Dark brown patch at the middle of LCD panel when black displayed.

Product : MacBook Pro 15" Unibody Late 2011

Context : I left my closed MacBook near my bed. When I woke up, I put one foot on it (at back of the screen assembly, where the apple is), with little pressure, as I was sitting at my bed and as I immediatly felt the MacBook under my foot.

Symptoms : Since then, when a black (or very very dark) color is displayed on the MacBook screen, there's a diffuse dark brown patch in the center, looking like dust patch the keyboard might left on your screen when it's left closed and opened later, except it doesn't dissapear with some cleaning.

Question : how can I make this patch disappear, or what piece of the display assembly can I change to repair that ?

I tried to clean the screen panel with a duster, I also tried to apply presure on the screen panel with my finger where the patch appears and I tried to run various software based dead-pixel fixers. Without any effect.