises. The tissue damage here is directly proportional to the received radiation dose, it is no longer accidental, on the contrary, it is predictable - we are talking about deterministic effects .

The basic goal of radiation protection is therefore to eliminate deterministic effects of radiation (we mean side effects here, in contrast to therapeutic effects, eg in radiotherapy) and to reduce the occurrence of stochastic effects to a minimum .

Basic methods of radiation protection

The radiation dose received is determined by several basic factors: the radioactivity we work with, the type and energy of the radiation emitted, the exposure time and the geometric conditions (distance, shielding). We have 4 basic ways of radiation protection:

Time : the dose received is directly proportional to the exposure time, so we do not stay in an area with ionizing radiation for an unnecessarily long time, and work with radioactive substances must be carefully prepared and carried out as quickly as possible.

Distance : the intensity of the radiation and thus the dose rate are inversely proportional to the square of the distance from the radiation source (this applies exactly to the point source). It is therefore necessary to stay as far away from radiation sources (ie also from patients with applied activity), when working with radiators it is useful to keep them as far away from the body and possibly. use suitable manipulators, tweezers, etc.

Shielding : A very effective protection is the shielding of radiation with a suitable absorbing material. For gamma radiation, these are materials with a high specific weight - especially lead , from building materials then concrete with or. admixture of barite and the like. Lead containers are used for the transport and storage of radiators, lead sheet metal screens, shaped lead bricks, etc.

The thickness of the shield required depends on the density (and nucleon number) of the shielding material, the radiation energy g and the attenuation required. The tables are sometimes called states values. Polovrstvy absorption, which is a thickness of the shielding material which reduces the intensity of the radiation by half (2 1/4 polovrstvy then 3 polovrstvy to ¹ / ₈ etc. - shielding effect increases exponentially with the thickness shielding).

Light materials (such as plexiglass) are sufficient to shield radiation b , preferably in combination with a subsequent thin layer of lead to shield the braking electromagnetic radiation.

Prevention of contamination : The risk of external irradiation in workplaces with open emitters is approached by the risk of contamination with radioactive substances - there may be both  surface contamination of the body and  internal contamination . Internal contamination is the most dangerous, because the organism is exposed to radiation for a long time and "from the inside" - the radionuclide enters the metabolism and, depending on its chemical nature, can accumulate in certain "target" organs, which are then directly exposed to radiation. Internal contamination can occur through the digestive tract, respiratory tract, or skin penetration. To prevent contamination, it is therefore necessary to follow the rules hy gdo not eat in the controlled zone, use protective gloves, work with volatile radioactive substances in the fume hood, etc. .......

How is radiation protection organized and legislatively ensured when working with ionizing radiation?

Everyone who uses sources of ionizing radiation is obliged, within the limits of their competence, to take all necessary measures to  protect the health of themselves, their co-workers and other persons.

The basic legislative framework for working with ionizing radiation is currently the so-called “ Atomic Act ” (Act No. 18/1997 on the Peaceful Use of Nuclear Energy and Ionizing Radiation) and related standards and regulations. It is primarily the SÚJB Decree No. 184/1997 - amended by the SÚJB Decree No. 307/2002 and finally by the SÚJB Decree No. 499/2005, then the SÚJB Decree No. 146/1997 and SÚJB No. 214/1997. The Atomic Act establishes the most general rules for working with ionizing radiation from j currencies are important objectives of radiation protection - exclusion of deterministic effects and limiting stochastic effects to a minimum, principles of working with IT - the justification of activities (risk versus profit), optimization (human exposure versus costs of its reduction), l imitigation (natural resources, medical exposures ...).

The State Institute of Nuclear Safety ( SÚJB ) was established to supervise and coordinate the entire set of measures for the safe use of ionizing radiation sources . In addition to legislative activities, SÚJB assesses projects of workplaces with sources of ionizing radiation, issues relevant permits and performs inspection activities at these workplaces.

In addition, a supervisory officer is set up at each workplace with ionizing radiation , who deals with radiation protection issues on the spot and keeps the relevant documentation. The supervisor participates in courses and seminars organized by SÚJB and other organizations and professional societies.

At the larger workplaces of nuclear medicine (such as KNM FNsP Ostrava), a Technical and Physical Department ( TFÚ ) has been established, which, together with other physical and technical issues of the workplace, also provides a radiation protection methodology from a professional point of view.

