R help - Jan 2006 - cox model test heterogeinity

I'm a young physician and i work on the breast cancer. I made a cox
     model and i want to do a test of heterogeneity. Do you know how i 
can
     do this with R. I'm sorry for this question wich seems to be easy 
for
     you. I read the manual and the help and i don't find it.
     Thanks for your help
     Bertrand  Billemont