## 4 The Importance of Biomarkers in Monitoring RT Toxicity
### Cell-Intrinsic Radiosensitivity Biomarkers
#### 4.1.2 DNA Damage Response
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Assay (Molecular and Cytogenetic Markers)** & **Sample/Patient Profile** & **Outcome** & **Study** \\ \hline \multirow{4}{*}{G2 micronucle (MN) assay} & Blood samples from 18 _BROCA2_ mutation carriers and 17 subjects from both _BROCA1_ (n = 9) and _BROCA2_ (n = 8) & Higher radiosensitivity in healthy _BROCA2_ mutation carriers compared with healthy volunteers by means of the G2 MN assay. No increased radiosensitivity was observed in non-carrier relatives of _BROCA1_ and _BROCA2_ families. & [50] \\  & Lymphocytes obtained from 16 patients that were  retrospectively examined 1 to 108 months & Detectable differences in interindividual  radiation sensitivity. & [54] \\  & after radiotherapy. & & \\ \cline{2-4} Fluorescence in situ & & Significant overall correlation was found between the frequencies of t(Ba) (painted chromosomes  bearing one centromere with a color junction) & [55] \\ (FISH) assay & Blood samples from 47 BC patients who received  exclusively radiotherapy after surgical lumpectomy. & & \\ (FISH) assay & & & \\ \cline{2-4}  & Patients with a localized T1-3 NOM0 prostate carcinoma, with and without severe side effects, who were treated  with three-dimensional conformal radiotherapy. & No statistically significant difference in the overall  aberration yield between healthy donors and  patients with and without severe side effects. & [56] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. without prior RT, and in 38 healthy female donors without recent diagnostic radiation exposure or any chronic diseases. The aim of the study was the examination of the usefulness of \(\gamma\)H2 AX foci analysis expressed as a percentage of residual damage (PRD), which referred to measurements at 3 and 6 h after in vitro irradiation of peripheral lymphocytes with 2 Gy of 120 kV X rays for predicting adverse reactions of IR at the level of normal skin. For confirming the in vitro results in a clinical setting, breast cancer patients subjected to RT were categorized as over-responders (ORs) and non-over-responders (NORs) with respect to the extent of acute adverse reactions of clinical irradiation in the skin, according to the Radiation Therapy Oncology Group (RTOG) criteria. A statistical difference between the healthy and the NOR groups, as well as between the healthy and the OR groups, has been found. The authors concluded that the slow rate of foci disappearance with respect to the initial DNA damage may be the cause of clinically radiosensitive phenotypes [58]. Accordingly, the H2 AX assay is adequate to quantify PRD while considering the degree of the initial DNA damage and is accordingly an informative predictive assay for monitoring RT. Similar results were obtained by [PERSON] et al., 2013. In this study, the \(\gamma\)H2 AX assay was performed in peripheral blood mononuclear cells (PBMCs) isolated from the blood samples of 57 breast cancer (BC) patients, collected before and during clinical irradiation (72 h after five radiation fractions), against 12 clinically healthy donors. In this clinical study, six retrospectively identified BC patients with an early adverse skin reaction to RT, developed at the level of the irradiation field, were used as a reference for clinical radiosensitivity according to the RTOG score, and showed striking differences in the results of the \(\gamma\)H2 AX assay compared with healthy individuals. The data obtained using the \(\gamma\)H2 AX assay identified BC patients with normal clinical reactions to RT based on the induced and residual DNA damage, but not on the background DNA damage. It was concluded that the \(\gamma\)H2 AX assay may have the potential for screening individualized radiosensitivity of BC patients, as the mean number of \(\gamma\)H2 AX foci after five clinical fractions was significantly higher than before RT, especially in clinically radiosensitive patients [61].