The function of a treatment planning program is to graphically simulate radiation dose distribution from proposed radiation therapy treatments. While many such programs are available which provide this much-needed service, none addresses the question of how to compare calculation and display techniques. This paper describes a program system described for support of research efforts, particularly development and testing of new calculation algorithms. The system emphasizes a modular flexible structure, enabling programs to be developed somewhat as interchangeable parts. Thus multiple variants of a calculation algorithm can be compared without undue software overhead or additional data management. Unusual features of the system include extensive use of command procedures, logical names and a structured language (PASCAL). These features are described along with other implementation details. Obstacles, limitations and future applications are also discussed.