Answer : Radiation therapy is a therapy which uses a high-energy x-rays to destroy cancer cells and shrink tumors. Radiation therapy mainly targets the rapidly dividing cells.
Internal radiation therapy is when they place a radioactive material near the tumor cell and it is radiated; whereas in External radiation is when the radiation comes from outside source and is bombarded on the affected tumor cells.
Depending upon the type and location of the tumors, the way of radiation can be decided. Although, for most tumors, external radiation is the recommended; but still there are quite a few cases where internal radiation is the better option.