Implementing a tantalum wire mesh to increase the skin dose in low-energy electron irradiation of the chest wall

Radiation treatment of the post-mastectomy chest wall is performed in our institution by straight-on electron irradiation. The chest-wall thickness is measured and the beam energy is chosen so that the chest wall is treated to therapeutic doses, while sparing the underlying lung tissue. The most commonly chosen energies are 6 and 9 MeV. The skin dose should be 90% of the dose prescribed to the chest wall, which is higher than can be achieved with 6- and 9-MeV beams because of the low surface dose. The addition of a bolus slab during part of the treatment can correct for this; however, the added depth means that a higher energy has to be chosen, which will increase the lung dose (the higher the electron energy, the slower the falloff of the electron depth-dose curve). A mesh of a high-Z material above the skin gives rise to obliquely scattered and low-energy electrons that effectively spoil the buildup zone. Dosimetric measurements of a Tantalum (Ta) mesh were performed using a dose scanner in a water tank and a film inserted in a humanoid phantom during a simulated treatment. Measurements were also done for the clinically relevant cases of oblique beam incidence and with the mesh placed 1 cm above the surface. The measurements demonstrate the spoiling of the buildup zone, while having only a moderate influence on the dose distribution beyond the dose maximum. The mesh also changes the absolute dose. In a fractionated regime, the first part of the treatment would be without the mesh, adding it only during the latter fractions. The total dose distribution gives 90% to the skin, while leaving the depth-dose characteristics beyond the dose maximum virtually unchanged.