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NASHVILLE, Tenn. — Gov. Bill Haslam has vetoed legislation that would have required state employee insurance to cover proton therapy, an alternative treatment for certain cancers.

In a statement Thursday, the Republican governor said the legislation could put patients at risk and expose them to excessive charges from out-of-network providers.

The bill sponsored by Sen. Mark Green and Rep. Bob Ramsey, both Republicans, would have helped a Knoxville-based proton therapy company that plans to open another facility in Franklin. Medical experts say the treatment reduces damage to surrounding tissue.

Haslam said the provider advocating for the bill rejected a medically appropriate plan to expand coverage and instead pursued a political mandate.

House Speaker Beth Harwell and Senate Speaker Randy McNally said it would be premature to call for a veto-override special session.