Using Case in Order by clause

How would I use the Case statement in the Order by clause in this query? The rest of the query works without the case statement. The DDLs are long, so I didn't post them. I think I just need help with the syntax. Let me know if you need the DDLs.

:DBLookupComboBox1 is alphanumeric

:DBLookupComboBox2 is numeric

BL.ChargeName and TT.Doctor are varchar

I've seen posts suggesting casting to numeric when using Case. I don't know if that is the issue here.

The error message is "Incorrect syntax near ','"

SELECT TT.DOCTOR, TP.Adatetime as 'Date of Service', TP.Patient_No, 
(TP.Last_Name + ', '+ TP.First_name) as 'Client Name', CC_Loc.description as Location, 
BL.ChargeName as Clinic, PS.Service_Name, PS.Service_Code, Convert(Int,REPLACE(PS.Service_Time, ',', ''))
 as 'Time Spent'
FROM treat TT
Inner Join typepatient TP on TT.ednum = TP.Ednum
Inner Join Live_Knowledge.dbo.CRIS_LOCATION CC_Loc on TP.location_id = CC_Loc.id
Left outer join Patient_Service PS On TP.Ednum = PS.Ednum
left outer join Billing Bl on TP.ednum=BL.Ednum     
Where TP.Adatetime >= convert(varchar(10), :ST, 121)+ ' 00:00:00.000'
and TP.Adatetime <= convert(varchar(10), :SP, 121)+ ' 23:59:00.000'
And TP.Sign = '1'  
and TP.Visit_Type in ('Outpatient', 'Inpatient', 'Substance Evaluation', 'Emergency', 'Triage')
and TP.Visit_Name <> 'Backfill'
 and (((:DBLookupComboBox2 = 0)  or (TT.Dr_ID =:DBLookupComboBox2)) and ((:DBLookupComboBox1 = 'QQZ') or (BL.ChargeName = :DBLookupComboBox1)))                         

ORDER BY
CASE WHEN :DBLookupComboBox1 = 'QQZ'
AND :DBLookupComboBox2 <> 0
THEN BL.ChargeName, TP.Date_ END asc,  
CASE WHEN :DBLookupComboBox1 <> 'QQZ'
THEN TT.DOCTOR, TP.Date_ END asc