Intensity modulated radiation therapy is also called IMRT. IMRT radiotherapy machines use hundreds of tiny devices called multi leaf collimators to shape the radiotherapy area. The radiotherapy machine aims the beams at the tumour from many different directions. IMRT can create an indented (concave) area within the radiotherapy field to avoid high radiation doses to structures that would be damaged by the radiotherapy. The machines also vary the strength of the beams during each dose of treatment so they give very precise doses to a cancer or to specific areas within the tumour.