Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
region [ˈriːdʒən] n (area, of country) райо́н, регио́н (ADMIN, ANAT) о́бласть*f
in the region of (fig, approximately) в райо́не +gen
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.