Question: What is the name of the geographical region that early Islamic usage often regarded everywhere east of western Persia and where Vistaham helped Khosrow regain his throne after the rebellion of Bahram Chobin?  
Sentence (from "Shirin"): After conquering Jerusalem in 614, amidst the Byzantine–Sasanian War of 602–628, the Persians captured the True Cross of Jesus and brought it to their capital Ctesiphon, where Shirin took the cross in her palace.
Relevant evidence: No