Abstract
BACKGROUND: Immune checkpoint inhibitors (ICI), alone or in combination with other agents, are now standard of care for a range of tumour types, including breast cancer. Radiological assessment of baseline tumour burden has been associated with prognosis in a variety of tumour types treated with immunotherapeutic agents. Whether baseline tumour burden is prognostic in breast cancer treated with ICI is unknown. METHODS: A retrospective review of patients from a single institution with advanced breast cancer treated with ICI was performed. The sum of baseline target lesion measurements (baseline tumour size [BTS]) and all lesions (total tumour burden [TTB]) were determined in accordance with RECIST version 1.1 and correlated with clinical outcomes. RESULTS: 82 patients were identified, treated between 2015 and 2020. BTS and TTB by quartile were significantly associated with overall survival (OS) (p < 0.001) and clinical benefit rate at 6 months (CBR6) in univariable analysis (p = 0.004 and p = 0.002 for BTS and TTB, respectively). Multivariable analysis confirmed a significant inverse relationship with OS for both BTS (p = 0.035) and TTB (p = 0.024). A non-statistically significant numerical inverse association between increasing tumour burden and progression free survival was also observed. CONCLUSION: Baseline tumour burden assessed by BTS and TTB is significantly associated with prognosis in advanced breast cancer treated with ICI and should be considered a potential biomarker to improve selection of patients for treatment with ICI. These results suggest that ICI treatment will be most effective in patients with lower tumour burden.