Survival outcomes from the onset of all index HNCs (A).

Survival outcomes HNCs with second SPMs, from the onset of initial HNC(B) and from the onset of SPM (C) according to the age (age <65 versus age ≥65). Survival outcomes from the onset of SPM, according to the site of SPM occurrence (D). Survival outcomes from the onset of initial HNC according to the occurrence of SPM (E).