Estimating the number of people living with metastatic breast cancer: a population-based retrospective cohort study using linked health data.

[BACKGROUND] The paucity of population-level data on people with metastatic breast cancer (MBC) means there is a critical information gap for planning cancer services tailored to meet the needs of people with MBC. The aims of this study were to estimate MBC prevalence and report the characteristics of people with MBC using linked routinely collected health data.

[METHODS] This was a population-based retrospective study. Residents of New South Wales (NSW), Australia, diagnosed with breast cancer from 1972 to 2020 were identified from the NSW Cancer Registry. People with MBC at diagnosis (de novo MBC) were identified using degree of spread at diagnosis recorded on the NSW Cancer Registry. Those with progressed or recurrent MBC were identified from linked cancer registry, hospital admission, radiotherapy and chemotherapy records. Vital status was obtained from the National Death Index.

[FINDINGS] There were an estimated 6692 people living with MBC on 31 December 2020, a third (32.5%) of those with de novo MBC. Half (52.6%) were aged 65 years and older, one quarter (25.3%) lived in regional and remote areas and one quarter (26.0%) were from non-English speaking countries of birth. Estrogen receptor positive/HER2 negative was the most common subtype (65.3%). Two out of five people (42.6%) had been living with MBC for five or more years. Nearly one quarter (23.3%) of people with progressed or recurrent MBC developed MBC ≥10 years after primary breast cancer diagnosis. The estimated number of people with MBC was 3821 in 2011 and increased 75% by 2020.

[INTERPRETATION] We identified a large and rapidly growing cohort of people with MBC. The majority were initially diagnosed with non-metastatic breast cancer. Their demographic and clinical characteristics can be used to understand their distinct needs for tailored and targeted treatment and supportive care. We have demonstrated that comprehensive population-level reporting of MBC is feasible using linked routinely collected health data.

[FUNDING] This work was funded through the operational budget of the Cancer Institute NSW. A.L.S. is supported by a Daffodil Fellowship. S.J.L. received funding from an NHMRC Project Grant ID: 1125433.