Alpha/beta (α/β) ratio is defined in radiobiology as a measure of intrinsic radiosensitivity of a specific tissue, measured in Gy, and often considered as being 3 Gy for late-responding normal tissues and 10 Gy for rapidly proliferating carcinomas.
Radiation cell killing, α and β are two constants representing two processes of radiation induced cell death, and their ratio represents a measure of the relative importance of the two processes.
A low α/β is consistent with a greater capacity for repair between fractions, with an accompanying greater relative sparing with small fraction sizes, than for tumors with their typically higher α/β ratios.
Thus, cell lines with a low α/β ratio should be more sensible to hypofractionated radiotherapy regimens (fractions of 2.1–3.5 Gy, five days per week) and to SBRT (usually characterized by higher dose per fraction, e.g 3.5–15.0 Gy).
# Alpha/Beta Ratio in Radiation Therapy
The alpha/beta (α/β) ratio is a fundamental concept in radiation oncology that describes how different tissues respond to radiation doses. It’s derived from the linear-quadratic model of cell survival and has significant implications for treatment planning.
## Basic Concept
The linear-quadratic model describes cell killing by radiation with this equation: – Survival fraction = e^-(αD + βD²) – Where D is the radiation dose
The α/β ratio represents the dose (in Gy) at which the linear (α) and quadratic (β) components of cell killing contribute equally to radiation damage.
## Clinical Significance
– **Low α/β ratio tissues** (1-5 Gy): Typically late-responding normal tissues like spinal cord, brain, kidney, and liver – More sensitive to larger doses per fraction – Show greater sparing effect with fractionation
– **High α/β ratio tissues** (8-10+ Gy): Typically early-responding tissues and most tumors – Respond similarly regardless of fractionation – Examples include skin, mucosa, bone marrow, and many rapidly proliferating tumors
## Treatment Applications
– **Conventional fractionation** (1.8-2 Gy/day): Traditionally used to spare late-responding normal tissues
– **Hypofractionation** (>2 Gy/day): More effective for tumors with low α/β ratios – Prostate cancer (α/β ≈ 1.5-3 Gy) – Some breast cancers (α/β ≈ 4 Gy)
– **SBRT/SRS** (very high doses per fraction): Leverages radiobiological advantages for certain tumors
Understanding the α/β ratio helps optimize the therapeutic ratio by maximizing tumor control while minimizing damage to surrounding normal tissues.