Point inside/outside area constructed by an expanded path

Hi,

I use postgreSQL.8.4.2 and don't have postGIS installed.
I have a geometric type polygon and I know that it is possible to determine
if a point it is inside or not of this polygon, using anything like that:
select
polygon('(-23.5672,-46.6510),(-23.5670,-46.65095),(-23.5672,-46.6508),(-23.5672,-46.6510)')
@> point('-23.567534,-46.64942')

The question:
Anyone knows an algorithm or have ideas how to determine if a point it is
inside an area constructed by geometric type path expanded for a known
distance?
Graphically:

the path
x=========x=======x

the path expanded by a distance

| | A |
| | |
-------x=========x=======x------- C
| | |
B | | |

the points A and B are inside and the point C are outside .

Suggestions are welcome.
Thanks in advance,
Josi Perez