## 4 The Importance of Biomarkers in Monitoring RT Toxicity
### Cell-Intrinsic Radiosensitivity Biomarkers
#### 4.1.1 Cytogenetic Markers
Particularly, significant differences were found in radiation-induced chromosomal damage, measured as breaks per 1000 mitoses ranging from 70 to 556 after 0.7 Gy and from 420 to 1210 after 2 Gy. Thus, the authors concluded that this technique may be useful to detect patients with severely enhanced radiosensitivity. [PERSON] et al., 2011 quantified chromosomal aberrations by means of FISH using specific probes for chromosomes 1, 4, and 12 in blood samples from 47 BC patients collected 6 weeks post RT. Meanwhile, clinical side effects of RT were evaluated weekly during the therapy, and the scoring was carried out according to the Common Toxicity Criteria (NCI-CTC scale; scale digits 0, 1, 2, 3, 4). The study's findings were increased chromosomal aberrations in patients with more severe side effects, a correlation between the frequency of a specific aberration (painted chromosomes bearing one centromere with a color junction type t(Ba)), and the occurrence of skin side effects. Based on the frequency of t(Ba) aberrations, the authors identified patients with short latency (early side effects) when no correlation was found for other aberrations (painted chromosomes bearing one centromere with a color junction type t(Ab), dicentrics(dic), color junctions(cj)) with side effects or latency. Results have indicated that the estimation of chromosomal radiosensitivity at the translocation level in peripheral blood lymphocytes can be proposed as a predictive assay for the detection of radiosensitive individuals [55]. The 2, 11, and 17 paint probes tested in localized prostate cancer patients treated with 3D conformal RT, with (PS) and without (P0) side effects, as well as in a healthy group, led to negative results in the study of [PERSON] et al., 2013 [56] as no pronounced differences in chromosomal radiosensitivity levels between donor groups, after a 2 Gy-irradiation, were mentioned. A dose-dependent increase in aberrations for all analyzed chromosomes, with smaller chromosomes 11 and 17 being less frequently involved in aberrations compared with the larger chromosome 2, was found. Nonetheless, the FISH assay did not manage to demonstrate significant differences in chromosomal radiosensitivity between donor groups when comparing the chromosomes with each other. However, the dose-dependent increase in aberrations suggests that higher radiation doses may be needed to reveal such differences. FISH has some limitations compared with the G2 and MN assay, as it is a laborious technique that requires specialized equipment, while the preparation of FISH probes, hybridization, and image analysis processes can be time-consuming, limiting its widespread implementation and applicability in routine clinical practice. In turn, the utilization of the FISH assay for assessing radiosensitivity offers several advantages, including the detection of chromosomal aberrations with high sensitivity, specificity, and accuracy. These strengths make it a valuable tool in detailing an individual's response to IR toward the development of personalized RT strategies. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Assay (Molecular and Cytogenetic Markers)** & **Sample/Patient Profile** & **Outcome** & **Study** \\ \hline \multirow{6}{*}{\(\gamma\)H2 AX IF assay} & Peripheral lymphocytes of 31 patients with resected head-and-neck cancer with different grades of oral mucositis undergoing adjuvant RT or RCT. & Patients with a proportion of unrepaired DSBs after 24 h higher than the mean value had an increased incidence of severe oral mucositis. & [57] \\  & Peripheral lymphocytes of 80 post-chemotherapy OR and NOR BC patients and 38 healthy female donors. & Statistical difference between the healthy and NOR groups, the healthy and OR groups, and the NORs and ORs, using PRD (percentage residual damage). & [58] \\ \hline \multirow{6}{*}{\(\gamma\)H2 AX FCM assay} & PBMCs isolated from 25 prostate cancer patients treated with RT (Expanded Prostate Cancer Index Composite [EPIC].)