Verification of a two-dimensional pencil beam arc electron dose calculation algorithm.

The dosimetry of arced electron beams is of increasing importance because of the increased capabilities of modern linear accelerators. A practical pencil beam algorithm has been developed for arc electron beams and is capable of using computed tomography information for heterogeneity corrections. For homogeneous phantoms, the maximum dose and bremsstrahlung components are predicted very accurately, that is, within 1% of the maximum dose. However, the depth of maximum dose (treatment depth) is predicted to be deeper than measurement, as much as 0.7 cm deeper. For a heterogeneous lung phantom, the discrepancies are as high as 30%, but the accuracy of dose calculation is consistent with conventional stationary pencil beam algorithms. It was concluded that improvements in the dose prediction are possible with more accurate calculations of the pencil beam widths and the incorporation of range straggling into the algorithm.