Can fulvestrant cause a tumor flare reaction in hormone receptor positive breast cancer?  

No.  Fulvestrant does not have agonist activity on the estrogen receptor, so it will not cause a true tumor flare.  When it is paired with an LHRH agonist for a premenopausal woman, the LHRH agonist can cause an initial tumor flare because of the rise in estradiol before full ovarian suppr...