Radiotherapy can be given in two different ways – from outside the body (external radiotherapy) or inside the body (internal radiotherapy). External radiotherapy usually involves using a machine called a linear accelerator, which focuses high-energy radiation beams onto the area requiring treatment. External beam radiotherapy is completely painless. External beam radiotherapy usually involves a series of daily treatments over a number of days or weeks. Internal radiotherapy can involve placing a small piece of radioactive material temporarily inside the body near the cancerous cells (known as brachytherapy), or the use of a radioactive liquid that's swallowed or injected. The radiation emitted by internal radiotherapy is painless, though the procedure to insert the source can sometimes cause mild discomfort.