Do you discontinue anti-hormonal therapy if a high-risk ER+ HER2- localized breast cancer patient develops blood clots on anti-hormonal therapy?

Tamoxifen has been associated with an increased risk for venous thromboembolic events, but there does not appear to be any increased risk with the aromatase inhibitors in the placebo controlled studies. These include MA.17, MA.17R in the extended endocrine therapy setting and MAP.3 and IBISII in the prevention setting. Thus, I would continue anti-estrogen (anti-hormonal) therapy. Presuming a venous thromboembolism event occurred on tamoxifen, I would change to an aromatase inhibitor (with ovarian suppression or removal if woman is premenopausal).

SERMS cause increased risk of thromboembolism, and this has been known for quite some time, even before the availability of alternative agents, such as the modern aromatase inhibitors.

My practice in the ancient times was to treat women with anticoagulants, and to continue the SERM if the risk of recurrent breast cancer was felt to be sufficiently high to justify the risk of long-term anti-coagulation, as I would recommend continuing anti-coagulation for the duration of SERM therapy.

Certainly, switching to a different endocrine therapy not associated with increased risk of thrombosis is an alternative option, if the patient has other treatments available.

Keep in mind that there was RCT of aromatase inhibitor use versus non-use that showed an increased risk of VTE, although not as high as tamoxifen.