Combination Strategies for Carbon Ion Radiotherapy: Biological Rationale, Clinical Evidence, and Future Directions

Carbon ion radiotherapy (CIRT), characterized by high linear energy transfer and increased relative biological effectiveness (RBE), offers distinct physical precision and radiobiological potency in the treatment of radioresistant and anatomically complex tumors. As clinical adoption expands, the focus has shifted from technical feasibility to rational integration with other treatment modalities to optimize the therapeutic ratio. This review summarizes the biological rationale and clinical evidence for 3 principal CIRT-based combination strategies. We compare these strategies across their respective sources of benefit, levels of evidence, toxicity constraints, and applicable clinical settings. Mixed-beam approaches employ photons or protons to cover larger treatment volumes and carbon ions to boost residual or high-risk disease. They should be distinguished from emerging multi-ion therapy, which integrates multiple ion species to jointly optimize linear energy transfer and RBE-weighted dose. Immunotherapy may convert local tumor control into systemic antitumor responses but carries immune-related toxicity and challenges in lymphocyte preservation. Chemoradiotherapy and mechanism-based radiosensitization may combine CIRT with systemic therapy or exploit specific biological vulnerabilities, although CIRT-specific evidence remains heterogeneous. Future studies should prioritize biomarker-guided patient selection, mechanism-matched combinations, immune-related dosimetric optimization, standardized toxicity assessment, and prospective validation to enable precise, scalable clinical translation.