No statistics . . . just found one personal story:

Sexual assault

On some nights, some prisoners would rape others for various reasons. Usually they were the result of personal fights caused by our intolerable conditions.

The sexual assaults did not end there, however. The Iraqi prison wardens also raped the prisoners. They favored the younger prisoners, especially volunteer soldiers who were sometimes as young as 10 or 11 years old.

There were some prisoners who, in return for privileges, would actually search for these boys and pick them out for the Iraqis. These mercenaries -- one of them had the nickname "Mohsen Amrikaie" -- would cloth the boys in Arabic garments, put make-up on them and hand them over for the sexual pleasure of the wardens.

http://www.iranian.com/Nov95/POW.html#Abuse