Beam modification devices are used in radiotherapy to modify the spatial distribution of radiation within the patient. The main types of beam modification are shielding to eliminate dose to some areas, compensation for tissue heterogeneity or beam obliquity, wedge filtration to tilt isodose curves, and flattening filters to modify the natural beam profile. Common beam modification devices include shielding blocks, compensators, wedge filters, and multileaf collimators. The choice of device depends on the treatment site and goals.