There have been circulating rumors that using THC for treating a hormonally active breast cancer might cause increased estrogen levels and perhaps increase tumor growth. As I looked for data, I came across an article discussing THC and estrogen where it seemed pretty clear that THC decreases estrogen effect. So, with decreasing estrogen effect, THC should actually be helpful in treating hormonally sensitive breast cancers. I would not be concerned about using THC in treating breast cancer and generally use it in a 1:1 ratio with CBD.
In addition, Cannabis smoke has anti-estrogenic effects, decreasing serum estrogen, presumably being helping for treating breast cancer.
So, I cannot find any data showing that THC increases estrogen levels, estrogen binding or estrogen effects.
In summary, THC should decreas estrogen effects and binding and this should have anti-tumor effects for hormonally active breast cancers