*1.1.1 Mechanism of action of RT*

For a normal cell to be able to multiply, the cell cycle takes approximately 10–20 hours [6]. Tumor cells tend to proliferate faster. During the G2 and mitotic (M) phases of the cell cycle, chromatin is more compact and hinders the action of repair enzymes, thereby increasing the probability of DNA damage [3]. Therefore, these are the two phases of the cell cycle (G2 and M) in which cells are the most radiosensitive [3, 4, 6, 7].

In addition to DNA damage, other mechanisms of cell damage can result from the use of ionizing radiation, which induces cell death. The type of cell death induced by ionizing radiation depends on the cell type, cell cycle stage, DNA damage repair capacity, ionizing radiation dose, and cellular microenvironment [3, 7, 8]. This can occur through direct or indirect mechanisms.

The direct mechanism of cell death induced by RT involves the absorption of energy by the cellular biological environment, and this energy interacts directly with DNA and proteins, causing damage that can occur up to a time after tissue irradiation [4, 6, 8]. In the indirect mechanism, ionizing radiation interacts with molecules that constitute the cell environment, primarily water, increasing the concentration of free radicals that can enhance radiosensitivity and promoting cellular damage [4, 6]. Double-strand DNA breaks can also be induced by reactive oxygen species, which are naturally produced during cellular metabolism [7].

The recognition of DNA damage induced by ionizing radiation promotes the activation of a cascade of signals that, depending on their function, will determine whether the fate of cell repair, cell cycle progression, or apoptosis [9]. Furthermore, increasing the concentration of reactive oxygen species can activate genes that induce tissue inflammation or increase oxidative stress, thereby affecting radiosensitivity [9]. The inflammatory cascade can also be induced by exposure to ionizing radiation [1].

Cellular response to radiation is also regulated by gene activation cascades and signal transduction proteins, which involve the *PI3K/AKT, MAPK/ERK, NF-κB* and *TGFβ* pathways [10]. The *MRE11-RAD50-NBS1* complex and *53BP1*, γ*H2AX*, and *MDC1* genes repair DNA end fragments [9]. The *ATR* and *ATM* genes are responsible for activating DNA repair processes by homologous recombination and non-homologous end splicing, respectively, after double-strand breakage [8, 9, 11]. These genes also interact with other genes that are essential checkpoints for verifying the integrity of genetic material in the phases of the cell cycle [9, 11]. If DNA damage is significant, cell death occurs [8]. Any alteration in the function of the genes that participate in the pathways, which regulate cellular responses to radiation, influences DNA repair, cell cycle progression, and cell death by apoptosis.

Considering that tumor cells multiply faster than normal tissue cells, they tend to go through the G2 and M phases of the cell cycle more often. For RT to be effective in controlling the growth and multiplication of tumor cells, the planned total ionizing radiation dose is subdivided into daily doses (dose fractionation). RT fractionation regimens aim to reach the largest number of tumor cells in the most radiosensitive phases of the cell cycle (G2 and M), thereby increasing the therapeutic effect of ionizing radiation. The dose of ionizing radiation absorbed per unit mass in RT is defined as Gray (Gy) [5]. From the first dose of ionizing radiation, free radicals, reactive oxygen species, double-strand DNA breakage, and recruitment of the inflammation cascade are generated [1]. Total dose fractionation also aims to minimize adverse effects on healthy tissues adjacent to the tumor [5].
