Heavy-ion therapy (HIT) is currently one of the most advanced forms of radiotherapy. Heavy ions, defined as charged particles heavier than protons, encompass a range of species, including carbon, helium, neon, silicon, nitrogen, and argon, among which carbon ions are the most widely used in clinical practice (carbon ion radiotherapy, CIRT) (1). When heavy ions penetrate tissue, the velocity of the heavy ions decreases near the end of their range, leading to a sharp increase in the probability of ionization collisions and a concentrated release of energy, thereby producing the Bragg peak (2). This results in highly precise energy deposition within the tumor region. Compared with conventional X−rays therapy (XRT), this characteristic not only significantly increases the energy delivered to the tumor, but also reduces radiation damage to the surrounding normal tissues. (Figure 1).

Heavy ions offer a distinct radiobiological advantage over X-rays. Their high linear energy transfer (high-LET) radiation induces complex, frequently irreparable DNA double-strand breaks (DSBs) (3), which catastrophically disrupt tumor cell replication. Compared to the predominantly single-strand breaks (SSBs) generated by photon radiation, these DSBs are considerably more lethal to cells (4, 5). Current research indicates that the repair of DSBs induced by high-LET radiation proceeds at a slower rate (6). Secondly, proton radiotherapy, which also exhibits the characteristic Bragg peak, produces a sharp dose fall-off at the distal edge. However, its LET is lower than that of heavy ions, resulting predominantly in SSBs (7, 8). The DSBs caused by CIRT generate fragmented double-stranded DNA (dsDNA) that activates the cGAS/STING–IFN axis (9). This activation enhances tumor immunogenicity and promotes a more robust anti-tumor immune response, which may even trigger rare abscopal effects (AE). This mechanism may fundamentally explain the superior immunotherapeutic efficacy of CIRT compared to conventional XRT. Second, this distinctive property of CIRT extends to their efficacy against tumors refractory to conventional XRT (7), particularly hypoxic cells (10, 11). Moreover, CIRT exert significant cytotoxic effects even on tumors that have developed resistance after prior XRT. The characteristics of CIRT and their advantages over XRT are summarized in Table 1. Due to these distinct properties, CIRT is increasingly recognized as a promising and emerging modality in oncology, particularly for the treatment of tumors that have been historically refractory.

Immunotherapy refers to therapeutic strategies designed to activate or enhance the patient’s immune system to combat malignancies (19). Although remarkable clinical efficacy has been achieved in specific types of cancer, significant challenges remain. Treatment benefits are often limited by tumor microenvironment-mediated resistance mechanisms and other factors (20)-{{-}}-low immunogenicity and immune escape. Currently, only a very small number of patients are eligible for clinical immunotherapy, and most patients respond poorly to such treatments. Before treatment, relevant clinical examinations are required to determine whether immunotherapy can be administered. Furthermore, whether immunotherapy will yield definitive efficacy remains uncertain. Additionally, immune-related adverse events are not uncommon in clinical observations. These may include immune-mediated rash, nephritis, pneumonia, myocarditis, hypophysitis, etc., which can be life-threatening in severe cases. On the other hand, the types of tumor immunotherapy drugs currently available on the market are limited, primarily consisting of immune checkpoint inhibitors (ICIs).

Conventional therapies exhibit limited efficacy for patients with recurrent or metastatic advanced malignancies. Currently, tumor metastasis and invasion remain among the principal causes of cancer-related mortality (21). Several studies have demonstrated that CIRT elicits a more robust immune response than conventional XRT. For example, it can increase the infiltration of tumor-infiltrating immune killer cells and induce stronger immunogenic cell death, bystander effects, and AE (Figure 2). This provides a feasible basis for developing combination therapies.

In a murine model of osteosarcoma metastasis, Takahashi et al. found that combining XRT (10 Gy) with immunotherapy significantly increased the distant tumor response rate compared to immunotherapy alone (complete response [CR] rate: 42% in the combination group vs. 9% in the monotherapy group). In a subsequent study (22), they further demonstrated that the combination with CIRT (5.3 Gy) also led to a significantly higher survival rate (CR rate improved from 20% with immunotherapy alone to 64% with the combination therapy) (23). Although equivalent biological doses were used, a merely literature-based comparison cannot provide an objective and comprehensive assessment of the immunological differences between CIRT and XRT, such as disparities in survival outcomes. Helm et al. established an osteosarcoma metastasis model using identical doses(10 Gy) of CIRT and XRT. They found that, for both radiotherapy modalities, combination with immunotherapy resulted in superior therapeutic efficacy compared to radiotherapy alone. While no significant difference was observed between CIRT and XRT in controlling distant tumors or in the metastasis model, the combination of CIRT with immunotherapy was strikingly superior to XRT combined with immunotherapy in controlling pulmonary metastases (6). A growing body of research substantiates the immunological advantages of CIRT over XRT. Yet, no study to date has undertaken a comprehensive comparative analysis of the two modalities.

