Sysdate and Current Month

Hello,
  I have an Oracle statement that returns product information for the past 120 days.

I need to modift the date part so that I get a current "Month to date" data set.
You can see my sysdate part below.  How do I modify this to get only the current month?

SELECT   invc.acct_dt, part.user_part_no, Sum(inv.qty) as QTY
    FROM invc invc, invc_ln_prod_fam inv, part part
   WHERE (    (invc.invc_no = inv.invc_no)
          AND (inv.part_no = part.part_no)
          AND (   (invc.invc_status_cd = 'OPEN')
               OR (invc.invc_status_cd = 'CLOSED')
              )
          AND (invc.invc_type_cd = 'INV')
              and trunc(invc.acct_dt) > trunc(sysdate -120)
         )
GROUP BY invc.acct_dt, part.user_part_no
ORDER BY part.user_part_no ASC