Abstract Background Immune checkpoint inhibitors (ICPIs) are being investigated in clinical trials for patients with glioblastoma. While these therapies hold great promise, management of the patients receiving such treatment can be complicated due to the challenges in recognizing immune-related adverse events caused by checkpoint inhibitor treatment. Brain imaging changes that are the consequence of an inflammatory response may be misinterpreted as disease progression leading to inappropriate premature cessation of treatment. The aim of this study was to, by way of a series of cases, underscore the challenges in determining the nature of contrast-enhancing masses that develop during the treatment of patients with glioblastoma treated with ICPIs. Case presentation We reviewed the clinical course and management of 4 patients on ICPIs who developed signs of tumor progression on imaging. These findings were examined in the context of Immunotherapy Response Assessment in Neuro-Oncology (iRANO) guidelines. Although all 4 patients had very similar imaging findings, 2 of the 4 patients were later found to have intense inflammatory changes (pseudoprogression) by pathologic examination. Conclusions A high index of suspicion for pseudoprogression needs to be maintained when a patient with brain tumor on immunotherapy presents with worsening in an area of a pre-existing tumor or a new lesion in brain. Our findings strongly suggest that pathological diagnosis remains the gold standard for distinguishing tumor progression from pseudoprogression in patients receiving immunotherapy. There is a large unmet need to develop reliable non-invasive imaging diagnostic techniques. Trial registration ClinicalTrials.gov NCT02311920. Registered 8 December 2014.
