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Carbon Ion Radiotherapy (CIRT)

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Particle beam therapy or particle radiotherapy is a type of cancer treatment that utilizes protons or heavy atoms (such as carbon) to precisely target tumors with high energy.1

Utilizing carbon ions allow for a characteristic energy distribution in depth (known as the Bragg Peak) where low energy is deposited in tissues proximal to the target and the majority of the energy is released at the target site.1

CIRT is mainly used for the treatment of solid malignant tumors and is becoming increasingly common for the treatment of various cancers including liver, lung, prostrate, head and neck.1

Motivation
  • Nearly one-half of patients with newly diagnosed cancer will undergo radiotherapy with most patients receive external-beam radiotherapy using photons (i.e. x-rays) and in recent years, there has been increased interest in the development of particle beam radiotherapy, the most popular being proton-beam radiotherapy. 2
  • Carbon ion radiotherapy has emerged as a promising new treatment technique, particularly for tumors resistant to conventional chemo-radiotherapy, compared to photon and proton beam therapy32
  • in 1994, CIRT was initiated at the National Institute of Radiological Science (NIRS) in Chiba, Japan, using the world’s first heavy ion accelerator complex 1. Since then, CIRT has been established and evolved through development in technology and clinical studies4, showcasing advantageous physical and radiobiological properties compared to photon and proton based therapy.

TERMINOLOGY

  1. Bragg peak = Carbon ion beams exhibit a depth–dose distribution in which energy deposition increases with penetration depth and reaches a sharp maximum at the end of the particle range, enabling highly localized dose delivery to the target while minimizing irradiation to adjacent normal tissues.15
  2. Linear energy transfer (LET) = LET is defined as the energy transfer by an ionizing particle traveling through matter.4
    • carbon ions have high LET (i.e. deliver larger mean energy per unit length) compared to low LET radiations such as photons or protons.5 This allows for carbon ion beams to commonly cause double-stranded DNA to break by one hit, resulting in the most significant event for cancer cell death.5
    • LET for neutrons remain uniform at any depth in the body, while for carbon ions, LET increases steadily from the point of incidence in the body with increasing depth to reach a maximum in the peak region. This property has a therapeutic advantage when carbon ions are used for deep-seated tumors5
  3. Relative biological effectiveness (RBE) = defined as the ratio of the amount of dose from a test radiation required to generate the same biological endpoint (usually cell killing) relative to a reference radiation (usually 250 kVp X-rays or Co-60 γ-rays)6
    • radiation beams with higher RBE values are more effective at producing biological effects at equivalent doses6
    • the measure of biological potency is termed RBE7
    • RBE is a complex entity depending on LET of the test radiation, physical dose, tumor type, tumor depth, and so forth.4 For carbon ions, RBE increases as a function of depth, with its highest value at the distal edge of the Bragg peak6
    • RBE for protons, calculated at 10% survival, is approximately 1.1, while RBE for carbon ions has been accepted to be in the 2 – 3.5 range46

PHYSICS

CIRT is an external beam radiation therapy, sending high-energy carbon ions to shrink or slow the growth of cancer cells by damaging their DNA.8 The heavy carbon ions pass through the body gradually losing their KE and depositing their dose via Coloumb interactions with atomic electrons.9 The Bragg peak position is adjusted to the depth of the tumor to reduce the radiation exposure to the normal tissue and side effects from the treatment9.

Carbon ion source = carbon ions are obtained from carbon dioxide (CO2) gas. First, subjected to impacts of highly accelerated electron beam, molecules dissociate and C+4 ions are extracted with applied electric field–Beam of C+4ions is accelerated to 10% of c (c = speed of light) via linear accelerator; the synchrotron further accelerates ions to 70% of c. The remaining two electrons are removed after beam passes through a sheet of carbon.

why do carbon ions experience Bragg peak = Carbon ions interact predominantly with electrons in matter through Coulomb forces, and their range is determined by the initial particle energy and the stopping power of the medium. As the particles slow down, the stopping power increases, resulting in maximal energy deposition near the end of the particle range and producing a narrow and sharp peak in the depth–dose distribution (Bragg peak). Although energy spread and range straggling contribute to some broadening of the peak, these effects decrease with increasing particle mass. 4

