field catalog restriction with the selection screen input

Hi,

I am doing ALV grid report.In that I am facing problem,fieldcat has to display according to the input given in the selection screen.

my selection screen value is s_gstrp+4(2) that is month given inthe input.

     PERFORM FIELD_CATALOG TABLES IT_FIELDCAT
       USING: 'IT_COL_PLAF' 'PWWRK' 'X' 'Plant' ' ',
             'IT_COL_PLAF' 'MATNR' ' ' 'Material' ' ',
             'IT_COL_PLAF' 'MAKTX' ' ' 'Material description' ' ',
             'IT_COL_PLAF' 'MEINS' ' ' 'Unit' ' '.
      PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
                 'IT_COL_PLAF' 'GSMNG_01' ' ' 'Jan-Qty' ' '.

      PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
                 'IT_COL_PLAF' 'GSMNG_02' ' ' 'Feb-Qty' ' '.

        PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
                 'IT_COL_PLAF' 'GSMNG_03' ' ' 'Mar-Qty' ' '.

   
      PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
          'IT_COL_PLAF' 'GSMNG_04' ' ' 'Apr-Qty' ' '.

         PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
          'IT_COL_PLAF' 'GSMNG_05' ' ' 'May-Qty' ' '.

          PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
          'IT_COL_PLAF' 'GSMNG_06' ' ' 'Jun-Qty' ' '.

        PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
          'IT_COL_PLAF' 'GSMNG_07' ' ' 'Jul-Qty' ' '.

        PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
         'IT_COL_PLAF' 'GSMNG_08' ' ' 'Aug-Qty' ' '.

          PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
          'IT_COL_PLAF' 'GSMNG_09' ' ' 'Sep-Qty' ' '.

         PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
         'IT_COL_PLAF' 'GSMNG_10' ' ' 'Oct-Qty' ' '.

         PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
         'IT_COL_PLAF' 'GSMNG_11' ' ' 'Nov-Qty' ' '.

        PERFORM FIELD_CATALOG TABLES IT_FIELDCAT USING:
        'IT_COL_PLAF' 'GSMNG_12' ' ' 'Dec-Qty' ' '.

if month given in select-option : sep to dec ie,1.09.2009 to 31.12.2009.

then my field cat has to display,sep qty,oct,qty,nov qty,dec,qty.

Suggest some ideas.

regards,

Bathri