**3.2 The need for assessment of biological risk**

Although up to the middle of the last century, practically only the physical quantities of radiation were used for assessing the harm caused by radiation exposure to persons, it was felt that for this purpose, another set of quantities had to be introduced. Such quantities were supposed to reflect biological effects regardless of the type of radiation and irradiation geometry. This was why several weighting factors were adopted to convert pure physical quantities into quantities, which would be better related to the biological response of the exposed human body to the most common types of radiation under typical exposure conditions. The values of applicable weighting factors were derived from the investigation of some radiation accidents and incidents, and especially from extensive epidemiological studies, including those carried out on the survivors of the atomic bombing in Hiroshima and Nagasaki. Of course, these data have never been considered final since more studies led to more relevant and reliable results of the weighting factors. This was why even throughout the last few decades, there had been certain biological quantities, which serve for the radiation risk assessment used for the control of radiation exposure in order to implement the basic requirements and philosophy of radiation protection known as *justification, limitation,* and *optimization*.

As mentioned above, for the assessment of the health risk related to exposure to radiation, other types of quantities should be used. These quantities are based on specific dosimetry quantities weighted by appropriate factors in order to reflect stochastic or deterministic biological effects.

Stochastic (probabilistic) effects are random phenomena and manifest as mutations of cells and not their death. It has been found that there is no threshold dose for these effects. This concept is known as *linear no threshold model.* In most cases, any cell mutations caused by ionizing radiation will be eliminated by the body's defense; however, when this does not occur, the mutations can induce cancers (**Figure 6**).

At higher doses, the deterministic effects (tissue reactions) take place. These are known as the biological effects, which are manifested after the dose exceeds the socalled threshold level. It is not the same for all organs; the susceptibility of cells to radiation damage is described by the term radiosensitivity.

*Basic Radiation Protection for the Safe Use of Radiation and Nuclear Technologies DOI: http://dx.doi.org/10.5772/intechopen.108379*

*Radiation-induced carcinogenesis occurs following interaction with ionizing radiation that leads to cell mutation (based on [6]).*

**Figure 7.** *An overview of biological consequences of radiation effects.*

The individual categories of radiation-induced biological effects are summarized in **Figure 7**.

## **3.3 Quantities reflecting stochastic effects**

Such quantities could be used only for relatively small doses where only probabilistic effects are expected.

The most frequently used quantities for this purpose include dose equivalent, equivalent dose, effective dose, committed effective dose, and specific operational quantities (introduced for external exposure only) approximating main radiation protection quantities.

One of the earlier quantities in radiation protection introduced for this purpose was the *dose equivalent (H)* defined at the point of interest in tissue as

$$H = D \cdot Q\_F \tag{4}$$

where *D* is the absorbed dose, and *QF* is the mean value of the quality factor for the specific radiation at this point. The unit of the dose equivalent is sievert (Sv), which corresponds to J.kg�<sup>1</sup> (multiplied by *QF*). The coefficient *QF* is one of those weighting factors mentioned above.

Since the dose equivalent is a point quantity, it itself has limited practical applications with the exception of its use in the definitions of so-called *operational quantities* (to be discussed later). More useful are the following main radiation protection quantities, namely the *equivalent dose* and the *effective dose.*

The first of these quantities *(HT)* is defined by the summation of the average of doses (*DT,R*) in a tissue or organ T caused by radiations of type R multiplied by the relevant radiation weighting factors (*wR*). This quantity is quantified by the unit Sv (sievert and is defined by the expression

$$H\_T = \sum\_R w\_R \cdot D\_{T,R} \tag{5}$$

While the equivalent dose represents the health effects in individual tissues or organs, the *effective dose (E)* is a measure of radiation exposure to the whole body, which may be exposed to radiation inhomogeneously, and various sensitivities should be taken into account. This is done by so-called *tissue weighting factors (wT)* recommended by ICRP [7, 8].

The effective dose (E) is the main quantity in radiation protection for the assessment of biological effects at low doses. It has been defined *only for stochastic effects*. The definition of the effective dose can be written in the form

$$E = \sum\_{T} w\_{T} \sum\_{R} w\_{R} \cdot D\_{T,R} \tag{6}$$

here *wT* is the tissue weighting factor, *wR* is the radiation weighting factor and *DT,R.* The unit of this quantity is sievert (Sv); more often, however, units such as mSv or μSv are used. The factor *wR* is related to the Linear Energy Transfer (LET), which reflects the average amount of energy transferred per unit of distance traveled). The values of LET are usually expressed in units of keV/μm. The values of *wR* for some radiations are as follows: low-LET radiation (photons, electrons, muons), 1; protons and charged pions, 2; and alpha particles, fission fragments, and heavy ions, 20. For neutrons, this factor depends on the energy [7, 8].

