Selenium is needed to make the internal antioxidant glutathione, and is very important to good health.
The antioxidant glutathione is made in the body and protects the mitochondria from oxidative damage.
Herceptin causes mitochondrial damage. -
http://www.ncbi.nlm.nih.gov/pubmed/15582322
Inhibition of ErbB2 causes mitochondrial dysfunction in cardiomyocytes: implications for herceptin-induced cardiomyopathy.
Grazette LP, Boecker W, Matsui T, Semigran M, Force TL, Hajjar RJ, Rosenzweig A.
I have been unable to find any indication if selenium is a positive or negative addition with herceptin as far as BC goes but may help reduce the risk of cardiac damage by helping reduce oxidative stress.
http://www.ncbi.nlm.nih.gov/pubmed/19017630?ordinalpos=1&itool=EntrezSystem2.PEntrez. Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.P ubmed_Discovery_RA&linkpos=3&log$=relatedarticles& logdbfrom=pubmed
ON a wider basis there are quite a number of suggestions that selenium reduces the risk of BC
Program in Cardiovascular Gene Therapy, Cardiovascular Research Center, Boston, Massachusetts, USA.
Shellfish, kidney, herring and lentils are reported as good sources in S Cunnane's book "Survival of the Fattest" which is about brains and evolution.
Brazil nuts are an excellent source of selenium but are high in Omega 6.