To improve the local rate of success or the conversion of cancers of the cavum oris and of the oropharynx which are inoperable because of their local situation, preoperative radiation is carried out in cases where this is appropriate. The radiation and the radical operation must be coordinated with each other. For this purpose the borders of the cancer are marked by tatooing them with ink. The tatooing makes it possible both to judge the effects of the radiation accurately and to locate the borders of the tumor with the naked eye after radiation treatment in apparently healthy tissues. In this way it is possible to perform the radical operation with an adequate safety margin around the pre-therapeutic borders of the tumor. With this method the rate of success in tongue-body, tongue-ground and tonsil cancers can be improved.