The LET values for some radiation are given in **Table 1**. The definition of LET is related to charged particles in any medium. As indirectly ionizing radiation, as gammas or X-rays, this quantity is associated with the secondary charged particle released by the interaction of indirectly ionizing radiation.

There is some relation between the LET and the Relative Biological Effectiveness (RBE). They both are important terms in radiation biology and reflect the relative damage that will occur under different circumstances. As LET increases, more energetic electrons are deposited closely together and thus, damage to DNA is more likely.

Since the LET is strictly speaking defined only for charged particles, its values for uncharged particles (photons and neutrons) are related to the secondary charged particles formed by this indirectly ionizing radiation.

*Basic Radiation Protection for the Safe Use of Radiation and Nuclear Technologies DOI: http://dx.doi.org/10.5772/intechopen.108379*


#### **Table 1.**

*The LET values of various types of radiations (based on [7]).*


#### **Table 2.**

*The* wT *values of various types of radiations (based on [8]).*

The weighting factor *wT* for calculating the effective dose represents a relative measure of the risk of stochastic effects that might result from exposure of a specific tissue T. It takes into account the variable radiosensitivities of organs and tissues in the body affected by radiation. The *wT* values for main tissues are shown in **Table 2**.

The remainder tissues include some 13 tissues that are significantly exposed. They comprise the following tissues: adrenals, extrathoracic region, gall bladder, heart, kidneys, lymph nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, uterus/cervix.

Both the abovementioned quantities can be related to the unit of time as the equivalent dose rate and effective dose rate where the same units, Sv.s<sup>1</sup> are used. More practical are widely used units such as mSv.h<sup>1</sup> or, in the case of the effective dose, even mSv.y<sup>1</sup> .

Both the equivalent dose and effective dose can be used to assess stochastic effects due to the external radiation as well as internal radiation emitted by radionuclides, which entered the body and exposed its tissues and organs from inside. The overall risk attributed to the component related to the internal radioactive contamination can be assessed by the quantities *committed equivalent dose –* HT(τ) and *committed effective dose*.

The *committed equivalent dose* represents the sum of the equivalent doses received in a particular tissue or organ of a person due to the intake of radionuclides during the period of τ, which is 50 years for adults or 70 years for children. This refers explicitly to the dose in a specific tissue or organ, in a similar way to the external equivalent dose. This quantity reflects the contribution of the internal exposure to the total equivalent dose. The committed equivalent dose *HT(τ)* in a tissue or organ *T* is defined by

$$H\_T(\mathbf{r}) = \int\_{t\_0}^{t\_0 + \tau} H\_T(t)dt\tag{7}$$

The c*ommitted effective dose –* E(τ), is the sum of the products of the equivalent dose a tissue or organ, T, received from the intake of radioactive materials by inhalation and ingestion, and the appropriate tissue weighting factors, wT, as shown in the following formula:

$$E(\mathbf{r}) = \sum\_{T} w\_{T} H\_{T}(\mathbf{r}) \tag{8}$$

The integration time τ follows the intake at time t0. Since the radiation weighting factor is considered to be a dimensionless factor, the unit of both the equivalent dose and committed equivalent dose is Sv (provided the dose is in Gy).

The quantity E(τ) is used rather rarely: only in the case of working with unsealed radioactive sources or an accident, which resulted in the release of substantial radioactive material contaminating the surrounding area. This may affect persons present especially by the inhalation of contaminated air.

Since the main radiation protection quantities mentioned above cannot be directly measured or monitored, specially defined quantities for assessing the risk due to external exposure have been introduced to assess this risk by means of measurable quantities. Such a set of so-called *operational quantities* have been introduced by the International Commission for Radiological Units and Measurements (ICRU) [9]. These quantities can provide an estimate or upper limit for the value of the protection quantities related to the external exposure or potential exposure of persons. They are characterized as follows:


An overview of operational quantities is presented in **Table 3**. The basic unit of all operational quantities is Sv.

**Figure 8** illustrates the position and the role of operational quantities in relation to physical quantities and radiation protection quantities. It should be noted that while operational quantities can apply only for the assessment of the exposure due to external radiation, radiation quantities represent general quantities for the quantification of the exposure resulting from both external radiation and internal exposure caused by the intake of radioactive material.


