Trump claims Biden has stage 9 cancer:

[Ms. Toad]

2. Unless you've dealt with cancer, the labels are confusing.

Someone on DU yesterday said he had stage 5 cancer. He doesn't have that either. He has a Gleason score of 9, which puts him in the grade group 5.

Stage has to do with size and spread.
Grade has to do with what the cells look like.
Most cancers/institutions use 4 stages and either 3 or 4 grades.
Prostate cancer uses 5 grades based on an evaluation of the two most prevalent appearances in the sample - giving individual stages up to 5 for each of two cell types, which are added together to create the Gleason score - which is use for a grade group.

Trump is really dumb - I'm not disputing that. But getting confused about the multiple ways in which cancer is labeled has nothing to do with being dumb. It's confusing even for people who have different kinds of cancers - and even for people with the same kind of cancer visiting different institutions which use a different grading scale.