Usually the initial treatment for small cell lung cancer is chemotherapy. With chemotherapy 80-90% of patients will have a positive response to treatment. The drugs given enter the blood stream and reach most parts of the body but only pass very low levels of chemotherapy to the brain. This means that if microscopic cancer cells (unable to be seen by a scan) have spread to the brain they can continue growing there despite the chemotherapy. In the past patients who have responded well to chemotherapy later developed problems from spread of the cancer to the brain. This is quite difficult to treat once it appears but can usually be prevented by giving a short course of radiotherapy to the head.

Although a CT scan gives very good pictures of the brain it is not sensitive enough to detect  microscopic cancer cells. So a scan does not rule out the possibility of cancer cells in the brain. If there is no cancer beyond the chest when treatment started and the disease responds completely to chemotherapy then radiotherapy to the whole brain is usually given as it has been shown to help people live longer.