**Table 3.**

*Operational quantities proposed for dose monitoring of external exposure.*

#### **Figure 8.**

*Relationship between quantities used in radiation protection.*

From physical quantities (exposure, kerma, dose), one can move to operational quantities using the quality factor *Q(L)* and to protection quantities through radiation weighting factor (*wR)* and tissue weighting factor (*wT*). The relation between operational and protection quantities is obtained based on measurement and calculation.

#### **3.4 Quantifies for assessment of deterministic harm**

While the quantities and units for the assessment of *stochastic effects* are well elaborated and defined, this is not the case with regard to deterministic effects. Quantities aimed at the estimation of stochastic effects include both the potential harm in selected individual organs (equivalent dose) and the health impact of the irradiation of the whole body, where contributions from the exposure of individual organs are taken into account (effective dose). The stochastic effects are of primary interest at low exposure, where there are no visible signs of the reaction of tissues or organs exposed. At sufficiently higher doses where the damage caused by radiation is apparent, more interest should be paid to *deterministic effects.*

Deterministic effects (nonstochastic effects, tissue reactions) are characterized by a threshold dose that must be exceeded for effect to occur. The severity of

deterministic effects increases with dose, which could result in such harms as cataracts, erythema, and sterility. The main role of radiation protection consists of keeping radiation exposure not only below the established dose limits to avoid the deterministic effects but ensure that the doses and radioactive contamination are as low as possible to achieve under the circumstances taking into account all possible specific conditions, including economic factors.

While for the assessment of stochastic effects, several quantities were defined, there has not been developed a similar approach to quantify deterministic effects [10, 11]. At present, a concept based on the RBE (Relative Biological Effectiveness) is being introduced. The relevant quantity, RBE-weighted dose (or, in short, RBE dose), is applied for this purpose [7, 12].

The RBE represents the relative absorbed dose of reference radiation (usually 250 kVp X-rays or cobalt-60 gamma rays) required to produce the same magnitude of the similar effect as the absorbed dose of the radiation in question (RBE >1 indicates that the radiation is more effective than the reference radiation). This factor is influenced by both the biological effects (cell killing, cell survival with mutations) and the LET of the radiation.

It looks like under present circumstances, the best way to call the main quantity for the assessment of the risk associated with the deterministic effects in terms of the *RBE dose* defined as

$$\text{RBE dose} = \text{RBE} \times \text{D} \tag{9}$$

with the unit Gy-Eq (gray equivalent). Therefore, a dose in Gy-Eq is the absorbed dose in Gy multiplied by a recommended RBE, which takes into account that ionizing radiation of different types and energies affects living organisms differently. The values of the RBE for some typical radiation are given in **Table 4**.

In this context, the RBE is analogous to the weighting factor *wR* used to define the equivalent dose, except that in this case, the RBE is a measured quantity for a specific deterministic endpoint. In this regard, there is no equivalent to the effective dose in the case of high exposure of many tissues or organs in the body. Although the term *RBE dose* would be an appropriate choice for the quantity expressing the harm following high exposure, it is still not widely used.

There are still some inconsistencies in using units for effective dose (Sv) and RBE-dose (Gy-Eq). In some cases, the unit Sv is also wrongly used for the assessment of deterministic effects.


**Table 4.**

*The RBE values for individual types of radiation (based on [8]).*

*Basic Radiation Protection for the Safe Use of Radiation and Nuclear Technologies DOI: http://dx.doi.org/10.5772/intechopen.108379*

#### **3.5 Contributions from external and internal exposure**

In general, radiation protection mechanisms have to provide adequate protection of persons against both external and internal exposure. The total exposure can be presented as a sum of the contribution from radiation incident on the surface of the body as well as radiation emitted by radionuclides, which enter the body through inhalation or ingestion and exposes the tissues from inside.

In order to control external radiation sources, some specific protective measures have to be in place. The radiation situation, including its impact on persons, is evaluated by appropriate quantities and other parameters characterizing the potential of the source, intensity of radiation field, and finally, the exposure of the affected person using appropriate quantities and units. The source is usually described by activity (number of radioactive decays per second) or emission (number of particles or photons emitted by the source in 1 second). The situation is illustrated in **Figure 9**.

In an analogous way, we may also characterize the circumstances in the case of personal exposure (**Figure 10**).
