Concierge medicine is expanding as more doctors—and patients— tire of assembly-line primary care, opting for something more personal, and pricey. The numbers are still very small —a survey commissioned by a congressional agency last year identified 756 concierge medical doctors in the United States, up from 146 in 2005. And Florida-based MDVIP, a company that helps physicians set up these practices, said it will add six new MDVIP doctors in the Boston area this year, increasing its physicians statewide to 16. But even a tiny number of doctors leaving traditional offices for boutique practices—out of thousands of primary care physicians—is enough to make some health care industry leaders nervous. They worry that more doctors will follow as insurers and government payers cut fees and hem in providers with regulations. And when even one doctor makes the switch, there are substantial side effects, leaving hundreds of patients to scramble for a new physician.