Palbociclib is a medication that works by blocking certain proteins that help cancer cells grow and divide. It is used to treat hormone receptor-positive breast cancer. In this trial, it is being studied in combination with hormone therapy for patients who have had a recurrence of breast cancer in the same area.

Endocrine therapy (also known as hormone therapy) is a treatment that blocks or lowers the amount of hormones in the body to stop or slow down the growth of hormone-sensitive breast cancer cells. This type of therapy can include different medications that either block hormones or stop the body from producing them.

Breast Cancer (HR-positive, HER2-negative) – A type of breast cancer characterized by the presence of hormone receptors (HR) and absence of human epidermal growth factor receptor 2 (HER2) on cancer cells. This cancer begins when abnormal cells in the breast tissue start growing uncontrollably. It can initially develop in the breast tissue and may spread to nearby lymph nodes. The cancer cells in this type respond to hormones like estrogen and progesterone, which can influence their growth. When this type of breast cancer returns after initial treatment in the same area or nearby regions, it is called locoregional recurrence.

Isolated Locoregional Recurrence (ILRR) of Breast Cancer – A condition where breast cancer returns in the area where the original cancer was located or in nearby lymph nodes, after a period of remission. This recurrence is limited to the local area and nearby regions, without evidence of spread to distant parts of the body. The recurrent tumor appears in the chest wall, skin, or lymph nodes near the original cancer site. This condition develops after the completion of initial treatment for breast cancer.