Radiotherapy combined with immunotherapy (24) has emerged as a promising strategy for managing metastatic disease. For instance, the combination of radiotherapy with pembrolizumab has been clinically implemented in the treatment of non-small cell lung cancer (NSCLC). This combined modality demonstrates superior therapeutic efficacy and prolonged overall survival in patients with advanced NSCLC compared to monotherapy approaches (25). Therefore, by leveraging the radiobiological characteristics of heavy ions and considering them as immunomodulators, we can combine them with immunotherapy to potentiate anti-tumor immune responses. The AE represents a potential therapeutic opportunity for metastatic cancers and is widely attributed to radiation-induced immunomodulation (26). Compared to conventional radiotherapy, CIRT demonstrates greater biological effectiveness and enhances post-irradiation tumor immunogenicity (27), thereby providing a mechanistic basis for investigating the systemic anti-tumor immune response induced by heavy ions. With advancing research on CIRT induced immunomodulation, the combination of radiotherapy and immunotherapy has emerged as a promising therapeutic strategy for advanced-stage malignancies (28).

Research into radiation-induced remodeling of the tumor immune microenvironment is now expanding rapidly. These immunological insights are crucial for elevating radiotherapy from a local intervention to a systemic treatment. This review summarizes key developments in combining CIRT with immunotherapy, with a focus on promising clinical strategies. It also evaluates the present state of both preclinical and clinical research in this field.

CIRT alters both the tumor microenvironment and subcellular structures, thereby enhancing the exposure of tumor antigens. As discussed previously, CIRT leads to increased DSBs. The cytosolic dsDNA is recognized by cyclic GMP-AMP synthase (cGAS) (29), which catalyzes the synthesis of the cyclic dinucleotide 2′3′-cyclic guanosine monophosphate–adenosine monophosphate (cGAMP). cGAMP activates the stimulator of interferon genes (STING), ultimately resulting in STING phosphorylation (30) (Figure 3). One of its hallmark features being the induction of type I interferon (IFN) secretion (31). IFN have been shown to inhibit tumor cell proliferation and disrupt tumor vasculature (32). Furthermore, studies indicate that STING signaling can also be associated with the activation of transcription factors such as nuclear factor-kappa B (NF-κB) (33). Hellweg et al. investigated the relationship between NF-κB and high-LET radiation, further demonstrating that both ATM inhibitors and proteasome inhibitors can suppress this activation. These results underscore the critical roles of ATM and the proteasome in mediating CIRT induced NF-κB signaling (34). The potential of STING signaling in cancer immunotherapy stems from its ability to promote diverse anti-tumor immune responses.

In addition, following CIRT, the activation of apoptotic genes and the release of immunogenic tumor-associated antigens work together to promote the death of tumor cells. A meta-analysis showed that exposure to high-LET radiation triggered substantial transcriptional changes within 6–24 hours, resulting in the upregulation of 2,689 genes and the downregulation of 1,839 genes. The most notable differential expression pattern featured the upregulation of autophagy-related genes alongside the downregulation of DNA repair-associated genes (35). Compared to conventional XRT, CIRT induces not only p53-mediated gene upregulation but also more substantial and more prolonged transcriptional activation at 24 hours post-irradiation (36). Furthermore, Sofia et al. showed that high-LET radiation upregulates a broad spectrum of immunologically relevant molecules at the cellular level, including immune recognition molecules (HLA, ICAM-1, CEA, MUC-1), surface-exposed calreticulin (CRT), damage-associated molecular patterns (HMGB1, ATP), and the immune checkpoint protein PD-L1 (37). A growing body of experimental evidence confirms that CIRT induce similar molecular and cellular alterations, which represent promising therapeutic targets for enhancing radiation-induced tumor immunogenicity. These findings collectively indicate that tumor cells acquire an enhanced immunogenic potential following CIRT.

