We evaluated water-equivalent slabs as an alternative to a bolus to reduce radiation dose to the underlying lungs during electron scar boost irradiation in breast cancer patients with a thin chest wall undergoing post-mastectomy radiation therapy.

Percent depth doses (PDDs) and attenuation factors were obtained for 6 MeV (the lowest electron energy in most clinics) with solid water slabs (1–10 mm by 1 mm increments) placed on top of electron cones. Scatter dose to contralateral breast caused by the solid water slabs was measured on a human-like phantom using two selective scar boost patient setups.

The PDD plots showed that the solid water slabs had similar dosimetric effects to the bolus with lower skin dose to ipsilateral breast for the same thickness. Slab attenuation and scatter dose to the contralateral breast were increased by ~220% and by a factor of 3 with a 5 mm slab, respectively.

Our results demonstrate the feasibility of using water-equivalent slabs to reduce lung dose for electron scar boost treatment in mastectomy patients with a thin chest wall. However, the increases in treatment time and scatter dose to the contralateral breast are the disadvantages of this approach.