Absorbed dose and the gray (Gy)

Absorbed dose (symbol D) is the mean energy imparted by ionising radiation to a mass of matter, divided by that mass: D = dε/dm. Its SI unit is the gray (Gy), defined as one joule of absorbed energy per kilogram of tissue (1 Gy = 1 J/kg); the older unit, the rad, equals 0.01 Gy. Absorbed dose is a purely physical quantity, defined for a point, organ, tissue or the whole body, and it applies equally to external-beam radiotherapy and to radiation delivered internally by a radiopharmaceutical.

In radiopharmaceutical therapy, absorbed dose to tumours and organs at risk (typically kidneys, salivary glands and bone marrow for PSMA and SSTR-targeted agents) is estimated from serial quantitative imaging using the MIRD formalism, and it is the physical quantity that dosimetry-guided treatment tries to optimise: enough gray to the tumour, not too much to the kidneys or marrow. It is distinct from equivalent dose and effective dose (measured in sievert), which apply a biological weighting on top of absorbed dose for radiation protection purposes; for prescribing and evaluating radiopharmaceutical therapy, absorbed dose in gray to the organ or tumour of interest, not sievert, is the clinically relevant quantity.

Because different radiation types (alpha, beta, gamma) deposit their energy very differently at the microscopic scale even at the same absorbed dose, gray alone does not fully capture biological effect; that is the role of linear energy transfer and relative biological effectiveness.