A Study of Real-World Treatment Patterns and Outcomes Among HR+/HER2- Metastatic Breast Cancer Patients Treated With First-Line CDK4/6 Inhibitors

Inclusion criteria:

* A diagnosis of mBC based on at least two diagnoses of breast cancer (International Classification of Diseases, 10th Revision, Clinical Modification \[ICD-10-CM\] code C50.xxx) along with a diagnosis of secondary malignant neoplasm (ICD-10-CM codes C77.xxx - C80.xxx except C79.8 or C79.81, reporting of a metastatic site by curation), in the period prior to or on the index date.
* Patients with HR+/HER2- status.
* Use of either ribociclib, palbociclib, or abemaciclib in 1L treatment for mBC.
* Continuous care at the contributing practices. Patients with a minimum of two visits up to and including the index date.

Exclusion criteria:

* Use of any prior CDK4/6i before the index date.
* Prior 1L treatment, other than CDK4/6i, in mBC including treatment with endocrine therapy (ET) either with tamoxifen, aromatase inhibitors (AI; anastrozole, exemestane, or letrozole) or fulvestrant. For it to be considered 1L in mBC, the diagnosis of mBC must have preceded treatment initiation, or was indicated so in unstructured data.
* Diagnosis of cancer other than breast cancer and/or mBC on or prior to the index.
* Evidence of participation in a clinical trial at any time during the study observation period.