radiationdose

Radiation dose is a measure of the energy deposited by ionizing radiation in matter per unit mass. The absorbed dose, D, is measured in gray (Gy), where 1 Gy equals 1 joule per kilogram. To relate dose to biological effect, the equivalent dose, H_T, is expressed in sieverts (Sv) and equals the absorbed dose multiplied by a radiation weighting factor that depends on the radiation type and energy. The effective dose, E, is the sum of tissue-weighted equivalent doses across all tissues, using tissue weighting factors, and provides a single value for stochastic risk to the whole body.

Dose rate denotes how quickly the dose is delivered, typically expressed in Gy per unit time. In radiation protection, air kerma or exposure historically served as measures in air, while at the tissue level the absorbed dose and the associated Sv quantities are used for risk assessment.

Applications and contexts vary widely. Diagnostic imaging (x-ray, CT) uses low to moderate doses, with CT generally yielding higher doses than plain radiographs. Radiation therapy administers high absorbed doses, often 50 to 80 Gy total, delivered in fractions over several weeks to target tumors while sparing normal tissue behind constraints on dose distribution. Environmental and occupational exposures are monitored to keep cumulative doses well within safety limits.

Safety and regulation emphasize the ALARA principle (as low as reasonably achievable) and established dose limits. Occupational exposure is typically higher than public exposure, with national regulations commonly setting annual limits in the tens of mSv for workers and about 1 mSv or less for the public, averaged over time.

Typical reference values include natural background radiation around 2–3 mSv per year; chest radiography about 0.02–0.1 mSv; abdominal CT around 8–10 mSv; and radiotherapy doses on the order of tens of Gy.