Earning his stripes?

It's been very dry here in the house and all of the dogs have been shedding and we ALL have dry skin, but Max seems to be developing some white stripes on his shoulders.

Today while I was brushing him I noticed that where he has these stripes, on both shoulders his hear is really thin, and actually, he doesn't have any under coat there, just a top coat.  I have no idea why this is happening.  The radiation never went anywhere near his shoulders, it took place 4 months ago, he doesn't lean and rub up against anything that would make the fur wear away there. 

His fur does seem very dry - so I'm wondering if it's a vitamin or mineral deficiency?  Will need to check ..

Although they look like a color change b/c of the shadows on his body - the white stripe on his shoulder is actually thinning hair.

You can see where the hair is thinned out in the stripe