PL/SQL populating a cursor from a pl/sql table

I want to populate a cursor from a pl/sql table.  The components are defined like this:

TYPE rtype is RECORD(
  the_key   another_schema.another_table.key_column%TYPE,
  ...);
TYPE ttype is TABLE OF rtype INDEX BY BINARY INTEGER;
TYPE ref_cursor_type is REF CURSOR;

CREATE OR REPLACE PROCEDURE dtp (cur_in IN OUT ref_cursor_type)
IS
dtp_temp_tab ttype := ttype();
dtp_temp_row rtype;
BEGIN
  -- Fill up the record, this happens lots of times in this procedure in varying ways under different conditions
  rtype.col1 := value1;
  ...
  rtype.coln := valuen;

  -- Insert the record into the table (this may happen many times in the procedure)
  dtp_temp_tab.extend;
  dtp_temp_tab(dtp_temp_tab.last) := dtp_temp_row;

  ...

  -- Finally at the end of the procedure and want to return the rows in the table using a cursor
  OPEN cur_in FOR 'SELECT * FROM TABLE(CAST(dtp_temp_tab AS ttype))';
END;
/

I get back:

ORA-00904: invalid column name

I'm running Oracle 8i 8.1.7.

I don't want to use a table of objects because I want the table in my procedure to be able to change as the columns in the tables in the database change.  I don't want to use a VARRAY because I cannot reasonably set an upper limit.  10,000 is probably the upper limit but that is an extraordinary amount of space to reserve.

I thought that the TABLE(CAST()) combo worked on collections, but I'm suspicious that the pl/sql table is not considered a collection, or I haven't called it properly.

Thanks in advance...This is my first question, so if I have been incomplete or screwed it up somehow, don't hesitate to tell me.