cGy

cGy, short for centigray, is a unit of absorbed dose used in radiology, radiation therapy, and dosimetry. Absorbed dose measures the amount of energy deposited by ionizing radiation in a unit mass of tissue or material. One centigray equals one hundredth of a gray: 1 cGy = 0.01 Gy = 0.01 joule per kilogram. The gray (Gy) is the SI unit of absorbed dose; therefore 1 Gy = 100 cGy. In clinical practice, doses are commonly expressed in Gy, with prescriptions for external beam radiotherapy typically in the tens of Gy delivered in fractions; brachytherapy dose rates may be described in cGy per hour.

cGy is a practical unit used in medical settings because many treatment doses are on the order of tens to hundreds of cGy per fraction. It is a non-SI unit but widely accepted for use with SI units in radiology and radiotherapy.

Absorbed dose is defined by energy deposition per unit mass, independent of radiation type; however, biological effect depends on factors such as linear energy transfer and tissue sensitivity. In dosimetry, devices such as ionization chambers are calibrated to estimate absorbed dose in water or tissue-equivalent material, with conversions from measured quantities to cGy or Gy using standard factors. In brachytherapy, dose rates are commonly described in cGy per hour, while external beam regimens are usually specified in Gy per treatment course. In summary, cGy expresses energy deposition per unit mass and is directly related to the gray by a factor of 100.