Could be both really - the fact that it's 'a metastasis' is about the closest thing to a description of a cancer's characteristics as you're going to get, but I could also see how it could reasonably be viewed as a measure of its severity. But, I'd lean more towards quality, personally.
It doesn't really matter which one it is, as long as you don't have those other HPI elements anywhere else in the note - 2 (unique elements of) HPI is 2 HPI, no matter which 2 you've got - for the phrase you gave, I'd count Quality and Location, if I were in your shoes.