ADVANTAGES / DISADVANTAGES

Advantage
Better dose localization and steeper dose penumbra allows for more precise targeting with less damage to surrounding tissue5The Bragg peak of carbon ions, in which the majority of the energy is deposited directly at the tumor site, allow for improved dose localization compared to photon beams.
carbon ions can be magnetically steered. They do not need to be physically collimated; a problem photon beams face 6By deflecting the charged beams laterally, the tumor can be painted with the planned radiation dose, resulting in a “spread-out Bragg peak” (SOBP) that results in additional benefits when compared to photon beam therapy, such as
- enhanced ratio of dose deposited to tumor relative to dose deposited to healthy tissue proximal to the tumor
- less of the dose is delivered to normal tissue on the back end of the Bragg peak
Carbon ions demonstrate a steeper lateral dose penumbra at greater depths, meaning the dose falls off more sharply at the edges of the beam, further reducing irradiation of surrounding healthy tissue 12The angular deflection of charged particles due to multiple Coulomb scattering decreases with increasing particle charge and mass. As a result, heavy ions such as carbon experience less lateral scattering along the beam path, leading to a more rapid lateral fall-off of dose and a sharper penumbra compared to lighter ions such as protons and electrons. This reduced beam spreading with penetration depth contributes to the superior lateral dose distribution observed in carbon ion radiotherapy. 4
- this is exploited in clinics by the fact that heavy ion beams can potentially be placed closer to at risk organs laterally, while maintaining a high degree of organ sparing6
Higher RBE resulting in greater biological damage to tumor cells per unit physical dosethe heavier carbon ions exhibit a higher linear energy transfer (LET) than photons and protons, leading to an increased relative biological effectiveness (RBE). The high LET of carbon ions produces clustered DNA damage that is more difficult for cells to repair, overwhelming cellular repair mechanisms17. As a result, carbon ion therapy is more effective at killing tumor cells per unit dose, which is particularly beneficial for radioresistant tumors.

Additional advantage is that particle therapy is effective at cell killing irrespective to cell stage, unlike photons, where cells are more resistant in late S and G2 phases 6

LET of neutron beams remains uniform at any depth in the body. In contrast, LET of carbon ion beams increases steadily from the point of incidence in the body with increasing depth to reach a maximum in the peak region. This property becomes a therapeutic advantage when carbon ion beams are used as cancer therapy for deep-seated tumours. 5
Heavy ions and the traversed matter undergo nuclear interactions that can produce positron emitting nuclei that can be imaged by PET scanners for dose verification 4 or by prompt gamma imaging (PGI) 9
high LET results in low OER and so CIRT is more effective agaisnt hypotoxic radioresistant tumorsoxygen enhancement ratio (OER) = ratio of the isoeffect dose (the dose of a certain radiation quality required to produce the same cell kill7) in hypoxic cells to that of aerobic cells 4
Weakness
Various sources of uncertainties may determine a mismatch between the planned position of the deposited dose and its actual position; this is also present in photon radiotherapy but carbon ion radiotherapy is more sensitive dye to its more peaked dose profile 9Uncertainties include anatomical variation of the patient (tumor shrinkage, weight loss, change in tissue density, etc.), physiological movement of organs, patient mispositioning, etc. 9 

These range uncertainties must be considered during treatment planning, because errors in the calculation of the depth of the Bragg peak can dramatically affect the dose delivered to the tumor and to the healthy tissue nearby 9
The nuclear interactions that take place in CIRT produces low Z fragements that have large forward momentum that can travel beyond the range where the incident theraputic nuclei stop, this creates the low dose and low LET tail beyond the bragg peak 4
The cost of developing and maintaining a heavy-ion center has been a challenge for its adoption in the US, with Pompos et el. (as cited by 1) estimates cost to be roughly twice as expensive as a proton centerDue to the high construction and operation costs of the accelerator system, there is still controversy on whether carbon ion RT is too expensive for the potential outcome improvements claimed. 5

Clinical use and evidence

  • The primary approach to managing glioblastoma (most prevalent and severe primary brain tumor in adults) is radiation therapy. However, a challenging aspect of delivering doses to the tumors is the prevention of radiation exposure to critical organs and tissues surrounding the tumor, which can lead to adverse effects to patient’s quality of life. In order to prevent this, CIRT has been proposed as a viable technique due to its high RBE, high LET, enhanced dose conformity and decreased total dose to normal tissues. Findings indicate that overall survival, progression free-survival and tumor local control improved, comparing carbon ion radiotherapy to alternative treatment, such as proton therapy. 10

