A new study published in Health Affairs has found “substantial” turnover among physicians participating in the Pioneer accountable care organization.

The research aimed to collect data on physicians who deliver care within ACOs since they had previously received “limited attention,” according to the study’s Massachusetts General Hospital-based research team.

More than half of physicians within the organization either joined (41%) or left (18%) the ACO during the two-year time period studied. The research team also found that nearly half of the growth experienced in the beneficiary population could be chalked up to new physicians joining the ACO.

“These findings may help explain the muted financial impact ACOs have had overall, and they raise the possibility of future gaming on the part of ACOs to artificially control spending,” the study’s authors said in their report.

John Hsu, M.D., MBA, the study’s lead author, told Modern Healthcare that the group’s findings are especially important since “physicians really are the crux of the ACO itself.”