Non-small cell lung cancer brain metastases and the immune system: From brain metastases development to treatment

Brain metastases (BM) are diagnosed frequently in non-small cell lung cancer (NSCLC) patients. Despite the high incidence of BM (up to 40% in unselected patients), patients with untreated and/or unstable BM were excluded from pivotal immune checkpoint inhibitors (ICI) NSCLC trials. Percentage of patients with stable and treated BM in these trials ranged from 9.1 to 14.7% and ICI benefit over chemotherapy was not always demonstrated. Only small trials have been completed that demonstrated ICI efficacy in locally untreated, selected BM patients. With 33%, cranial objective response rate (ORR) was comparable to extracranial ORR and responses were often durable. With the promising survival benefits of ICI, in daily practice also unstable and/or untreated BM patients will often receive treatment with ICI and extrapolating clinical trial data to these patients can be challenging. In this review, we will summarize the preclinical rationale and potential concerns for the use of ICI in BM patients. Furthermore, we will summarize BM subgroup data from the pivotal NSCLC trials, retrospective series, the NSCLC BM specific ICI trials and the use of cranial radiation and ICI. Last, we provide an overview of response measurement criteria and future directions.