How to change Bounding volume from box to region?

I have a problem when changing Bounding volume from box to region.I use Cesium.Cartographic.fromCartesian() to convert cartesian coordinates to radians,but the result is wrong.The center of box volume is [ -4.65661287307739e-10, 0.000116813229396939, -0.000936155673116446],and the result is [height: -6357078.545744079,latitude: -1.4474808437139701,longitude: 1.5708003131694683],
the transformed Bounding volume is shifted a lot!