In the frame of a pilot project for percutaneous tumor treatment using a reactor produced fission beam, a small therapy planning prestudy has been performed. The fundamental data for the depth dose distribution in a phantom are derived from transport calculations for neutrons and from published measurements for the gamma contribution. Simple decrementline approaches are used to represent the beam profiles. Tissue-air-ratios known from Co-60 irradiations, are adjusted by simple density corrections to fit the required depth dose characteristics. It is shown that for the calculation of the dose in a material which is different to tissue, with respect to composition, a unique kerma ratio in the depth region of interest can be applied. Some examples of tumor treatment in the head and neck are planned. They show that the fission neutron beam is applicable in the treatment of superficial and half deep tumors with respect to dose homogeneity, but there are clear limitations as far as tumor size and position are concerned.