Prognostic index for patients with parotid carcinoma: external validation using the nationwide 1985-1994 Dutch Head and Neck Oncology Cooperative Group database - Vander Poorten, VL M; Hart, AA M; van der Laan, BFA M; Baatenburg de Jong, R J; Manni, J J; Marres, HA M; Meeuwis, C A; Lubsen, H; Terhaard, CH J; Balm, AJ M (Cancer, 2003)