External beam radiotherapy targets the radiation at the tumor to kill cancer cells. There's no lingering radioactivity. You don’t need to worry about being radioactive at any time during or after treatment.
If you have internal radiation therapy (brachytherapy), you may need to be cautious. Brachytherapy involves placing a contained dose of radiation near the tumor. While it's active in your body, you may give off a bit of radiation.
While it’s active, you may need to limit contact with all people. You may just need to avoid pregnant people and young children. This will depend on the radiation dose.