Write CASE condition for one column in the Query

I have a query in which, I want to apply CASE condition. This is my query:

Select 
    a.mkey, a.party_name, a.doc_no Inward_No, 
    c.type_desc Doc_Type, a.ref_no, 
    convert(varchar(25),a.ref_date,103) Ref_date,
    a.Inward_amt, b.first_name + ' ' + b.last_name ToUser --- here
from 
    inward_doc_tracking_hdr a
inner join 
    user_mst b on a.To_User = b.mkey
inner join 
    type_mst_a c on a.doc_type = c.master_mkey  
                 and a.mkey =227423

What I want is: if I get ToUser as NULL then I want to take Last_To_User in place of that.

How to do this in SQL Server?