Improved abutment dosimetry in segmented-field electron conformal therapy

Purpose: Segmented-field electron conformal therapy is characterized by dose heterogeneity due to unmatched penumbra of abutted fields of differing energy. The present work investigates the potential to decrease dose heterogeneity by approximately matching beam penumbra using energy-specific source-to-collimator distances (SCDs). It was hypothesized that a clinically practical, variable-SCD method that utilizes Cerrobend® custom inserts can deliver segmented-field electron conformal therapy in the energy range of 6-20 MeV with less than ±5% variation in dose spread in the abutment regions of hypothetical planning target volumes (PTVs), i.e. constrain the PTV dose to 85%-105%. Methods: A Varian 15x15-cm2 electron applicator was modified to allow energy-dependent SCDs resulting in energy-dependent air gaps. Air gaps were chosen based on theoretical calculations to approximately match penumbra for 6, 9, 12, 16, and 20 MeV beams at a depth of 1.5 cm in water. Treatment plans developed for four simulated PTVs and a single patient using the variable-SCD applicator were compared to identical plans using the current constant-SCD applicator. Dose plans for the simulated PTVs using the variable-SCD applicator with electron inserts cut with diverging edges were delivered to film in a polystyrene phantom to assess feasibility. Results: Treatment planning results in the four simulated PTVs showed that dose homogeneity in agreement with the hypothesis can be achieved using the variable-SCD applicator. Minimum dose was increased by an average of 4%, and maximum dose was decreased by an average of 4%. On average, the standard deviation of the dose decreased by 29%, and D90-10 decreased by 32%. Measured dose in the abutment regions for all four simulated targets using the modified applicator agreed well with TPS predicted dose. For the patient PTV, the variable-SCD applicator plan predicted a 14% increase in minimum dose, a 10% decrease in maximum dose, and a 22% reduction in both the standard deviation of the dose distribution and D90-10 as compared to the standard applicator plan. Conclusion: The results of this study demonstrated that dose homogeneity in segmented-field electron conformal therapy can be substantially improved by using energy-dependent SCDs to match beam penumbra