Investigating changes in the tumor microenvironment (TME) after CIRT and combining this modality with immunotherapy to promote an anti-tumor immune milieu constitutes a core research paradigm. Enhancing the immune system’s ability to recognize and eliminate tumor cells is a central objective in tumor immunology. Sofia et al. further investigated radiation-induced alterations in the tumor immune microenvironment and revealed a series of coordinated changes at the macro level: increased infiltration of cytotoxic T lymphocytes (CD8⁺) and helper T cells (CD4⁺), enhanced natural killer (NK) cell activity, maturation of DCs, decreased populations of regulatory T cells (Tregs), and suppressed function of myeloid-derived suppressor cells (MDSCs) (37). These alterations collectively demonstrate a significant enhancement of anti-tumor immunity following irradiation. The established mechanism of immune-mediated tumor control operates precisely through these pathways (Figure 4).

Experimental evidence from post-irradiation immunological profiling suggests that CIRT have a significant immunomodulatory effect on host immune competence. For instance, localized CIRT delivery to cerebral targets has been shown to recruit non-targeted microglia to the treatment volume and enhance their immunophagocytic capacity (38). A recent study revealed that the epidermal growth factor receptor ligand amphiregulin (AREG) is associated with distant metastasis following radiotherapy (39). This phenomenon was observed in both murine and human models. Using a mouse model, the researchers further demonstrated that anti-AREG therapy could inhibit radiation-induced metastatic tumor growth and enhance the efficacy of radiotherapy. This finding provides a feasible mechanistic rationale for suppressing metastatic proliferation after radiotherapy. Patients with metastatic cancer may thus benefit from combined radiotherapy and immunotherapy. Takeshima et al. compared the efficacy of immunotherapy combined with either CIRT or XRT in mice (40). They made a critical observation: the attenuation of tumor volume reduction after CD8⁺ T cell depletion was far more pronounced in the heavy-ion group, implying that CIRT relies more extensively on CD8⁺ T cell-mediated immunity. This finding suggests that heavy-ion radiation may have superior compatibility with immunotherapy and the potential for enhanced therapeutic outcomes. Further exploring this synergy, Huang et al. examined the combined effect of CIRT and ICIs in a murine tumor model (41). The study demonstrated that the combined treatment significantly inhibited both local and distal tumor growth and prolonged survival in mice. Transcriptomic sequencing and bioinformatic analyses revealed a significant enrichment of immune-related pathways and genes in the combination group, accompanied by enhanced infiltration of immune cells and increased secretion of cytokines and chemokines. These findings provide experimental evidence supporting the combined use of CIRT and ICIs in cancer therapy. Hartmann et al. established a bilateral tumor model using Her2+ EO771 murine breast cancer cells. Their findings demonstrated that the combination of CIRT with anti-CTLA-4 therapy yielded superior antitumor efficacy compared to either monotherapy and induced AE. Notably, when mice that had undergone combined radioimmunotherapy were rechallenged with tumor cells, they exhibited complete resistance to tumor establishment. This observation suggests that the combined treatment can induce durable, tumor-specific immune memory (42). While such findings collectively demonstrate the potential of combination therapy, numerous challenges remain in its translation to clinical application. Taken together, these studies indicate that CIRT exerts synergistic effects with immunotherapy through enhanced immune reliance, activation of the tumor immune microenvironment, and the induction of immune memory. This provides a strong experimental basis for developing combined treatment strategies in the clinic.

Radiation-induced lymphopenia (RIL) is a known side effect of radiotherapy (43). When XRT with large radiation fields encompasses tumor-draining lymph nodes as well as hematopoietic and lymphoid organs such as the bone marrow, spleen, and thymus, both the lymphatic circulation and hematopoiesis can be compromised, leading to a reduction in lymphocytes. Lymphocytes are essential for mounting effective anti−tumor immune responses. In contrast, proton and heavy ions beams exhibit a sharp dose fall−off at the field edges, resulting in less damage to surrounding lymphoid tissues and thereby preserving a greater number of functional lymphocytes. A recent statistical analysis further demonstrated that patients treated with X-rays exhibit consistently lower relative lymphocyte counts upon treatment completion than those receiving ion therapy. This finding solidifies the aforementioned perspective (44).

Investigating the AE represents the key to evolving radiotherapy from a localized treatment into a systemic therapy. Brenneman et al. reported a patient with metastatic retroperitoneal sarcoma who exhibited AE following proton therapy (90). Through comprehensive immunohistochemical analysis of both primary and metastatic lesions, they identified a SMARCB1 nonsense mutation along with loss of INI1 protein expression, suggesting these molecular changes may be linked to the observed AE. However, no in-depth mechanistic investigations were conducted to clarify the underlying biological pathways. To decipher the mechanisms of AE, studies examining the immunohistochemical features of primary and metastatic tumors, alterations in the TME, and systemic immune changes following its occurrence are critically needed.

