Lung cancer is one of the leading causes of cancer-related deaths. Recently, radiotherapy is being used extensively for the treatment of patients suffering from cancer. Proton therapy is a very suitable form of radiation therapy for tumors that occur in the vicinity of critical tissues such as lung, due to the unique characteristics of the protons used in the treatment. In this study, a geometry model with lung, heart, tumor and bone structures in a water phantom is modeled with the Monte Carlo simulation tool vGATE, version of 7.2. With this simulation, the accumulated dose are calculated for each organ.