The 33-year-old former Wales captain, who was diagnosed back in October, is in line to play against Ulster on Saturday.
Rees last featured for the Blues in October during a 29-12 win over Edinburgh.
"To get over what he's got over and to come back to this level of rugby is absolutely unbelievable," caretaker coach Dale McIntosh is quoted as saying by BBC Sport. "It's an inspiration to us all."
Rees returned to training in February.