**1. Introduction**

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality in the Western world. It accounts for nearly one-third of all deaths worldwide. There are multiple contributory risk factors for heart disease. Some are of a controllable nature, such as lifestyle, dietary factors, and metabolic disorders, such as high cholesterol levels and hypertension. Others are noncontrollable risk factors, such as gender, age, and genetic predisposition [1, 2]. In addition, there are environmental factors affecting the risk of CVD, ionizing radiation being one such factor.

**1.1. What is ionizing radiation?**

**1.2. Radiation metrics**

been introduced [18].

From natural to manufactured sources, life on earth is exposed on a daily basis to ionizing radiation. Defined as a type of energy released by atoms that travel in the form of electromagnetic or particles, this energy can eject tightly bound electrons from the orbit of an atom, causing the atom to become ionized [12]. In nature, one can distinguish three main types of ionizing radiation: alpha (α), beta (β) particles, and gamma (γ) rays. They are all produced by naturally occurring substances with unstable nuclei (e.g., cobalt-60 and cesium-137) that spontaneously undergo radioactive decay. During the decay process, energy is lost via emission of ionizing radiation in the form of electromagnetic γ-rays and/or charged particles (e.g., α- and β-particles) [13]. One of the most common manufactured forms of ionizing radiation is X-ray radiation. X-rays are in most aspects similar to γ-rays but differ in origin. While γ-rays are derived from the natural decay of radioactive elements, X-rays are artificially produced in X-ray generators by directing a stream of high-speed electrons at a target material, such as gold or tungsten [14]. When electrons interact with atomic particles of the target, X-radiation is produced [12]. In addition to the most common forms listed above, there are many other forms of ionizing radiation of human or natural origin. Examples are neutrons, accelerated ions and fission fragments [15, 16]. These less common forms can have different biological effects, which

Selected Endothelial Responses after Ionizing Radiation Exposure

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Biological effects of ionizing radiation are related to energy deposition in matter. To assess the impact of ionizing radiation on human health and to set guidelines in radioprotection, units

The absorbed dose is defined as the amount of energy, originating from any type of ionizing radiation and any irradiation geometry that is absorbed per unit mass of material. The international SI unit for absorbed dose is gray (Gy). One Gy represents the absorption of 1 joule of energy in 1 kilogram of mass (1 J/kg). This definition is pure physical, as it does not consider the quality of the ionizing radiation type and the extent of biological damage it inflicts to certain tissues and/or organs. As a result, the terms equivalent dose and effective dose have

The equivalent dose takes into account the ability of a particular kind of ionizing radiation to cause damage. It is obtained by multiplying the absorbed dose (Gy) with a radiationweighting factor (wR) attributed to each different radiation type (e.g., the wR of photons and electrons is 1, the wR of protons and charged ions is 2, and the wR of α particles and fission

The effective dose is defined as the weighted sum of all tissue and organ equivalent doses multiplied by their respective tissue-weighting factor (wT). It expresses the biological effect that a certain type of ionizing radiation has on the human body. wT values have been defined to represent the contributions of individual organs and tissues to overall radiation effects on the human body. Similar to the equivalent dose, the effective dose has sievert (Sv) as international SI unit. Care should be taken with wT values because they constitute an average

fragments is 20). The international SI unit for equivalent dose is the sievert (Sv) [18].

can be exploited, for example, in hadron therapy for cancer treatment [17].

to measure a dose and its biological effects are required.

It has been known for a long time that high doses of radiation, such as those delivered during radiotherapy, cause damage to the heart and vasculature and thus increase the risk of CVD. Data from animal experiments have strongly supported this observation [3–6]. However, for doses <0.5 gray (Gy), epidemiological data are suggestive rather than persuasive. Therefore, the magnitude of CVD risk in the low-dose region where issues of radiation protection usually operate is not clear [3–6].

Various issues, such as occupational radiation exposure, future of nuclear power, manned space flights, and threat of radiological terrorism, call for a thorough understanding of lowdose health risks [7]. The main concern is, however, an increasing use of ionizing radiation for diagnostic medical purposes (**Figure 1**) [8]. For instance, since 1993, the number of computed tomography (CT) scans has increased four times in the United States, and a similar trend is observed in Europe [9]. Medical radiation is the largest source of radiation exposure in Western countries, accounting for a mean effective dose of 3.0 millisievert (mSv) on average per capita per year from diagnostic procedures only, corresponding to a radiological risk of 30 chest X-rays [10]. Of note, doses from therapeutic procedures are not taken into account in this number. Although the health benefits of these improved diagnostic procedures are huge, concerns are raised regarding "overuse" and potential associated health risks [11].

**Figure 1.** Average annual effective dose per person received in 1980 (left panel) and in 2006 (right panel) in the United States. The large increase in the use of ionizing radiation for medical purposes, in the period 1980–2006, contributed to a total increase from 3.0 mSv in 1980 to 6.2 mSv in 2006. Similar trends are observed in other industrialized countries [1].
