UnitedHealth's Optum plans to acquire Atrius Health, the largest independent physicians network in Massachusetts. The deal was first reported by The Boston Globe and is the latest in a succession of consolidations in the state's health care market. The nonprofit physician group operates 30 medical practices that offer primary care, more than 50 specialties, imaging, lab and pharmacy services to the Boston area. Atrius officials said they signed a definitive agreement with Optum on Monday, according to The Boston Globe.