ABSTRACT
Background: Studies evaluating COVID-19 in cancer patients beyond the effects of the infection itself are generally from single institutions, voluntary surveillance registries, or surveys. To extend the limited evidence available, we analyzed both the incidence and one-year mortality of breast cancer (BC) female patients at a population level in Lombardy, the first Italian region affected by the pandemic and the most populous one. Method(s): The regional COVID-19 database, including all SARS-CoV2 cases based on a positive swab result, was integrated with the Regional Health Information System, collecting data from 10 million habitants on primary medical care;hospitalization;pharmaceuticals;and survival status. From the database, we extracted data of newly-diagnosed not previously treated BC patients, including patient characteristics and comorbidities (respiratory insufficiency, diabetes, chronic kidney disease, cerebral vasculopathy, hypertension and cardiovascular disease), BC stage, and treatment. Result(s): The study population consisted of 12912 newlydiagnosed/ not previously treated BC patients, 7349 in 2019 and 5563 in 2020. There were two drops of newly diagnosed cases, one in the first wave (March-May 2020;-37.2%), the other in the second wave (October-December 2020;-15.8%). No major differences were found between characteristics of cases occurring in 2019 and 2020;with the exception of a reduced use of both chemotherapy (86.2% vs 53.4%) and radiotherapy (65.7% vs 42.1%) in 2020. One-year overall survival was 97.6% in 2020 vs 98.3% in 2019, Hazard Ratio [HR] (95% Confidence Interval [95%CI]): 1.51 (1.18-1.93);p=0.0010 at univariate analysis;HR 0.91 (0.71-1.17), p= 0.47, after adjusting for age, stage, BC treatment and comorbidities at multivariable analysis. COVID-19 occurred in 250 of 5563 (4.5%) newly-diagnosed BC cases in 2020. Notably, the time-dependent COVID-19 effect was significantly associated with mortality (multivariable Cox analysis HR 2.25 (1.35-3.74);p=0.0018) even after adjusting for age, stage, treatment and comorbidities. Conclusion(s): Breast cancer incidence and survival were both reduced in 2020, and COVID-19 was an independent predictor of death in BC patients. While follow-up is ongoing to assess long sequelae of COVID-19, these results encourage prevention of infection regardless of BC stage;and at the same time warn against suboptimal treatment and overlooking new diagnoses to ensure a favourable prognostic outcome.