I think you forgot to add rewrite
parameter in arguments,
Use this arguments:--
$args = array(
'hierarchical' => true,
'labels' => $labels,
'show_ui' => true,
'show_admin_column' => true,
'query_var' => true,
'rewrite' => array( 'slug' => 'cancers' ),
);
Note:- After changing this, please update your Permalinks
from Settings --> Permalinks
once...