Imaging the Cancer Immune Environment and Its Response to Pharmacologic Intervention, Part 1: The Role of 18F-FDG PET/CT.

Immunotherapy agents are now entering the clinic in a wide array of malignancies and have provided a valuable addition to the therapeutic armamentarium. These agents enhance the global immune response by modulating the tumor microenvironment but can lead to unconventional patterns of response, challenging the conceptual framework that imaging is a robust surrogate for therapeutic efficacy. There is also increasing evidence that an effective antitumor response requires a systemic immune response in primary and secondary lymphoid tissues. However, an enhanced systemic immune response can lead to disruption of immunologic hemostasis in healthy tissues, causing adverse events. Better understanding of the complex interplay between tumoral and systemic immune response has been provided through tissue and liquid biopsy. However, the applicability of these methods is constrained by the biologic, spatial, and temporal heterogeneity of the processes involved. There is a growing interest in molecular imaging of cell-specific lineage markers of the immune system using biomolecules. However, the ongoing role of the more widely available 18F-FDG PET/CT for response assessment is being recognized through ongoing refinement of interpretative guidelines and emerging evidence. These noninvasive methods provide insights into the biologic basis of the global immune response to maximize potential therapeutic benefit. In this review, we aim to provide an overview of the current status of 18F-FDG PET/CT in the monitoring of tumoral and systemic immune response. In a companion review, the role of other imaging probes that might complement 18F-FDG PET/CT will be discussed.