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The clinical neutron therapy system (CNTS) at the University of Washington Medical Center (UWMC) has been treating patients since 1984. Its new control system retains the original safety philosophy and delegation of functions among nonprogrammable hardware, PLCs, microcomputers with programs in ROM, and finally general-purpose computers. The latter are used only for data-intensive, prescription-specific functions. For these, a new EPICS-based control program replaces a locally-developed C program used since 1999. The therapy control portion uses a single soft IOC for control and a single EDM session for the operator's console. Prescriptions are retrieved from a PostgreSQL database and loaded into the IOC by a Python program; another Python program stores treatment records from the IOC back into the database. The system remains safe if the general-purpose computers or their programs crash or stop producing results. Different programs at different stages of the computation check for invalid data. Development activities including formal specifications and automated testing avoid, then check for, design and programming errors.
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