A wellhead assembly for an injection tubing string is provided which allows a master valve to be closed without damaging the injection tubing string while still allowing for the use of a back pressure valve to isolate the tree. Wellhead assemblies and related methods of the present invention include a flange adapted to be connected between a wellhead and a Christmas tree. The assembly also includes a mandrel adapted to being inserted into the longitudinal bore of the flange, the mandrel having a port for communicating with an injection port which extends radially through the flange. The assembly further includes a hanger adapted to connect to the upper end of the injection string, wherein the hanger is further adapted to land in the longitudinal bore of the mandrel. The hanger includes a communication passageway for facilitating fluid communication between the port of the mandrel and the injection tubing string.