The ISOCHECK test object consists of two cylindrical POM plates, each of 20 cm in diameter and 5 cm in thickness with a segment cut away to form a base. Both plates have circumferential line marks and front side crosshairs to perform easy adjustment by means of the LASER beams. The LASERs should be adjusted to the mechanical rotation axis of the accelerator gantry. An X-ray film, packed in an opaque envelope, is put between the plates and fixed by means of plastic screws. The mechanical isocenter is marked by a needle on the test film through the hole in the center of the test object.