A COMPARISON OF 3 ELECTRON PLANNING-ALGORITHMS FOR A 16 MEV ELECTRON-BEAM

Purpose: We report results of a comparison of three electron planning algorithms, an Age-Diffusion Pencil beam algorithm and two (2-D) and t hree dimensional (3-D) Hogstrom pencil beam algorithms, using simple 2 X 2 cm air and hard bone inhomogeneities and a complex anthropomorphi c head and neck phantom. Methods and Materials: The simple inhomogenei ties have variable dimensions outside the plane of calculation to test the effects of out of plane scattering on 2-D algorithms, compared wi th dose measured by film below the inhomogeneity in the dose fall-off range. Comparisons are also made of a parotid treatment field for 16 M eV electrons, and the dose measured by high sensitivity thermoluminesc ent dosimeters in the head and neck phantom. Results: Behind the simpl e inhomogeneities, the electron algorithms are found to underestimate the dose behind the air cavity by up to 40% and overestimate the dose behind bone by up to 30%. In the head phantom, the presence of inhomog eneities also presents problems for the algorithms, with overestimatio ns of dose of up to 20% found behind bone-tissue interfaces, apparentl y due to shielding by high density bone. Overestimations of up to 17% are also found beside interfaces parallel to the beam. Underestimation s of dose of up to 10% are found on the beam-side of interfaces, due t o under-prediction of backscattered electrons. All three investigated algorithms underestimate the dose by up to 20% behind extreme surface curvature. One algorithm is found to underestimate the dose in the fal loff region while another overestimates the dose around the 90% isodos e. Conclusion: Clinicians should be aware of the limitations of their planning systems.