Radiosensitivity is a clinical issue for oncologists who face early and late post-RT abnormal tissue response with a variety of severity syndromes quantified with a grading system notably the CTCAE (Common Terminology Criteria for Adverse Events) of the National Cancer Institute: from 15 to 20% of patients present significant tissue effects (grades 2 to 5). The radiosensitivity is related to cell deaths and thus quite different from individual radiosusceptibility to cancer related to transformed cells and radiodegeneration related to tissue degeneration. These events are mostly encountered after RT of head and neck carcinoma, breast, lung, prostate and rectal cancer. Radiation oncologists already take into account known factors (diabetes, smoking…) and they wish to know unknown individual factors (abnormal DNA damage response, genetic …) and require predictive assays to adapt radiation therapy protocols to prevent these adverse events.
Clinical radiosensitivity has been quantified for years with in vitro cell surviving fraction at 2Gy (SF2). It has demonstrated intrinsic radiosensitivity since cell survival curves never cross with a continuum in radiation responses from normal to highly abnormal. So far up to 30 genetic diseases have been identified to present individual radiosensitivity. A statistically significant correlation between clonogenic cell survival (SF2) and CTCAE grades has been established for both these genetic diseases and for patients of the Copernic cohort referred for adverse events after RT (Le Reun et al. 2022).
However, because clonogenic assays are too time-consuming, predictive functional assays simpler than SF2 are needed for routine practice. They should reflect the continuous spectrum of responses and the dose dependence over the relevant clinical dose range, establish a quantitative relationship between clinical radiosensitivity (from CTCAE grade 0 to grade 5 whatever the early or late nature of tissue reaction) and cellular radiosensitivity and identify patients with moderate radiosensitivity (CTCAE 2 to 5) and without a known radiosensitive genetic disease.
Among the numerous functional assays tested for demonstrating individual intrinsic radiosensitivity: 1- cell death assays (mitotic death, micronuclei, cellular senescence, apoptosis); apoptosis is the most documented with significant cell type dependence, e.g., lymphocytes / fibroblasts, no general correlation between apoptosis and radiosensitivity, and one inverse correlation (with no mechanistic rationale) for late complications reported in CD8 T-lymphocytes at 8 Gy; 2- chromosome assays (Staining assay of chromosome breaks and aberrations, Premature chromosome condensation, Fluorescence in situ hybridization, Comparative genomic hybridization) are also time consuming and with no correlation with CTCAE grades; 3- DNA damage assays (PFGE, Halo, Comet, Cell free, H2AX foci, other immunofluorescent biomarkers) are not sufficient to predict moderate radio-sensitivity; 4- Genomic approaches (Micro-array techniques, Single nucleotide polymorphisms, Genome wide association studies) do not allow so far an individual assessment of radiosensitivity; CDKN1A (p21) decrease relative gene expression is a marker of severe early radiation toxicity; 5- RIANS assay (radiation induced ATM nucleo shuttling) investigates the shuttling of the ATM protein from cytoplasm to nucleus induced by IR (2Gy) and demonstrated by immunofluorescence. The delay is mechanistically explained by the cytoplasmic interaction of ATM with (abnormal) proteins in excess in the cytoplasm. Quantification is made with the maximal number of pATM foci in the nucleus of skin fibroblasts from 10 to 60 min. RIANS demonstrates significant quantitative correlation between: i. the maximal number of pATM foci and radiosensitivity evaluated by CTCAE grade (Granzotto 2016, Le Reun 2022), ii. the maximal number of pATM foci and SF2. ROC curves used for intercomparison show the superiority of RIANS over apoptosis and genomic approaches.
In conclusion, clinical radiosensitivity after radiation therapy exists in a significant number of patients (up to 20%), is a continuous phenomenon between normal and highly abnormal and has an intrinsic individual component. Radio-oncologists require a predictive functional assay applicable in routine. SF2 provides the best correlation with CTCAE grades but is not clinically applicable. RIANS is the only other functional test, based on a significant mechanism that provides a correlation of maximum pATM with CTCAE grades and explains the linear-quadratic model.