Radiation therapy has evolved beyond conventional X-ray treatment. Particle therapy, including proton therapy and carbon ion therapy, uses charged particles to deliver radiation to a defined treatment area.
Both proton and carbon ion therapy have a finite range in tissue and can produce a Bragg peak, where the radiation dose is concentrated near the end of the particle’s range. However, protons and carbon ions differ in their physical and biological characteristics.
So, how is carbon ion therapy different from proton therapy?
What Is Carbon Ion Therapy?
Carbon ion therapy, also known as carbon ion radiotherapy (CIRT), is a form of radiation therapy that uses accelerated carbon ions to treat cancer.
Like protons, carbon ions have a finite range in tissue. They deposit dose along their path and release a greater proportion of it near the end of their range, producing the characteristic Bragg peak. Treatment planning uses this property to define where the dose is delivered and to limit dose deposition beyond the particle range.
Carbon ions are heavier than protons and have a substantially higher linear energy transfer (LET). This gives carbon ion therapy different radiobiological characteristics from proton therapy.
Carbon Ion Therapy vs Proton Therapy
| Feature | Proton Therapy | Carbon Ion Therapy |
|---|---|---|
| Particle | Proton | Carbon ion |
| Bragg peak | Yes | Yes |
| LET | Lower than carbon ions | Higher than protons |
| RBE | Relatively low | Higher and variable |
| Clinical use | Established across a broad range of indications | Used and studied for selected tumours |
What Is LET?
LET (linear energy transfer) describes how densely radiation deposits energy as it travels through tissue.
- Lower LET: energy is deposited less densely.
- Higher LET: energy is deposited more densely.
Carbon ions have higher LET than protons, so their energy is deposited more densely along their tracks.
What Is RBE?
RBE (relative biological effectiveness) compares the biological effect of one type of radiation with that of a reference radiation under defined conditions.
Carbon ions can have a higher RBE than photons and protons under defined conditions. However, RBE is not a fixed value. It depends on several physical and biological factors, and different calculation models can give different estimates.[1] For this reason, RBE is taken into account in treatment planning for carbon ion therapy. It is not a simple measure of how well a treatment works.
In Which Tumour Types Has Carbon Ion Therapy Been Studied or Used?
Carbon ion therapy has been studied or used for a number of tumour types, including:
- Skull-base tumours, such as chordoma and chondrosarcoma
- Selected head and neck cancers
- Bone and soft-tissue sarcomas
- Prostate cancer
- Liver cancer
- Pancreatic cancer
- Selected lung cancers[3]
âš Important: Inclusion of a tumour type in this list does not mean that carbon ion therapy is a standard treatment, is suitable for every patient with that cancer, or has been shown to be superior to other treatment approaches. Treatment decisions depend on individual clinical circumstances and the available clinical evidence.
What Does the Current Evidence Show?
The clinical evidence for carbon ion radiotherapy continues to develop. Clinical outcomes have been reported across several tumour types.
A 2024 systematic review and meta-analysis pooled 34 comparative studies of carbon ion radiotherapy against other radiation treatments. Overall local control rates were comparable between the groups, and results differed by comparison group and tumour type.[2]
For many tumour types, studies directly comparing carbon ion therapy with other radiation treatments remain limited. Differences in patients, treatment protocols and study design also make comparisons difficult.
Therefore, the higher LET and RBE of carbon ions should not automatically be interpreted as proof of better clinical outcomes for every cancer. Further prospective and comparative research is needed to define where carbon ion therapy may have a clinically meaningful role.
Where Does Multi-Ion Therapy Fit In?
Multi-Ion therapy refers to an approach in which different particle types can be considered for particle therapy. This is relevant because particles differ physically and biologically: protons have lower LET than carbon ions, while carbon ions have higher LET and different RBE characteristics.
Multi-ion approaches allow different particle types to be considered according to the clinical and physical requirements of an individual treatment plan.
However, having access to multiple ion species does not itself establish a clinical benefit. The choice of particle still needs to be guided by tumour characteristics, treatment objectives, treatment planning and available evidence.
Key Takeaway
Carbon ion therapy and proton therapy both use charged particles and can produce a Bragg peak, but carbon ions have higher LET and different RBE characteristics.
These properties have led to clinical use and ongoing research in selected cancers. Carbon ion therapy is best understood as a different particle therapy option with distinct physical and biological characteristics, not as a replacement for proton therapy.
As particle therapy evolves, multi-ion approaches may allow different particle types to be considered according to the requirements of individual treatment plans.
Explore Particle Therapy
Want to learn more about proton therapy, carbon ion therapy and emerging particle therapy approaches? Explore our particle therapy resources to understand how these technologies are being studied and used in modern radiation oncology.
Frequently Asked Questions
What is carbon ion therapy?
Carbon ion therapy, also known as carbon ion radiotherapy (CIRT), is a form of particle therapy that uses accelerated carbon ions to deliver radiation to a defined treatment area. Carbon ions have a finite range in tissue and can produce a Bragg peak, while also having higher linear energy transfer (LET) than protons.
What is the difference between carbon ion therapy and proton therapy?
Both proton therapy and carbon ion therapy use charged particles and can produce a Bragg peak. The key difference is that carbon ions are heavier and have substantially higher LET, resulting in different radiobiological characteristics and generally higher relative biological effectiveness (RBE) than protons under defined conditions.
Is carbon ion therapy better than proton therapy?
Carbon ion therapy should not automatically be considered better than proton therapy. Although carbon ions have higher LET and different RBE characteristics, clinical benefit depends on the tumour type, treatment plan, patient characteristics and available evidence. Comparative evidence remains limited for many tumour types.
Why do carbon ions have a higher RBE than protons?
Carbon ions have higher LET than protons, meaning they deposit energy more densely along their tracks. This can result in greater biological effects under certain conditions. However, RBE is not a fixed value and depends on physical and biological factors as well as the calculation model used.
Which cancers can be treated with carbon ion therapy?
Carbon ion therapy has been studied or used for several tumour types, including skull-base tumours such as chordoma and chondrosarcoma, selected head and neck cancers, bone and soft-tissue sarcomas, prostate cancer, liver cancer, pancreatic cancer and selected lung cancers. Its role varies by tumour type and clinical setting.
Is carbon ion therapy suitable for every cancer patient?
No. The suitability of carbon ion therapy depends on factors such as tumour characteristics, location, treatment objectives, the patient’s clinical circumstances and the available evidence. A radiation oncology team determines whether particle therapy is appropriate for an individual patient.
What is multi-ion therapy?
Multi-ion therapy refers to an approach in which different particle types can be considered for treatment planning. Since protons and carbon ions have different physical and biological characteristics, access to multiple ion species may allow the treatment approach to be considered according to the requirements of an individual treatment plan.
References
- Karger CP, Peschke P. RBE and related modeling in carbon-ion therapy. Phys Med Biol. 2018;63(1):01TR02. doi:10.1088/1361-6560/aa9102
- Yun JE, Kim S, Park KY, Lee W. Effectiveness and Safety of Carbon Ion Radiotherapy in Solid Tumors: A Systematic Review and Meta-Analysis. Yonsei Med J. 2024;65(6):332–340. doi:10.3349/ymj.2023.0439
- Clinical indications and future directions of carbon-ion radiotherapy: a narrative review. Ewha Med J. 2024;47(4):e56. doi:10.12771/emj.2024.e56
