De or het radiumbehandeling?

The correct Dutch article is:

de radiumbehandeling

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

How to use radiumbehandeling

the radiumbehandelingde radiumbehandeling
this radiumbehandelingdeze radiumbehandeling
that radiumbehandelingdie radiumbehandeling
our radiumbehandelingonze radiumbehandeling
your radiumbehandelingjouw radiumbehandeling
every radiumbehandelingelke radiumbehandeling
which radiumbehandeling?welke radiumbehandeling?
a big radiumbehandeling / the big radiumbehandelingeen grote radiumbehandeling, de grote radiumbehandeling