Definition – Linear-Quadratic (LQ) model
The most widely used model to describe cell survival after radiation.

The equation:
SF = e^-(αD + βD²)

• SF = surviving fraction

• D = dose

• α = linear (single-track) component

• β = quadratic (two-track interaction) component

α/β ratio

• Dose where αD = βD².

• A measure of how “curved” the survival curve is.

• High α/β (≈10 Gy) → early-responding tissues, most tumors.

• Low α/β (≈2–3 Gy) → late-responding tissues (spinal cord, rectum) and prostate cancer.

Clinical meaning

• High α/β → less sensitive to fraction size.

• Low α/β → very sensitive to fraction size (why hypofractionation may benefit prostate).