Kefah Mokbel, Chair of The Multidisciplinary Breast Cancer Program at London Breast Institute, shared on LinkedIn:

“Estrogen-only HRT reduces breast cancer risk and women who had prior hysterectomy:

The biological rationale and the totality of the randomized clinical trial evidence supports a conclusion that use of estrogen-alone, among post-menopausal women with prior hysterectomy, significantly reduces breast cancer incidence with findings perhaps not limited to CEE-alone.

Regarding potential biological mechanisms, I propose the following:

Estrogen promotes apoptosis in breast cells, particularly following short-term estrogen deprivation.
Estrogen prompts breast cancer cells to enter dormancy after a period of estrogen deprivation.
Hysterectomy may contribute through decreased levels of anti-Müllerian hormone (AMH) and inhibin B, and increased levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Estrogen reduces the size of the human breast stem cell pool.”

Kefah Mokbel summarizes estrogen-alone use and breast cancer incidence by Rowan T. Chlebowski et al

Additional information.
Source: Kefah Mokbel/LinkedIn