Room: Exhibit Hall
Purpose: In the treatment of prone breast patients, it is common for the medial tangent to pass through the breast board, which is typically close to the medial portion of the breast. Photons passing through this portion of the board will create contaminant, low energy electrons. These low energy electrons will increase the surface dose to the medial portion of the breast. This increase in dose will not be shown in treatment planning systems. In this study, measurements are made to determine the relative increase in surface dose.
Methods: To measure the increase in surface dose, a setup is constructed to approximate clinical treatment, where the photon beam passes through a portion of the breast board. Measurements are taken with a parallel plate chamber at various distances from the breast board. The breast board was angled at 45 degrees to simulate a typical treatment. Both 6 MV and 15 MV photon energies were measured.
Results: Surface dose measurements showed a 3.08 and 2.53 fold increase in dose when passing through the breast board, for 15X and 6X photons respectively, with the breast board closest to the surface. Even with the board 7 cm from the surface, the surface dose was 2.09 and 1.73 times higher for 15x and 6x respectively.
Conclusion: Large increases in surface dose can be expected when photons pass through material at close distance. When treating breast patients in the prone position, consideration should be made to reduce the amount of breast board that may be in the medial beam. This can be done with patient positioning, beam angle, and padding thickness on the breast board. Reducing the padding thickness brings the sternum closer to the breast board and typically reduces the amount of board in the beam.