sql median in grouped query

hi, i have the sql query below for a report, i need to calculate the median Concentration for the itemtype. any body know how i can do that? the median is the middle Concentration number of the group if it has old number of rows, and if its even, it is the middle 2 number divided by 2.

SELECT [itemType], SUM(CASE WHEN Concentration = '-999' THEN 0 ELSE 1 END) as [Tsample]
    ,SUM(CASE WHEN Concentration = '-1' THEN 1 ELSE 0 END) as [Tnd]
    ,SUM(CASE WHEN Concentration != '-1' AND Concentration != '-999' THEN 1 ELSE 0 END) as [Tnnd]  
    ,MIN(CASE WHEN Concentration = '-1' OR Concentration = '-999' THEN NULL ELSE Concentration END) as [Min]

    , ----   Median Concentration for itemtype

    ,MAX(CASE WHEN Concentration = '-1' OR Concentration = '-999' THEN NULL ELSE Concentration END) as [Max]
    from dbo.vwSampleDetails as a
  GROUP by [itemType]

this will calculate the median if i was working with the whole table, but i need it to calculate inside the group by query

  Select ((
        Select Top 1 Concentration
        From   (
                Select  Top 50 Percent Concentration
                From    vwSampleDetails
                Where   Concentration != '-1' and Concentration != '-999'
                Order By Concentration
                ) As A
        Order By Concentration DESC) +
        (
        Select Top 1 Concentration
        From   (
                Select  Top 50 Percent Concentration
                From    vwSampleDetails
                Where   Concentration != '-1' and Concentration != '-999'
                Order By Concentration DESC
                ) As A
        Order By Concentration Asc)) / 2