Radiation induced diarrhea is a whole different animal from the usual suspects.
In “normal” cases, a bout has a reason, which we can correct using fiber, probiotics, etc. But radiation produces an inflammation which is very hard to combat with products we are familiar with.
You really have to turn to the Immodium or Kaopectate to try to get it under control. Or if it’s still not responding, get an RX for Lomotil - it contains an opioid which basically stops the contractions of the gut. They also add a compound which makes it non addictive, but you should still follow dosage recommendations.
Phil