I'm a caucasion with hypopigmented blotches across my upper back. What is it, and how can it be treated? (photo)

It's spreading across my shoulders and the front of my neck. I've been to 2 dermatologists who only gave me bleaching cream for my normal skin (which is somewhat sun-damaged and acne-prone). I doused myself for a month in dandruff shampoo--nothing.

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