A follicular cyst during tamoxifen therapy in a premenopausal breast cancer woman.

A large ovarian follicular cyst developed on day 43 of low-dose tamoxifen adjuvant therapy for a premenopausal breast cancer (total drug intake: 860 mg). The serum estradiol level increased to almost ten times the pretreatment level. However, the FSH and LH decreased slightly, and the serum progesterone level remained almost unchanged. It is hypothesized that tamoxifen acted directly on the ovaries to provoke excessive proliferation of the granulosa cells in a follicular cyst, causing it to grow very large, and resulting in elevated E2 levels.