The computational aspects of the design of an optical distance indicator (O
DI) for use with a radiation therapy accelerator are discussed. The specifi
cs of the optical and mechanical design of an ODI for use with a supercondu
cting cyclotron used for fast neutron therapy are presented. (C) 1999 Ameri
can Association of Physicists in Medicine. [S0094-2405(99)01802-7].