Purpose: To quantify and compare the effectiveness of electron treatment with cutout and skin blocks for high and low energy electrons in terms of required physical margin and dose uniformity.
Methods: Monte Carlo simulations were performed in a cube water tank phantom (30 cm sides). RayStation 8A, modeling electrons in Elekta Versa HD and Eclipse v 15.5, modelling electrons in Varian Truebeam STx were used for dose calculation. Each TPS performed 12 different simulations including three different circular cutouts of 3 cm, 5 cm and 7 cm in diameter and their equivalent skin blocks for two different electron energies of 6 MeV and 15 MeV. For all setups, an SSD of 100 cm and a 10 cm x 10 cm cone was used. Doses were calculated by delivering predefined 100 MUs for each setup and uncertainty of 0.5%. Dose was prescribed to 2 cm for 6 MeV and to 4 cm for 15 MeV electrons. The calculated isodose distributions were compared based on the physical margin required for each method, and dose uniformity within the prescribed isodoseline.
Results: Both TPS systems produced comparable dose distributions for all 12 setups. Our results showed that a margin of 5 mm on the skin surface provide enough coverage at 50% of prescribed dose for both 6 MeV and 15 MeV electrons. Larger margins should be used for treatment of deeper targets.
Conclusion: Although skin collimators can reduce the required margin on the skin level, no significant difference was observed for treating deep lesions. Electron cutouts still provides sufficient dose coverage laterally to the targets. Smaller cutouts and lower energy electrons may use an additional 1 mm margin as compared to higher energies and larger cutouts. Skin collimator creates hot spots that are more pronounced for high energy electrons and smaller diameters.