@ddbrown61 Welcome to Mayo Connect, I hope your treatment so far is going well. That is a very important question, one many people would not know to ask!

Radiation, by itself, should have little impact on your blood sugar. However, steroids are often used to reduce swelling in the brain during radiation, and can cause elevated blood sugar, which can start suddenly and without warning.
This is even more probable if you already have Type I or Type II diabetes, and are on insulin or medications.
So, prior to the star of radiation, your blood glucose and A1C should be checked. And you blood sugar should be monitored. The best way is for you to test it yourself daily with a glucometer, but if that is overwhelming with everything else going on, it should be checked before every radiation treatment, and with every doctor or nurse visit.
We did not know this when my mother-in-law was having radiation for a tumor, she had no history of diabetes or pre-diabetes, and we didn't realize what was going on until she had ketosis. It was at that point when the hospitalist told us what the radiology clinic should have been doing.
Sue