The invention relates to an after loader apparatus for treatment of tumours in an animal body comprising a housing with at least one exchangeable radiation source connected to a first end of a transport wire, which wire is connected with its other end to transport means for advancing said transport wire and said radiation source through a guide tube to and from a tumour in said body, where at least the radiation source and the transport wire are mounted in an exchangeable radiation shielded after loader cartridge, which cartridge can be placed in a suitable receptable opening present in said housing.It is an object of the invention to provide an after loader apparatus wherein in a more safe manner to the environment a cartridge containing a radioactive source can be replaced by another cartridge containing a new radioactive source and according to the invention the after loader apparatus is characterized in that, for exchanging said radiation source after use an exchanging device is present for, in a first step, removing the cartridge with the used radiation source from said receptable opening and, in a second step, inserting a further cartridge containing a new radiation source in said receptable opening.The invention relates furthermore to a device for exchanging an after loader cartridge in an after loader apparatus according to the invention.