Alicia:  I worked with a 39 yo woman who got preg with baby #4 3 yr
post-mastectomy after having been assured she was infertile due to
 chemo-therapy induced menopause.  Had a healthy preg. home birth and well
baby.  I saw her at 2 wk pp for fissured nipple in remaining breast.
 Basically it was a matter of positioning problems and needed to rest on pump
for a few days to heal.  That was easily resolved and she went on to
successful bfg that was still taking place at my last contact at about 2 mo
pp.  Her remaining breast functioned normally, and the only odd thing that
occurred was phantom pain in the area of the missing breast during let-down.
  When I attended the meeting in Chicago last Feb. on breast cancer during
preg. and bfg. one of the oncologists mentioned that cancer typically doesn't
spread from breast to breast.  It spreads from breast to lymph nodes to other
organs.  A 2nd cancer in the other breast would be a primary cancer.  I think
we all have to continue to be watchful for failure of lumps to resolve with
conservative care whether a woman has prior hist. of cancer or not.  While
its true mammography is not able to 'see' much in a lactating breast, its
also true that you can't get a good view of a preg. or even of a young
(dense) breast with mammograms.  If there were reason to, ultrasounds could
be employed.  These are not good for broad screenings, but can be used
diagnostically if there is a reason to suspect that an at-risk woman be
monitored.  Estrogen (as others have pointed out) is surpressed during
lactation.  Some cancers "bind" to different hormone receptors -- not just
estrogens.  This is such a dreadful and sad position for a young woman with
children to be in.  The woman I worked with hadnt reached the 5 yr survival
mark, and her future was by no means assured.  It meant the world to her to
bfeed her baby as she desperately wanted the baby to have a secure start
emotionally in case she had to go on without her mother.  It was a hard case
for me to counsel, as my sister-in-law had just died of breast cancer several
months previously leaving a 15 yr old daughter.  I think I have an obligation
to counsel women to do whatever they need to survive.  If it means weaning to
take chemo or radiation, I'd say wean so the baby has a mother.  If the
future is uncertain and there is no one who can prove there is added risk,
then the mother must decide.
Barbara Wilson-Clay, BSE, IBCLC
priv. pract. Austin, Tx