The goal of surgery for lung cancer is the complete removal of the tumor located in the lung. The tumor is removed along with the surrounding tissue, known as the resection margin. We say the tumor has clear margins if we do not find traces of cancer at the resection margin, which is determined when we examine the portion of the lung that has been removed after surgery. At surgery, lymph nodes near the lung and in the mediastinum are also removed. This is done to check for the presence of cancer and to decide if additional therapy is needed. It is important in every operation to remove at least 4-5 lymph node groups regardless of whether they appear clear on tests or to the naked eye.