Answering my own question.
A letter from the Russian hospital outlining the course of illness, the treatment taken, the remission length and lack of adverse effects was enough. With a notarized translation into Spanish. The AME was mostly interested in the part where it says there is no adverse impact and no risk of incapacitation. She also double checked that the tumor was NOT in the brain.
So I got my Class 2 with no problems.
I'm wondering if it'd be as simple for Class 1...