The α/β ratio provides the dose in Gray where cell killing from linear and quadratic components of the linear quadratic equation are equal. At points below this value, linear cell kill is dominant; in those after quadratic cell kill takes over. This ratio may be used in many areas of radiotherapy.
Differentiating early and late responding normal tissue
Early responding tissues typically have a high α/β ratio, making them sensitive to total dose but with minimal effect with fractionation. Late responding tissues typically have a low α/β ratio, demonstrating increased survival at low doses and significantly greater toxicity at higher doses.
Determining sensitivity of tumour cells to fractionation
Most tumours are rapidly growing and have high α/β values, making them sensitive to overall dose with less effect from fraction size. Some tumours, notably prostate cancer and melanoma, have low α/β values. This makes them more sensitive to large fraction sizes and resistant to small fraction sizes. This distinction is one reason high dose rate brachytherapy, which delivers a small number of large doses to the vicinity of the prostate, has such potential for increasing cure rates of prostate cancer.
Choosing fractionation schedules
Low α/β tissues typically suffer less toxicity at low doses compared with high α/β tissues. Most tumours also possess high α/β values. In areas where late toxicity requires high dose, such as in the skin or limbs, it is possible to shorten treatment times without significant late normal tissue toxicity. This becomes problematic in areas with organs sensitive to late effects; in these cases a more fractionated regime is recommeneded to exploit the cell kill between the tumour cells and late responding cells.
