FIG. 1.

A sketch illustrating the calculation of Φ(X_b, θ_b) as a function of the leave position and the beam angle. Given that the dose contribution of each beamlet to each voxel is precalculated by $D_{i,j}^k\left({\theta }_b\right)$, Φ^k(X_b, θ_b) can be obtained by summing up on $D_{i,j}^k\left({\theta }_b\right)$ over all open beamlets at station (X_b, θ_b). These open beamlets are labeled with “1” in this figure.