  • By December 2020, approximately 37,500 patients had been treated at 12 CIRT centers across Asia and Europe3

  • As of 2025, around 13 active CIRT center operate world-wide, mainly in Japan, Germany, China, Italy and Austria, contrast with the 80 operational proton therapy centers worldwide, with over 100 under development3

phases of a clinical trial

Common phases for clinical trials for cancer are11

  1. Phase 1 = used to see how safe a treatment is and what the best dose is. 15 - 30 people in the trial
  2. Phase 2 = used to show how well a treatment works for a certain type of cancer, seeing how safe a treatment is, and possible side-effects. fewer than 100 people in the trial
  3. Phase 3 = compares the promising new treatment to the standard treatment. several hundred to several thousand people
  4. Phase 4 = gather more information of possible effect after the treatment has been approved for use. several hundred to several thousand people

essay

Thesis: Carbon ion radiotherapy represents an advanced treatment modality that combines superior physical dose localization with enhanced biological effectiveness, making it particularly for radio-resistant and deep-seated tumors.

Footnotes

  1. Malouff TD, et al. 2020. Carbon Ion Therapy: A Modern Review of an Emerging Technology. Front Oncol. 10, 82. (URL: Frontiers | Carbon Ion Therapy: A Modern Review of an Emerging Technology) ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8

  2. Lazar, A. A., Schulte, R., Faddegon, B., Blakely, E. A., & Roach III, M. (2018). Clinical trials involving carbon-ion radiation therapy and the path forward. Cancer, 124(23), 4467–4476. https://doi.org/10.1002/cncr.31662 ↩ ↩2 ↩3

  3. Sumaida, A. B., Shanbhag, N. M., AlKaabi, K., Balaraj, K., Sumaida, A. B., Shanbhag, N. M., AlKaabi, K., & Balaraj, K. (2025). Cost-Effectiveness of Carbon Ion Radiotherapy in Oncology: A Systematic Review. Cureus, 17(5). https://doi.org/10.7759/cureus.84008 ↩ ↩2 ↩3

  4. Kim, J., Park, J. M., & Wu, H.-G. (2020). Carbon Ion Therapy: A Review of an Advanced Technology. Progress in Medical Physic, 31(3), 71–80. https://doi.org/10.14316/pmp.2020.31.3.71 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9

  5. Ohno, T. (2013) Particle radiotherapy with carbon ion beams. EPMA Journal 4, 9. Particle radiotherapy with carbon ion beams | EPMA Journal | Springer Nature Link) ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  6. Mohamad, O., Sishc, B. J., Saha, J., Pompos, A., Rahimi, A., Story, M. D., Davis, A. J., & Kim, D. W. N. (2017). Carbon Ion Radiotherapy: A Review of Clinical Experiences and Preclinical Research, with an Emphasis on DNA Damage/Repair. Cancers, 9(6), 66. https://doi.org/10.3390/cancers9060066 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  7. Mohamad, O., Yamada, S., & Durante, M. (2018). Clinical Indications for Carbon Ion Radiotherapy. Clinical Oncology, 30(5), 317–329.   https://doi.org/10.1016/j.clon.2018.01.006  ↩ ↩2 ↩3

  8. Radiation Therapy for Cancer—NCI. (2015, April 29). [Article]. [https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy](https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy ↩

  9. Idrissi, A. B., Borghi, G., Caracciolo, A., Riboldi, C., Carminati, M., Donetti, M., Pullia, M., Savazzi, S., Camera, F., & Fiorini, C. (2024). First experimental verification of prompt gamma imaging with carbon ion irradiation. Scientific Reports, 14(1), 25750. https://doi.org/10.1038/s41598-024-72870-6 ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  10.  Koosha, F., Ahmadikamalabadi, M., & Mohammadi, M. (2024). Review of Recent Improvements in Carbon Ion Radiation Therapy in the Treatment of Glioblastoma. Advances in Radiation Oncology, 9(5).   https://doi.org/10.1016/j.adro.2024.101465  ↩

  11. cancer, C. C. S. / S. canadienne du. (n.d.). Types and phases of clinical trials. Canadian Cancer Society. Retrieved February 8, 2026, from https://cancer.ca/en/treatments/clinical-trials/types-and-phases-of-clinical-trials ↩

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