Marginal Failure

Hi, I'm translating an article for a friend, an AVM survivor. There's an unclear term - "marginal failure," which everyone in the Net seems to know but nobody wants to explain. Could someone explain it to me, please? Here's the context:
"Persistent out-of-field nidus (marginal failure) was identified in XX previously embolized versus..."

I did som research and it appears in reference to radiation surgery and I saw a substitution of ‘marginal target miss’ which leads me to believe that it is referring to being slightly off target with the radiation. I.e. In layman’ terms if one circle is the AVM and another is the radiation. They would overlap but not exactly line up.

Thank you so much, Jeremy!