A set of main principles, measures and methodology of measuring procedures to ensure the optimal level of radiation protection at a particular workplace are written in the so-called Workplace Monitoring Program (what is measured, how often it is measured, where it is measured, how and what it is measured, interpretation of results measurement). Part of the monitoring program is the determination of reference levels - recording, investigation, intervention.

Another related material is the Quality Assurance Program for diagnostic and therapeutic activities of the workplace of nuclear medicine, which is a set of control and adjustment activities to ensure the proper functioning of devices and the required quality of radiopharmaceuticals; this is a condition for accurate and reliable measurement and examination results. The issue of protection against radiation that is linked by optimizing the benefits and risks, application of ionizing radiation: what is more valid in ý results of diagnosis and better therapeutic effects, the more prevalent health risk profit patients over the adverse effects of ionizing radiation - and vice versa.

The set of measures, including decontamination procedures and control measurements in the event of radiation accidents and other extraordinary events at the workplace, are summarized in the Workplace Emergency Code . The   Operating Rules of the workplace also contain a number of specific principles for correct and safe work with sources of ionizing radiation.

What is the absorbed dose of ionizing radiation and what are its units?

The absorbed dose of ionizing radiation is the amount of ionizing radiation energy absorbed by a unit of mass of the irradiated substance at the site in question. The unit is 1 Gray (Gy), representing the absorbed radiation energy of 1 Joule per kilogram of substance. As it is too high a value, decimal parts are used in practice - miligray (1 mGy = 10 ^-3 Gy) and microgray (1 m Gy = 10 ^-6 Gy).

The size of the dose is directly proportional to the intensity of radiation and the time of exposure, it also depends on the type and energy of radiation and the composition (especially the density) of the irradiated substance.

The absorbed dose is directly proportional to the number of ions, free radicals formed and thus the risk and extent of damage to biologically active substances in the body.

Since the biological efficiency of different types of radiation may differ, a so-called quality factor Q (also called “radiation weighting factor” or “relative biological efficiency”) is introduced for each radiation , indicating how many times a given type of radiation is more biologically effective than photon radiation. The value of the quality factor Q depends on the type and energy of radiation: for radiation X, g and b is Q = 1, for neutrons Q is » 2 (slow neutrons) to 10 (fast neutrons), for radiation a is Q » 20. For more objective assessment of the effect of radiation (especially late stochastic effects), the absorbed dose is multiplied by the value of the quality factor, thus creating the so-called dose equivalent(equivalent dose), the unit of which is 1 Sievert (Sv). Dose 1 S of any radiation has the same biological effects as a dose of 1 Gy X-ray or gamma radiation (for which the quality factor is equal to 1). To assess the long-term effects of internal contamination by a radioactive substance - radiotoxicity - a so-called dose-line is introduced , which is the absorbed dose of ionizing radiation caused by a given radioactive substance in a certain organ or tissue for 50 years from its intake. Radiotoxicity depends not only on the physical parameters of the radionuclide (half-life, type and energy of radiation), but also on the chemical properties of the contaminant, which determine its metabolism, distribution to individual organs .biological half-life, route of excretion.

How are radiation doses limited for workers with ionizing radiation? What is protective dosimetry?

Any dose of ionizing radiation can be associated with a certain risk of harmful effects, so care must be taken to keep the doses as low as possible. For the purpose of assessing and controlling radiation exposure, certain dose limits have been set for a quarter, a year and 5 years - limits (maximum permissible doses) for workers with ionizing radiation sources, which are still associated with a very low probability of radiation damage.

The current value of the annual limit for workers is 50 mSv , the five-year limit is 100 mSv . Dose limit values, including recording, examination and intervention levels, are given, for example, in the “Monitoring program ”. The basic limits for the rest of the population are set at 1 mSv / year.

