Door Pillar, B pillar??

[wolfthing2000]

I think it is called the B pillar, at any rate it is where the door hinges bolt to.
I was under the truck cutting out a small rusted area at the bottom of the front cab support that goes across to the under side of the pillar and bolts up under there through the inner rocker panel. After I cut out the bottom of the floor brace I was cleaning out the sound metal on the inside and noticed the old ugly bubble of rust on the surface of the inner rocker panel and right between the channel where it bolts up. A telltale sign of trouble on the other side!!
I poked around there with a screwdriver long enough to brake through and Holy Cow the dirt and old mud and flaky rust that came out would have filled up a beer can ( I measure a lot of things with this world wide standard that doesn't have to be English or Metric, we just all KNOW how much a beer can amount is!!!)
Question is, where does this dirt get in there from and how do I prevent future accumilation there to avoid future problems ( there is a fair amount of cancer removal and repair ahead in this area and the inner kick panel)??
I seem to remember a photo on this forum of a truck that was patched up in the kick panel area and it seems like I remember them leaving an opening about 1 inch or so along the bottom where the kick panel meets the floor.

Any thoughts????