## 4 The Importance of Biomarkers in Monitoring RT Toxicity
### Secreted Soluble Protein Biomarkers
In this way, VEGF may function as an independent predictor of prognosis in the case of cervical cancer. In another study [163], it was shown that the expression of VEGF was increased in HeLa cells in vivo and in vitro after IR exposure. Increased apoptotic cell death and knockdown of VEGF in these cells increased cellular sensitivity to IR in cervical cancer therapy. [PERSON] et al., 2015 have reported increased VEGF levels in culture media of non-irradiated multipotent mesenchymal stromal cells (MSCs), but irradiated ones have shown no increase in VEGF [164]. [PERSON] et al., 2020, showed that the IR-induced senescence of CCR6+Th17 T-cells promotes VEGF-A secretion at 48 h after exposure to 2 Gy irradiation [107]. A meta-analysis for the evaluation of the relationship between serum VEGF levels and radiosensitivity of patients with non-small cell lung cancer (NSCLC) among Asians revealed that these levels were higher than those of the healthy controls. There was also a significant difference in the VEGF levels before and after RT. Meanwhile, no correlations between the tumor node metastasis (TNM)-stage and histologic grade in these patients were found. It was concluded that NSCLC patients with positive VEGF expression had shorter overall survival than those with negative ones [165]. The association between IR exposure and variable VEGF levels has been shown to be cell-type dependent. However, this observation does not exclude the promising perspective of VEGF as a biomarker closely related to radiosensitivity. High sensitivity C-reactive protein (hs-CRP) was evidenced as an indicator of skin toxicity for breast cancer patients having undergone RT. A well-documented study has found that patients with increased levels of hs-CRP had a significantly higher risk of grade 4 skin toxicity, which was not related to radiation dose or the patient's BMI [166]. hs-CRP may also be a biomarker for radiation-induced cardiac dysfunction, as shown in the study by [PERSON] et al., 2020 in the serum of patients after thoracic RT [167]. Most of the above-mentioned studies have been focused on the variations in the cytokine serum/plasma concentrations before and after RT, correlating them with subsequent radiotoxicity events and different tissue radiosensitivity. Moreover, a clear distinction between cytokine levels and the used dose during RT is necessary, as higher doses delivered in larger volumes around the tumor might be closely associated with different levels of cytokine biomarkers [168]. It is increasingly clear that the reported changes in the concentration of cytokines are strongly related to biochemical mechanisms initiated during or after the RT and not before, and this could be a deterrent for their use as predictive markers, except for the cases when multiple studies have reported strong evidence in this respect (e.g., IL-6, IL-18, and TGF-81). On the other hand, the predictive assays utilized so far for assessing normal tissue radiosensitivity have some major advantages: (i) blood collected non-invasively from patients and controls is an important source of soluble biomarkers; (ii) soluble biomarkers are assessed using validated, easy-to-perform, and cost-efficient methods that are commonly used in clinical practice (i.e., ELISA). Instead, the screening for new biomarkers requires sophisticated high-throughput techniques for protein multiplexing. In the case of the plasma/serum concentration of these biomarkers being too low to be assessed by ELISA(such as particular soluble stress proteins), the new Quanterix technology for ultrasensitive biomarker detection is now available \"[[https://www.quanterix.com/](https://www.quanterix.com/)]([https://www.quanterix.com/](https://www.quanterix.com/)) (accessed on 2 September 2023)\". Ultimately, there is a high need for extended and well-documented studies to validate for clinical use those soluble protein biomarkers that represent a reliable radiosensitivity signature of normal tissues in connection with the RT regimen and tumor characteristics.