Baseline tumour burden and prognosis in breast cancer treated with immune checkpoint inhibitors

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.

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.

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.

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.