Four employees of a shuttered outpatient surgery center in Buena Park were convicted Monday in Orange County Superior Court in a $154 million medical fraud case that prosecutors called the largest of its kind in the U.S.

The charges of tax fraud stemmed from accusations that Unity Outpatient Surgery Center billed insurers for payments for unnecessary surgeries, according to a report published on the Insurance and Financial Advisor Web site.

Convicted on multiple counts of tax fraud were: Roy C. Dickson, an attorney for Unity Outpatient; Andrew Harnen, an accountant for the center; and administrators Dee Francis and Rosalinda Landon.

Prosecutors said Unity Outpatient recruited more than 2,800 patients for unnecessary surgeries that were billed to insurance companies. Insurers paid out more than $20 million during a nine-month period, they said.

Dickson, Harnen, Francis and Landon were convicted for their roles in recruiting more than 250 patients.

They each face another 101 charges for allegedly taking more than $2.5 million each as part of the scheme, according to prosecutors. They will be tried on those charges at a later date.

Several other defendants also face charges.

The case has already led to convictions of six other defendants, including Unity Outpatient owner Tam Vu Pham, who received a 12-year prison sentence and an order to pay restitution.