CIRT has demonstrated promising clinical outcomes in some tumor types, achieving excellent local control rates, survival benefits, and manageable toxicity profiles (91, 92). However, there remains a paucity of research investigating the immunomodulatory effects of CIRT or its combinatorial potential with immunotherapy. In addition to investigating the potential of combination therapies, research into their dosage, sequencing, and adverse effects is equally critical. Hu et al. compared peripheral blood samples from prostate cancer patients before and after treatment with either CIRT or XRT. Their analysis confirmed that, unlike XRT, CIRT did not induce significant lymphocytopenia. Notably, most lymphocyte subsets (except Tregs) exhibited enhanced proliferative capacity following particle therapy, demonstrating an immunological advantage over XRT. Additionally, elevated TNF-α levels were detected post-treatment, which may promote NK cell proliferation and thereby enhance NK cell-mediated antitumor immunity (93). Although relevant studies have provided some basis for understanding RIL, the current knowledge remains limited and contains gaps. Ronchi et al. treated 40 patients with head and neck mucosal melanoma using CIRT, among whom 18 received immunotherapy either before or after irradiation. Statistical analysis revealed a significantly prolonged median survival time of 17 months in the combination therapy group compared to 3.6 months in the CIRT-alone group (p < 0.001) (91). Mizoguchi et al. analyzed 34 patients with newly diagnosed head and neck mucosal melanoma treated with primary CIRT (total dose 64 Gy). The patients were divided into three groups: Group A (radiotherapy alone), Group B (radiotherapy followed by ICIs therapy with Nivolumab), and Group C (concurrent radiotherapy and ICIs therapy). Post-treatment follow-up revealed a two-year overall survival rate of 100% in Group C (11/11), compared to 66.7% in Group B and 50.8% in Group A. Differential analysis confirmed a statistically significant difference between Group C and Group A, whereas the difference between Group B and Group A was not statistically significant (94). Undoubtedly, this study provides clinical evidence to inform the timing of combination therapy, thereby addressing one of its key challenges. However, the small sample size limits the generalizability of the findings. Given that head and neck mucosal melanoma is a rare tumor, applying similar research approaches to diseases with higher incidence would yield more clinically actionable guidance. Clinical trials investigating the combination of CIRT with immunotherapy are being conducted actively at major particle therapy centers. At present, most of the studies on CIRT combined immunotherapy are retrospective studies, or immunotherapy that is received before and after CIRT, without concurrent treatment. Therefore, prospective clinical studies on CIRT combined immunotherapy are being actively carried out (Table 2). These also indicate that the primary reason for the clinical trial’s failure was an insufficient number of enrolled patients.

After combination therapy, research on treatment-related toxicities has emerged as another critical focus area. A meta-analysis of animal studies comparing immunotherapeutic agents (ipilimumab and IL-2) as radiosensitizers versus conventional radiosensitizers (cisplatin) demonstrated that immunotherapy significantly enhances local tumor control when combined with radiotherapy, without exacerbating radiation-induced toxicity (98). There are data that suggest that CIRT-immunotherapy synergy may confer a significant survival advantage over CIRT alone (41). Cavalieri et al. conducted a retrospective analysis of adverse events in 33 patients treated with combined CIRT and immunotherapy, revealing no significant increase in toxicity incidence (99). Currently, there remains a paucity of relevant studies. However, existing evidence has not identified significantly additive toxic effects when combining immunotherapy with radiotherapy. Should future large-scale studies conclusively demonstrate that combination therapy does not exhibit substantially greater toxicity than either modality alone, this would provide compelling evidence to support broader clinical adoption of combined treatment approaches.

The studies mentioned above are based on investigations of radiation toxicity. Whether the combination of CIRT and immunotherapy would exacerbate immune-related adverse events remains unexplored. A retrospective study systematically evaluating the safety and toxicity of stereotactic body radiotherapy (SBRT) combined with ICIs for lung cancer revealed that the incidence of grade 3 or higher pneumonitis in the SBRT + ICIs group was 10.7%, significantly higher than that in the SBRT monotherapy group (0%; P = .007). However, the incidence of pneumonitis of any grade showed no statistically significant difference between the two groups (100). Unfortunately, the study did not report the median overall survival or tumor control rates for either group, leaving it unclear whether the observed pneumonitis was associated with antitumor efficacy.