Proton Beam denied by insurance

Anyone ever have proton beam therapy denied by their insurance?
Blue cross and blue shield of Arkansas denied my dad's treatment despite the significant improvement in the level of radiation damage to outlining organs ( based on tumor location) in comparison to regular radiation. Mind blowing differences yet insurance company says no. Despite regular radiation and proton beam therapy costing the same...
My dad had to pay out of pocket and is still working with insurance company on some sort of reimbursement, if any... to actually start treatment.