GSTM1 and nasopharyngeal carcinoma: Stratified analyses demonstrated that heavy smokers with low post‐RT Hb had HRs of 2.295 (P < 0.001) for death, 2.222 (P < 0.001) for disease failure, and 2.267 (P < 0.001) loco‐regional recurrence compared with light smokers with high post‐RT Hb levels, and post‐RT Hb level is an important predictor of survival in patients with NPC.