Poster — Thurs Eve‐20: Analysis of dosimetric differences between dose‐to‐water vs. dose‐to‐medium calculations for electron beams

Purpose: With the advent of commercial Monte Carlo based treatment planning systems (TPS) typically calculating dose‐to‐medium, Dm, as opposed to dose‐to‐water, Dw, calculated by conventional TPS, a thorough analysis of differences between these treatments plans is required. Such an analysis has not yet been carried out. The purpose of our study was to evaluate dosimetric differences between such plans generated with a commercial MC based TPS. Materials and Methods: The analysis included plans of 53 breast cancer patients treated with electron beams ranging from 6–20 MeV. These plans were originally calculated using the Dm approach. Keeping the original beam arrangements and the same calculation parameters, the plans were recalculated using the Dw approach. The comparison between Dm and Dw plans was performed by means of dose volume histograms and isodose distributions on the corresponding CT slices. Results and Conclusions: The plans calculated using Dm vs. Dw show some differences, with magnitudes depending on the location of the tumor and organs at risk and the beam energy. The largest difference was found for the treatment of the chest wall after complete mastectomy with 13 MeV beam. The dose to internal mammary nodes was 43.6 Gy and 39.0 Gy for Dm and Dw approach, respectively. This amounts to 8.1% difference in the maximum dose delivered to that volume. For the same patient, the dose received by the right lung was 13.5 and 15.4 Gy for Dm and Dw approach, respectively, which amounts to 3.6% difference in dose delivered to this lung.