Treatment usually indicates something medical used with the intent to cure ( medicines, surgery) ; therapy is used for other treatments that will help restore a person to his optimal health e.g., physical therapy, massage. Very often used in the field of psychology too, i.e., couples therapy, Gestalt, etc.

I would say, as a physician in America, they are often used interchangeably. Therapy could be historically thought to be more connected to psychological troubles. It is now often used to describe pharmacological treatment as well as any intervention requiring prolonged or repeated interventions. An example of difference may be limited to surgical correction or treatment of injury. Anything (like most surgeries) with a distinct end-point would be a treatment, rather than therapy. Example: "Appendectomy is the only proven treatment for appendicitis" "Therapy (or treatment) with blood-pressure lowering medicine is the best approach to reduce your risk of heart attack or stroke" "I would recommend physical therapy for your recovery after knee replacement surgery." -due to multiple required visits, therapy would be used rather than treatment. "physical therapy" is the "treatment" does that help? is there a specific use you are concerned about?