Protective dosimetry deals with the measurement, evaluation and regulation of doses of ionizing radiation in workers . The determination of doses of ionizing radiation in individual workers is based primarily on personal dosimeters (film or thermoluminescent) of ionizing radiation, which determine (more or less accurately) the whole-body dose from external radiation sources. Personal dosimeters are worn by workers attached at a reference point (on the chest - a representative place for whole-body irradiation, or finger dosimeters for determining the doses on the hand when handling radioactive substances) throughout the stay in the controlled zone. At defined intervals (currently 3 months) are dozimet r y exchanged and sent to evaluation of benefits at the workplace of the National Personal Dosimetry Service in Prague. In the event of a radiation accident and a suspicion of receiving a high dose, a personal dosimeter can be sent for evaluation immediately . For immediate reading of doses, measurement of dose rate and optimization of work procedures from the point of view of radiation protection, hand-held intimeters calibrated in dose units (mSv) are available at the workplace.

In workplaces with unsealed sources are also to be checked and contamination working Iku and working environment (see below).

What is a controlled zone and how is it defined in the workplace of nuclear medicine?

The controlled zone is called those areas of the workplace where radioactive substances (or other sources of ionizing radiation) are handled and where the regime of protection of persons against ionizing radiation must be observed . *)

In general, the entire building of a nuclear medicine workplace is considered a controlled area, but certain areas such as changing rooms, study rooms, living rooms, inactive laboratories, relevant corridors and staircases are excluded.

Entrances to the controlled area must be marked with warning signs. Only employees of the Department of Nuclear Medicine have free access to it, other persons only with the permission of the head of the relevant workplace and their stay is registered.

*) Decree No. 184 SÚJB, §35, specifies: “The controlled zone is defined wherever it is expected that during normal operation or with foreseeable deviations from normal operation, the exposure could exceed three tenths of the basic limits for workers” .

Arrangement and equipment of workplaces with sources of ionizing radiation

The construction, layout and equipment of the workplace must be carried out in such a way as to ensure adequate protection of workers, other persons and the environment. In the event of an accident, it must be possible to decontaminate people and the workplace as quickly and efficiently as possible. Projects and eligibility of workplaces for ionizing radiation are approved by SÚJB employees.

Workplaces are divided according to whether they are designed to work with closed emitters (such as radiological or radiotherapy workplaces) or with open emitters. Workplaces with open emitters are divided into 3 categories according to the processed activities (Decree No. 184 SÚJB, §6, §40). Workplace I. Category are for working with low activities Radion cleaning with low radiotoxicity (minor source IZ) and after the construction or equipment is no different from chemical laboratories. Category II workplaces process medium activities of open radionuclides, have a controlled zone and are equipped with protective aids incl. hoods, or separate sewerage of active waste.

The workplace of the 3rd category is intended for the most demanding work even with high activities of open radionuclides. Therefore, considerable requirements are placed on its construction and equipment in order to ensure the fastest and most effective cleaning in the event of contamination. There should be 3 types of rooms in the controlled area: for demanding work with high activities, for routine laboratory work and measuring rooms. In addition, a special room or to the rectory for storage of radionuclides and radioactive wastes. Floors and walls of laboratories must be smooth and washable, floors are further sloped and provided with waste. Intensive ventilation with active air filtration with an outlet above the roof should also be provided .Liquid radioactive waste is led to extinction tanks. The workplace must be equipped with suitable shielding, manipulators, fume hoods and devices for protective dosimetry. Controlled area is separated from other areas hygienic loops to measure c COMES device and a washroom.

What are the main principles for collecting and handling open emitters?

The collection and use of radioactive emitters requires a SÚJB permit, provided that their safe transport, storage and use is ensured. Radioactive emitters must be transported in strong packaging, ensuring protection of the environment from radiation and contamination, marked with a warning symbol and accompanied by a certificate of radioactive substance. Reception, storage and dilution of large activities is performed in a properly equipped room. All preparations and samples containing radionuclides must be transported at the work place properly labeled appropriately shielded (e.g. stored in a lead pot) and secured against uncontrolled release of radioactivity (closed above by on a tray). All handling of open radionuclides can only be performed in the controlled zone with appropriate safety precautions.

What is radioactive contamination and how to deal with it?

When handling open radioactive substances, their leakage and subsequent contamination (contamination) of objects, the working environment and persons with these radioactive substances can occur .

Surface contamination of work surfaces, aids or people most often occurs   . For continuous control of surface contamination during and after work, radiometers with large-area probes are used, which should be located in all exposed workplaces and in hygienic loops. A sensitive method of contamination control is also the method of swabs , where we wipe off with a cotton swab dipped in a suitable solvent (alcohol gasoline) or contamination from a defined area of ??the exposed site and then measure it in a test tube with a well scintillation detector.

