A stencil for guiding the placement of an optical marker for radiotherapy onto a surface is a sheet 20 with a hole 22 therein of a size and shape to snugly receive the marker. An upper surface of the sheet has markings 24,26 (such as crosshairs) for aligning with an image projected onto the target surface. The upper surface of the sheet may carry identification markings such as a barcode 30 or text 32. The sheet may be flexible or rigid but is typically thin with releasable adhesive on its underside. In use, an image is projected onto the target surface, the sheet is aligned with the projected image, and an optical marker is placed into the hole in the sheet. The sheet may then be removed. The optical marker has a reflector, the location 28 of which coincides with the centre of the crosshairs, and may be part of a photogrammetric or optical tracking system.
