CMS announced on December 19 that it has selected the first 32 health care organizations nationwide to participate in the Pioneer Accountable Care Organization (ACO) program. The Pioneer ACO program is designed to test a rapid transition to outcomes-based contracting and is an accelerated version of ACO model in the Medicare Shared Savings Program under section 3022 of the Affordable Care Act (ACA). The inaugural class of Pioneer ACOs consists of distinguished medical organizations with the ability to show what actually can be achieved through better coordinated, patient-centered care for Medicare fee-for-service beneficiaries.   Massachusetts-based organizations selected by CMS in this first round are Atrius Health, the Beth Israel Deaconess Physician Organization, the Mount Auburn Cambridge Independent Practice Association, Partners Healthcare, and the Steward Health Care System. Other New England organizations selected by CMS in the first round include the Dartmouth-Hitchcock ACO and the Eastern Maine Health Care System. The Medicare ACO contract period of performance for these leading organizations began on January 1.