>but the whole region and even country when it comes to specialized treatment

Seriously, my FIL visits us 2-3 times a year to see a Parkinson specialist at MGH. I live out in Western MA now and will be bringing my baby into Childrens in a few weeks to see a plastics specialist and specifically made sure my PCP out here was MGH affiliated so if i need a specialist I can see one in Boston and not in Springfield. When we lived in NH Dad got his hip replaced in Boston. Everyone is making the same decisions my family does.

I'd say it's even larger then the standard Boston metro region. Essentially the entirety of Vermont, Maine, NH, MA, RI and parts of CT and NY are gonna end up in Boston for "big" stuff.