The biological effect of inhomogeneous dose distributions in fractionated radiotherapy.

The linear quadratic (LQ) model is applied to an organ receiving a fractionated course of radiotherapy with an inhomogeneous dose distribution. It is shown that the gradient in the extrapolated response dose (ERD) will be steeper than the gradient in the physical dose. This effect will be greatest for an organ with a small alpha/beta ratio treated with large dose fractions. Clinical implications are discussed with an emphasis on radiation myelitis.