There are 2 basic types of depression: "Situational" and "chronic". Chronic is long term, sometimes even forever, more difficult to manage and the medications have to be carefully managed and tweaked by a psychiatrist.

Some inexperienced doctors will prescribe chronic depression meds (like Prozac) when anti-anxiety meds, (like Zanax) would do the trick.

I never had a depression rating test, but as Charm said, it is a normal response to having cancer.

Get a referal to a psychiatrist - managing brain chemistry is what they do. Oftentimes cognitive therapy by a psychologist is used in conjunction with the psychiatry.