Immune system plays a very important role in preventing cancer, as indicated by the increase in the cancer risk when the immune system is suppressed in organ-transplant recipients and AIDS patients (Oliveira Cobucci, 2012).

Immune system response declines rapidly with age (Levin, 2014), qualitatively explaining the well-known age-related increase in cancers risk.

Low-dose radiation boosts the immune system response (see graph below) (Yang, 2014)

and so would reduce cancer risk.

References

Levin, M.J., Immune senescence and vaccines to prevent herpes zoster in older persons, Curr Opin Immunol. Aug;24(4):494-500, 2012 . http://www.ncbi.nlm.nih.gov/pubmed/22857823

Oliveira Cobucci, Comparative incidence of cancer in HIV-AIDS patients and transplant recipients, Cancer Epidemiol. Apr;36(2):e69-73, 2012 http://www.ncbi.nlm.nih.gov/pubmed/22236649

Yang, G., Low-dose ionizing radiation induces direct activation of natural killer cells and provides a novel approach for adoptive cellular immunotherapy, Cancer Biother Radiopharm. Dec;29(10):428-34, 2014. http://www.ncbi.nlm.nih.gov/pubmed/25402754