Loss of electronic equilibrium in lung tissue causes a build-up region in the tumor. Increasing the photon energy increases the depth at which electronic equilibrium is reestablished within the lung tumor. This study uses the Monte Carlo code PENELOPE for simulations of radiation treatment of tumor surrounded by lung. Six MV photons were compared to 15 MV photons using four beam arrangements in both homogeneous and heterogeneous media. The experimental results demonstrate that for every beam arrangement in heterogeneous media 15 MV photons delivered 5% to 10% lower dose to the tumor periphery than 6 MV photons. The simulations also show that in axial coplanar treatment plans, the loss of electronic equilibrium was greatest in the coronal plane. In conclusion there is a tumor sparing effect at the tumor-lung interface that is a function of beam energy. As an alternative to increasing beam energy, the addition of multiple beam angles with lower energy photons improved target coverage. If higher energy beams are required for patients with large separation, then adding multiple beam angles does offer some improved target coverage. The non-coplanar technique with the lower energy photons covered the tumor with a greatest isodose at the tumor periphery without tangential sparing in the coronal plane.

Key words: Lung, Radiation, Monte Carlo, Dosimetry.