"Decision tree boundaries"

I am new to rapidminer and coding in general and I have a simple question. Usually, decision boundaries for decision trees are rectilinear. Does anyone know a very basic way that the boundaries can be a circle instead? I don't have to come up with the algorithm or anything. Just on a basic level. Any help is appreciated as I am having a hard time trying to figure this out. I was told to assume that we are using the entropy measure to measure impurity.