My thinking is that the back button should follow the same metaphor as the browser bar. Its the same rationale that apple follows for their IOS interfaces where the back button is above next to the title.
This is because of context. Anything below the title is encounter specific (or page specific data) but a back button is outside of this context.
Long story short, i'd put the back button at the top. You may want to run some user testing to validate that premise. Show people the design and ask them how they would go back. If it takes more that 1-3 seconds, something is wrong with the interface.
You can also experiment with the look of the button. A back arrow, or a BACK / RETURN / CANCEL in words could help make it clearer (and is slightly more semantic)
Comments about 1-3 seconds. Normally navigation elements are expected to be in specific areas of the page, taking excessive amounts of time to find them is a serious usability issue.