Sometimes Radiation Oncologists, doctors who prescribe and are specialized in radiation therapy, can do brachytherapy treatments. In the case of a lung tumor, the physician would introduce a strand of seeds in through the lungs via the mouth. It is a relatively simple procedure that is very quick and can be done usually in a Radiation Oncology Department. The Radiation Oncologist should go over the side effects that he should experience during the treatments. Any side effects your doctor explains generally don't start happening for a few weeks into the treatments.
Radiation is cumualitive.
That's why radiation is given during a course of 4-6 weeks typically instead of just a few treatments. Radiation relies on the cell cycle. Radiation is only given 5 days a week because the cells need a few days off to recover. Radiation not only kills bad cells, but it also kills good cells too. The hopes are that the bad cells won't re-generate, but the good cells will with time.