The most frequently mentioned advantage of early neutering of female dogs is protection against mammary cancer (MC)
[12]. However, a recent meta-analysis of published studies on neutering females and MC found that the evidence linking neutering to a reduced risk of MC is weak
[13].
And this is not the only study to find the evidence weak for Mammary Cancers....
I am left scratching my head on this. But of course we must remember there were no hall mark studies to show that in all cases for all breeds of dogs S+N is best done at 6months of age or so.