How does Radiation cause cancer?
Cancer is the stochastic effect of radiation i.e. the probability of occurrence increases with effective radiation dose, but the severity of the cancer is independent of dose.
Sometimes radiations deposit enough energy in organic tissues to cause ionisation. This tends to break bonds and thus alter the molecular structure of the irradiated molecules. Radiations penetrating the cells hits molecules within the cells randomly. Molecules broken by the radiations can become highly reactive free radicals that cause further chemical damage.
These direct or indirect changes, may eventually impact chromosomes and epigenetic factors that control the expression of genes. Cellular mechanisms will repair some of this damage but some repairs may be incorrect and some chromosome abnormalities will turn out to be irreversible.
Latest research suggest that mutagenic events do not occur immediately after irradiation. Instead, surviving cells appear to have acquired a genomic inability which causes an increased rate of mutations in future generations.