Back injuries suffered in a fall early last week from a swing near Maurer's garage, east of Nesquehoning, proved fatal today to Peter Semanoff, 18, son of Mr. and Mrs. John Semanoff of Nesquehoning, who died at 5 p.m. at the Temple University Hospital, Philadelphia. The youth thought his injuries were superficial until X-rayed at the Coaldale State Hospital. He was admitted to the Philadelphia hospital where he underwent surgery.