The Thoron shield was a new prototype defence system that was deployed on the Frontier-class starbase Deep Space 9 (II). This system was a fully independent defence field generator from the standard shield generators and offered stronger protection against enemy attack, both energy and physical, as well as dampening all subspace communications and sensor scans.
Starfleet intended to deploy thoron shields on the next generation of starships once they could determine how to deploy thoron-based technology on warp-capable starships without interfering with their engines. As of 2385, Deep Space 9 was the only known facility equipped with this defense system. (DS9 - The Fall novel: Revelation and Dust)