Radiation therapy has long been recognized to exert immunomodulatory effects beyond direct tumor cell killing, including antigen release, inflammatory signaling, and modulation of the tumor microenvironment. In this context, Yttrium-90 (Y-90) transarterial radioembolization (TARE) represents a distinctive form of internal radiation therapy, delivering high-dose beta radiation selectively to liver tumors via intra-arterial microspheres. Despite its extensive clinical use in hepatocellular carcinoma (HCC) and liver-dominant metastatic disease, Y-90 TARE has historically been conceptualized as a locoregional cytotoxic therapy with minimal systemic immunologic consequences (Ruohoniemi et al., 2020; Salem et al., 2013).

This view is increasingly challenged by emerging clinical data. Multiple studies employing longitudinal immune profiling in treated patients now suggest that Y-90 TARE induces reproducible changes in circulating immune cell populations, inflammatory mediators, and tumor-associated immune programs (Yu et al., 2024; Lee et al., 2023; Malone et al., 2025). A seminal Gut study demonstrated that sustained clinical responses to Y-90 TARE in HCC are associated with enhanced adaptive immune activation and increased effector cell signatures in both tumor tissue and peripheral blood, directly linking immune engagement with clinical outcomes (Chew et al., 2019). Furthermore, a related Gut commentary elaborated on this study’s own data, emphasizing that Y-90-induced immune responses call for early application of multiple immune checkpoint blockers to overcome emerging adaptive resistance and maximize clinical benefit (Rivoltini et al., 2023). These observations raise an important question with direct pharmacologic relevance: should Y-90 TARE be considered an in vivo immune modulator, and if so, how can this property be therapeutically exploited?

In this Perspective, we argue that sufficient human in vivo evidence exists to support the classification of Y-90 TARE as an immunologically active intervention. We synthesize key findings from clinical studies, discuss the dual nature of immune activation and suppression following treatment, and outline implications for combination strategies and biomarker-guided therapy.

This Perspective is based on a focused narrative synthesis of published human clinical studies evaluating immunologic effects of Y-90 TARE. Relevant literature was identified through targeted searches of PubMed and Google Scholar up to March 2025, using combinations of the terms “Yttrium-90,” “radioembolization,” “immune,” “immunomodulation,” “immunotherapy,” “checkpoint inhibitor,” and “hepatocellular carcinoma.”

Priority was given to prospective trials, translational clinical studies incorporating immune profiling, and high-quality retrospective analyses reporting systemic or intratumoral immune endpoints. Studies limited to dosimetry, technical optimization, or purely preclinical models without patient immune correlates were not included. This article does not aim to provide a systematic or exhaustive review but rather to synthesize representative in vivo human evidence relevant to the question of whether Y-90 TARE functions as an immune-modulating intervention with pharmacologic implications. We acknowledge that heterogeneity among the included studies, including differences in patient populations, treatment settings, immune assays, and sampling timepoints, may influence the observed immune signals. In addition, the possibility of publication bias toward studies reporting positive immunologic findings cannot be fully excluded. These limitations should be considered when interpreting the conclusions of this Perspective.

For the purposes of this discussion, immune modulation refers to treatment-induced changes in immune state that are measurable in patients and plausibly relevant to antitumor immunity. In the context of Y-90 TARE, these endpoints include: systemic immune changes, such as shifts in circulating lymphoid and myeloid populations, cytokine profiles, and activation markers; local immune remodeling, encompassing alterations in tumor-infiltrating immune cells, immune gene expression signatures, and checkpoint pathway activation within the liver or tumor tissue; and temporal dynamics, including the onset, magnitude, and duration of immune perturbations following treatment.

Importantly, immune modulation does not imply durable immune memory or clinical benefit per se. Rather, in the context of this Perspective, immune modulation refers to measurable and often transient immune changes observed in patients following Y-90 TARE, and should not be interpreted as definitive evidence of immune-mediated clinical efficacy.

While immune activation following Y-90 TARE is increasingly well documented, it is equally important to recognize accompanying counter-regulatory mechanisms. Several studies report increased expression of immune checkpoint molecules, including PD-1 on T cells and PD-L1 on myeloid or tumor cells, following treatment (Yu et al., 2024; Lee et al., 2023; Kaya et al., 2023). These findings suggest that immune priming may be rapidly tempered by adaptive resistance pathways.

This dual response mirrors observations made with other radiotherapeutic and locoregional interventions, where inflammatory signaling and antigen release coexist with immunosuppressive feedback loops. From a pharmacologic perspective, this balance underscores why immune activation alone may be insufficient for durable tumor control and why rational combination strategies are required.

Despite growing interest in the immunologic consequences of Y-90 TARE, it is important to acknowledge that immune activation following treatment is heterogeneous and not uniformly associated with clinical benefit. Large randomized trials comparing Y-90 radioembolization with sorafenib in advanced hepatocellular carcinoma, including the SARAH and SIRveNIB studies, failed to demonstrate an overall survival advantage for Y-90 in unselected patient populations, underscoring that biologic activity does not necessarily translate into improved outcomes (Vilgrain et al., 2017; Chow et al., 2018).

Moreover, immune perturbations observed after TARE are often transient and may reflect a combination of tumor-directed immune priming and nonspecific hepatic inflammation or lymphocyte depletion. Prospective immune monitoring studies have reported variable trajectories, including delayed lymphopenia, raising the possibility that post-TARE immune landscapes may in some cases constrain rather than enhance responsiveness to immunotherapy (Kitsel et al., 2024). Together, these findings emphasize that immune modulation by Y-90 TARE should be viewed as context-dependent and hypothesis-generating, rather than intrinsically predictive of therapeutic synergy.

To advance the field, future studies should integrate immune monitoring as a core component rather than an exploratory endpoint. Harmonized sampling schedules, paired tissue analyses, and standardized immune assays will be essential. Importantly, immune findings should be correlated with clinical outcomes to establish predictive rather than merely descriptive biomarkers.

Accumulating human in vivo evidence suggests that Y-90 TARE induces measurable and biologically meaningful immune modulation in patients with liver tumors. These effects are characterized by transient immune activation accompanied by adaptive resistance mechanisms. Recognizing Y-90 TARE as an immunologically active intervention reframes its role within modern oncologic pharmacology and provides a rationale for biomarker-guided combination strategies. As clinical development continues, integrating immunologic endpoints into Y-90-based trials will be critical to fully harness its therapeutic potential.