A Study of HR+/HER2- Metastatic Breast Cancer Patients Treated With Palbociclib Together With an Aromatase Inhibitor From 2017 to 2023 in Denmark.

The purpose of this study is to describe the effect of the medicine palbociclib when given together with an aromatase inhibitor for treatment of breast cancer. The study will consider participants who: * have advanced or metastatic breast cancer that is spread to other parts of the body. * have HR+/HER2- (hormone receptor positive* / human epidermal growth factor receptor 2 negative**) breast cancer types. * Hormone receptor positive (HR+): are cells that have a group of proteins that bind to a specific hormone. For example, some breast cancer cells have receptors for the hormones estrogen or progesterone. These cells are hormone receptor positive, and they need estrogen or progesterone to grow. This can affect how the cancer is treated. Knowing if the cancer is hormone receptor positive may help plan treatment. * Human epidermal growth factor receptor 2 negative (HER2-): cells that have a small amount or none of a protein called HER2 on their surface. In normal cells, HER2 helps control cell growth. Cancer cells that are HER2 negative may grow more slowly and are less likely to recur (come back) or spread to other parts of the body than cancer cells that have a large amount of HER2 on their surface. Checking to see if a cancer is HER2 negative may help plan treatment. * have started treatment in the period between January 2017 and December 2021. The study will describe the treatment effect for different patient groups in terms of age and comorbidities. Comorbidity is the condition of having two or more diseases at the same time. The data is collected by the Danish Breast Cancer Group in the period between 2017 to 2023.