Thanks to everyone for your responses. I suspected that cornstarch was something they recommended to people who were having radiation somewhere where there are skin folds (I think my cancer center sees a lot of breast cancer). I'm finding, as I wend my way through all the information I receive from the various health professionals, that there isn't a organized view on how to deal with the range of side effects. For instance, I am up to eight different "home" remedies (from doctors, periodontists, nurses, radiation therapists, etc., all of whom are reportedly on my "care team") with which to swish my mouth, some of which seem to be in direct contradiction to each other.

In any case, thanks for confirming my suspicion that cornstarch didn't make sense.