Tumors as malignant as lung cancer can be cured by radiation therapy. Local control is the primary requirement for complete cure of the tumor, but the continued presence of the mass demonstrated by imaging techniques does not necessary mean persistence of viable tumor cells. The mass shadow persisting in the region irradiated at a cure dose may simply represent delayed absorption of dead cells so that the patient should be observed for at least 3 months after the therapy. This requires patience on the part of clinitians, but erroneous additional irradiation must be avoided, because excess irradiation can do harm but no good. Radiosensitivity, radioresponsiveness, and radiocurability are all independent characteristics of the tumor. In general, the effects of radiation therapy must be evaluated comprehensively in consideration of these three characteristics of the tumor. Understanding of the natural history of the disease is essential for the treatment of malignant tumors. Therapeutic approach cannot be determined if the natural history of tumor is disregarded.