Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)
Ionizing radiation includes both high-frequency, short-wavelength electromagnetic energy such as x-rays and gamma radiation, and high-energy subatomic particles such as electrons, protons, neutrons, and alpha particles.
Measures of ionizing radiation:
- Radiation release: 1 Curie (Ci) = 3.7 x .0.0 decays/second
- 1 Becquerel (Bq) = 1 decay/second
- Roentgen: the amount of radiation, e.g., gamma, producing a charge of 2.58 x 10–4 coulomb in 1 kg dry air
Effects of radiation on tissue:
- Exposure to 1 Roentgen ~1 rad (radiation absorbed dose)
- 100 rads = 1 Joule/kg tissue = 1 Gray (Gy)
- 100 rem (radiation effect in man) = 1 Sievert (Sv)
- High-dose exposure: ionizing radiation at doses of 500–2000 mSv is known to be carcinogenic, potentially causing several malignancies, e.g., acute leukemia, thyroid cancer. Much of the data comes from the atomic bomb survivors of Hiroshima and Nagasaki.
- Low-dose exposure: much of the data comes from epidemiological studies of miners and studies of second malignancies in patients previously treated for cancer.
- The average per-capita dose from all sources of ionizing radiation is 3.6 mSv per year, with 82% from natural sources, of which two-thirds is from radon exposure. The remainder is primarily from medical exposures.
- Extrapolation from data on exposure to .500 mSv suggests that .%–3% of all cancers may be attributable to radiation arising largely from natural sources.
- The initiating factor for radiogenic cancers is probably a mutation in a tumor suppressor or proto-oncogene and aberrant loss or gain of function.
- 3% of the population shows undue sensitivity to conventional doses of ionizing radiation without any obvious pretreatment phenotype apart from the presence of cancer. This group is at risk of excess toxicity from standard radiation therapy regimens. Greater understanding of underlying defects in DNA repair, cell cycling, and DNA damage signal transduction would allow appropriate tailoring of therapy.
- Rare radiosensitivity syndromes also exist, predisposing to early development of cancer, e.g., ataxia–telangiectasia and Bloom’s syndrome. Susceptibility to DNA damage is enhanced, and both radiotherapy and chemotherapy regimens need adjustment accordingly.