Includes patients who are receiving care for their breast cancer at participating Seattle area IO clinics.

For each IO clinic patient, an average of two (up to four) matched comparison cases will be recruited from the Washington State Cancer Surveillance System (CSS). Matched comparison cases from CSS will be identified by CSS and confirmed by the FHCRC investigator.

Will also describe how long after diagnosis women with breast cancer seek IO care and differences in IO care prescriptions associated with when, in a woman's course of treatment, she seeks care. This patient cohort will consist of all female patients with breast cancer seeking IO clinic care. Our analyses will describe how long after diagnosis women with breast cancer seek IO care and describe differences in IO care prescriptions associated with when, in a woman's course of treatment, she seeks care.

Comparison will be between breast cancer patients living in the same geographic area with similar prognostic characteristics (age, stage of cancer at diagnosis, estrogen receptor/ progesterone receptor (ER/PR) status, race, ethnicity, and marital status) at the time of their initial diagnosis with breast cancer who did not receive IO care. We will identify an average of three (up to four each) matched controls for each eligible IO clinic cancer patient from the Western Washington CSS database and to invite them to enroll in the outcomes study.

Collect pilot data on disease-free, progression free, and overall survival, as well as comorbidities and abnormal laboratory values at 6 months post-enrollment for all Cohort 1 and Cohort 2. We will collect data at 1 year post-enrollment and then yearly on disease-free, progression free, and overall survival, as well as comorbidities for as long as funding is available in a nested cohort of IO breast cancer patients (Cohort 1A) and their matched comparison group who did not receive care at any of the IO clinical sites (Cohort 2).