De or het tumorvorming?

The correct Dutch article is:

de tumorvorming

Rule Words ending in -ing: almost always de. See the rule

How to use tumorvorming

the tumorvormingde tumorvorming
this tumorvormingdeze tumorvorming
that tumorvormingdie tumorvorming
our tumorvormingonze tumorvorming
your tumorvormingjouw tumorvorming
every tumorvormingelke tumorvorming
which tumorvorming?welke tumorvorming?
a big tumorvorming / the big tumorvormingeen grote tumorvorming, de grote tumorvorming