PGA usage/content

Hello,

Is there a way to find out what the PGA currently used by a process is made of? For example:

USERNAME        SID   SPID     LOGON_TIME         PROGRAM                PID_REMOTE                Used MB Allocated MB Freeable MB   Max MB
--------------- ----- -------- ------------------ ---------------------- ------------------------ -------- ------------ ----------- --------
USER1           1056  20789    06-JUL-12         PROGRAMx (              18000                       622.0        714.0         7.0    714.0

The current session is using 622M, allocating 714 but it can only free 7Mb

How do I know what it is using the memory in and how can I force it to release it? (killing the session wouldn't be an option)

thanks in advance!

Daniel