The BED concept helps compare different fractionation schemes by accounting for total dose and fraction size via which model?

• A. Exponential decay model
• B. Michaelis-Menten kinetics
• C. Linear-Quadratic model (alpha/beta)
• D. Hill equation

Answer: (C)

Biologically effective dose uses the linear-quadratic model to relate dose to effect by splitting cell kill into two components: a linear part and a quadratic part. In this model, the cell-killing effect scales with αD plus βD^2, so the survival after a given total dose D delivered in fractions can be described as S = exp(-αD - βD^2). When doses are given in multiple fractions of size d (with n fractions, D = nd), the BED is nd [1 + d/(α/β)]. This provides a common scale to compare regimens with different total doses and different fraction sizes, because the α/β ratio reflects how sensitive a tissue or tumor is to changes in fraction size (tissues with high α/β are less sensitive, while those with low α/β are more sensitive). By converting regimens to an equivalent dose in standard fractions, clinicians can optimize and compare treatment plans. Other models listed describe other phenomena and don’t traditionally apply to modeling fractionation effects in radiotherapy.