For workers with higher activities of open emitters, it is also necessary to examine whether there has been  internal contamination . This is done by measuring gamma radiation using a sensitive scintillation detector over critical (target) organs. In ¹³¹ I it is the thyroid gland, so in workplaces performing thyroid therapy with this radionuclide, it is necessary to periodically measure the activity of the thyroid gland in all workers involved in these therapies.

How to proceed in the event of a radiation accident at a nuclear medicine workplace?

A radiation accident at a workplace with open emitters means an uncontrolled leakage of a radioactive substance into the working environment (eg by spilling or spraying a radioactive solution) with subsequent contamination of the working environment or workers.

In the event of contamination of the working environment , the worker is obliged to prevent the spread of contamination, mark a visibly contaminated area, report this incident to the manager or supervisor and, under his guidance, cooperate in decontamination . When decontaminating, it is necessary to first aspirate as much of the active liquid as possible with filter paper or pulp and then wash and wipe the contaminated area with a suitable cleaning or decontamination agent. The generated waste must be stored in plastic bags and contaminated objects decontaminated or stored in plastic bags for radiation. Contaminated water must be poured into the waste connected to the extinction sumps. The effectiveness of decontamination is continuously checked by measuring with a radiometer. If the activity cannot be completely eliminated, the site should be marked and covered with protective paper or foil; the manager will then decide on the next steps and resume operation.

In the event of personal contamination , the worker must remove contaminated clothing or protective equipment, check the contamination of the body surface and, if necessary, clean it by washing or showering . It is also necessary to check whether the worker has been internally contaminated. In cases of suspected internal contamination and exceeding the maximum permissible dose of radiation is necessary to take the necessary health measures in cooperation with SÚJB and sanitary authorities, including d očasného removing the worker from the environment with ionizing radiation.

Protection of patients during diagnostic and therapeutic procedures in nuclear medicine

Radiation protection of patients is based on the basic ethical requirement that the risk of radiation damage during diagnostic or therapeutic procedures be balanced (or better, if possible, outweighed) by the expected health benefits for the patient.

When diagnosing  in nuclear medicine, it is therefore necessary to apply such a necessary amount of radioactive substance (required quality and purity) that guarantees sufficient diagnostic information with the lowest possible radiation exposure of the patient. In pregnant women , radiation-related radiodiagnostic procedures should be performed only when absolutely necessary, choosing the most gentle methods possible with regard to fetal protection . To optimize the amount of applied radioactivity of various radiopharmaceuticals for individual examination methods, tables of guideline values were issued, which also enable the recalculation of the applied activity for individual patients (even non-standard ones - eg children, overweight people, etc.).

In therapeutic applications of radionuclides, a precisely determined amount of radioactivity is applied - it is determined either according to verified empirical formulas or as a lump sum according to the given diagnosis and the desired therapeutic effect.

The activity of each radioactive substance administered to a patient (especially therapeutic applications) must be measured on a properly calibrated and metrologically verified activity meter. The value of the applied activity must be recorded in the diagnostics or therapy documentation .

What is radioactive waste and how is it disposed of and disposed of?

Radioactive waste is waste generated using radiation sources that contains radioactive substances. This waste can be solid, liquid or gaseous.

Solid rad ioaktivní wastes are collected in appropriately labeled plastic bags, classed according to the activities and half-lives decay in the room where they are stored until they naturally decay of activity falls below a predetermined level. At appropriate intervals , they are measured and disposed of as no longer active waste (eg by incineration). Long-term radioactive waste, which would have to be stored in a central repository, does not occur at nuclear medicine workplaces (used Mo-Tc generators are returned to the supplier).

Liquid radioactive waste, especially ¹³¹ I, is led (separately from the inactive sewer) to the extinction tanks , from where it is discharged into the sewer or treatment plant only after sufficient decomposition (approx. 60 days) - it is necessary to comply with the limit value for wastewater activity from the institute. is currently 450 Bq / liter.

Monitoring program

Quality control program

Emergency and decontamination regulations

From work or:

RNDr. Vojtěch Ullmann, physicist
Zdenek Puchálková, Ludmila Ullmann
technical-physical section KNM

Vojtech Ullmann