We used an object-oriented design to build a large scientific application: simulation of radiation therapy treatments for cancer. We provide features familiar in the graphics workstation world, including graphic editing of the proposed treatment, multiple views of the treatment in different windows, and computations which proceed concurrently as the input data are being edited. To make our system practical for the typical clinic we used a popular minicomputer and the vendor's operating system and compiler. This paper describes how we implemented objects, inheritance, message passing, windows, and concurrency in (almost) standard Pascal on a VAX under VMS.