CARBON ION THERAPY: THE PARTICLE THERAPY TREATMENT THAT PACKS A POWERFUL PUNCH AND REALLY HITS CANCER WHERE IT HURTS

Radiation therapy is currently used to treat approximately 50% of all cancers (Baskar et al, 2012). Conventional radiation therapy such as photons is not typically used for tumours that are considered radio-resistant or recurrent diseases where radiotherapy has been used before. As they are unlikely to effectively treat cancer and would cause unnecessary damage to healthy tissue. This is where carbon ion therapy is starting to make its mark on the radiotherapy world.

Physical and Biological Considerations

Carbon ions fall into the category of particle therapy, the same as protons. Protons have become more established in mainstream cancer treatments since their introduction in the 1940s and are recognized for their high precision targeting of tumours, healthy tissue sparing, and reduction in side effects of treatment.

Carbon ions share many similarities to protons in that both exhibit energy distribution at depth in the same characteristic pattern known as the “Bragg Peak”. This shows a slow initial absorbed dose as it passes through the tissue and then a very steep rise to maximum absorption followed by a sharp fall towards the end of the particle range.

This energy distribution means that they deposit most of their energy at the end of their trajectory through tissue when they have started to slow. Meaning when planning treatment with heavy particles such as carbons if you plan the energy correctly you can ensure that the highest energy transfer coincides with the target area, the tumour, whilst the surrounding healthy tissue receives very little dose due to the steep rise and fall of dose deposition.

Carbon ions differ from protons in a few ways. The lateral fall-off and penumbra (sharpness) for carbon ions are smaller than protons resulting in superior dose distribution. Carbon ions have a higher RBE (relative biological effectiveness) at 2-3.5 compared to protons’ RBE of 1.1. An important factor in radiation therapy is something called the oxygen enhancement ratio (OER). Tumours that are well-oxygenated typically have an OER of 1 for photons and protons and respond well to radiation treatments. Tumours that are deemed to have low radiosensitivity or poor oxygenation typically have high OER and to reduce this and effectively treat that tumour they need a high LET particle to achieve this. Fortunately, carbon ions have the highest LET out of photons, electrons, and protons and are 12 times heavier than a proton. They also have an increased probability of direct cell death through double strand break of DNA rather than requiring the generation of free radicals to do this such as with photon treatment. So carbon ions are deemed to be the most effective in treating hypoxic radioresistant tumours, whilst protecting surrounding healthy organs or tissue (Kim et al, 2020).

Current Carbon Ion Technology and its Limitations

The first centre to offer carbon Ions as a form of treatment was The National Institute of Radiologic Sciences in Japan in 1994. As clinical trials have continued to showcase the benefits of this treatment, its popularity has increased and to date 5 countries now offer it and more are starting to plan installation of facilities to support carbon ion machines (Malouff et al, 2020).

Carbon ions are produced using a synchrotron which accelerates the carbon ion particles to the speed and energy that is required for treatment. One of the main reasons that carbon ions haven’t been utilised in the same way that photons have is the size and expense associated with them. The majority of centres that currently treat carbon ions use fixed beam lines and a supine treatment couch which limits the treatment positions and cancers that can be treated, as patients have to be re-setup between beams if they require multiple beam treatments. There are only a few centres in the world that have a rotating gantry requiring very large treatment rooms.

Treatment can be delivered using passive scattering whereby a collimator is used to shape the beam in the lateral direction and a range compensator is used to shape the beam distally. Another way to deliver the treatment beam is using active scanning, using a narrow pencil beam that does not require collimators or compensators.

A Potential Solution to Making Carbon Ions More Efficient and Accessible

Carbon ion therapy in an upright position will overcome a number of the limitations currently identified with this treatment option. An upright patient positioning system such as ‘Eve’ produced by Leo Cancer Care will allow fixed beam machines to deliver multiple treatment angles including posterior beams without the need to reposition the patient between each beam improving treatment accuracy and significantly reducing treatment time. By removing the need for a rotating gantry, treatment rooms will be smaller and more compact.

See below the video: A Look Into the Future of an Upright Radiotherapy Treatment Using Our Marie? Solution.

References

Baskar, R., et al. (2012). Cancer and Radiation Therapy: Current Advances and Future Directions. International Journal of Medical Sciences. ((3): 193-199. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3298009/

Kim, J., Park, M, JM., Wu, HC. Carbon Ion Therapy: A Review of an Advanced Technology. Progress in Medical Physics. 31(3): 71-80. Available at: https://www.progmedphys.org/journal/view.html?uid=870&&vmd=Full

Malouff et al (2020). Carbon Ion Therapy: A Modern Review of an Emerging Technology. https://www.frontiersin.org/articles/10.3389/fonc.2020.00082/full

Radiotherapy. 2003. How particles can be therapeutic. https://physicsworld.com/a/how-particles-can-be-therapeutic/

Wang, X., et al (2021). Application of Carbon Ions and its Sensitizing agent in cancer therapy: A systematic review. Frontiers in Oncology. 11. (Pages 1-21) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8287631/

This article is also available to view on the Leo Cancer Care website: https://www.leocancercare.com/more-news-and-blogs-0/carbon-ion-therapy-the-particle-therapy-treatment-that-packs-a-powerful-punch-and-really-hits-cancer-where-it-hurts

*Please note: The Leo Cancer Care technology is not commercially available and will not treat patients until the required regulatory approval has been achieved.

Written by Kate Yip , Clinical Engagement Manager at Leo Cancer Care.