Hi, if the X ray
is showing the abutment or the stump getting decayed, the cap must be removed, the tooth treated( a root canal treatment
) and the cap must be replaced. Remember, a filling will never look like a decay. A decay appears as a black shadow or radiolucency and a filling is always radio opaque or appears white. If you leave the tooth unattended, it will decay completely and the cap will fall off. Root canal treatments are mostly successful . So do not worry and go ahead with the treatment. Take care!