Reconciling the discrepancies in randomized data of combining immunotherapy and radiation therapy: Not all radiotherapy is created equal

It remains highly unclear and debatable whether combining radiotherapy (RT) and immune checkpoint blocker (ICB) therapy yields improved outcomes compared to either modality alone. Whereas some randomized data have shown improved outcomes, others have not. As a result of these conflicting data, it is essential to reconcile differences in the data and postulate reasons thereof. This work seeks to address these discrepancies, and uses the lessons learned from both positive and negative trials, including the most cutting-edge data available, in order to guide future clinical trial design and clarify the ideal/expected role of combinatorial therapy going forward. Because RT offers two distinct contributions (cytoreductive (local) effects & immune-stimulating (systemic) effects), RT should complement immunotherapy by addressing immunotherapy-resistant clones, and immunotherapy should complement RT by addressing RT-resistant or out-of-field clones. RT is not merely a single "drug", but rather a constellation of diverse "drugs" that can be varied based on dose regimens, previous systemic therapy regimens, number of irradiated sites, treatment intent/location/timing, tumor biology, and individual patient immunological circumstances. These factors are discussed as an important explanation for the discrepancies in results of various randomized trials in heterogeneous populations and clinical settings, and these discrepancies may continue until trials of more uniform circumstances are designed to use particular RT paradigms that meaningfully add value to systemic therapy.