Radiation is often used in prostate cancer treatments. Radiotherapy treatment planning is a complex decision making process. In order to make a patient well, the oncologist has to make a trade-off between the benefit of radiation i.e. delivering a high enough dose to fight the cancer cells, and risk that refers to the side effects of the treatment. In this research, a case based reasoning system is developed for dose planning in prostate cancer treatment. A case similar to the new patient is retrieved. An adaptation of the suggested plan is performed taking into account trade-offs made in the past to determine the extent to which the maximum dose limits of the new patient (case) can be violated. The performance of the proposed method is validated on the real data sets obtained from the City Hospital Nottingham, University Hospital, NHS Trust, UK.