Figure 1.

An abstract vector space with 1 unitless spatial coordinate as the horizontal axis, the unitless dose coordinate as the vertical axis and the origin at the location of the reference value for which we wish to find the minimum magnitude of u (Eq. 6). The tangent and normal vectors are t and n, respectively. These vectors are drawn for the value of x where the normal vector passes through the origin and at this position γ = |u|. In three spatial dimensions r⃗ ≡ (x, y, z).