Problem with string concatenation

declare @Topicid dtid
Declare @LevelID varchar(10)

select @Topicid = 'L10000'
select @LevelID= substring(@Topicid,1,2)  -- output - L1

select QID,Marks,Question from QBObjective	
where @LevelID+'SubjectID' = @TopicID -- FAIL

--L1SubjectID is my column name

//-- not giving any results, actually data is in table with specified id

print @LevelID+'SubjectID'  // output is L1SubjectID

-------actual syntax

select QID,Marks,Question from QBObjective	
where L1SubjectID = @TopicID -- Success