How should accountable care organization be structured? If public comments submitted to CMS are any indication, physicians groups have been worried enough to urge the administration to stay flexible on organizational models that might serve for such organizations. In a letter the American Medical Association sent to CMS, it called on the administration to "do everything possible to facilitate participation by all types of provider structures authorized under the Affordable Care Act, and not inadvertently bias participation in favor of large health systems and hospital-dominated networks." FierceHealthcare