Lufthansa would consider a backpack a personal item if it does not exceed maximum dimensions of 11.8 x 15.7 x 3.9 inches (30 x 40 x 10 cm). If it is larger than this allowance then there is a chance you could be asked to check your carry-on piece or backpack. Why does Lufthansa only allow an 8kg (17lb) carry-on bag?