Abstract: The collimator assembly comprises a collimator shielding block for blocking undesired X-rays and an insert piece inserted into the shielding block. The insert piece which is made of a material of high atomic number, has an inner conical surface which defines the X-ray cone transmitting the insert piece and therefore the maximum field size which can be obtained for medical treatment. In order to supply different maximum field sizes, there are provided means for easily interchanging the insert in the collimator shielding block with another insert having a different interior conical dimension. By using inserts of different cone angles, which can easily be inserted into the shielding block, for instance by means of a screw thread from below the shielding block, morbid tissues of different sizes can be treated while the surrounding healthy tissue is fully protected.