By CoperNick

Valerio Pagliarino

October 30, 2018

How particle accelerators can be used for advanced cancer therapy.

When science-enthusiasts think about the frontiers of modern physics, they surely imagine elaborate particle physics experiments running in big laboratories such as the CERN (European Centre for Nuclear Research) that, investigating the properties of the fundamental particles, try to unravel the mysteries of the universe. However, only few people know that high-energy physics can be game-changing in many applied science fields as well: from microelectronics and astronomy to medical imaging and revolutionary cancer therapy.

Among the most important tools employed by experimental particle physicists, there are the famous particle accelerators. These devices can accelerate elementary particles (like electrons or various kinds of ions) near the speed of light using electric fields, while magnetic fields force the particles to follow particular trajectories.

The following formula describes the acceleration received by a charged particle immersed in an electric field:

where ‘a’ is the acceleration, ‘q’ is the charge of the particle, ‘E’ is the electric field and ‘m’ is the mass of the particle.

License: This file is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication
Ref: https://commons.wikimedia.org/wiki/File:Linear_accelerator_animation_16frames_1.6sec.gif
Description: Schematic showing how a particle beam is accelerated in a particle accelerator by the electric field.

The bending effect of the magnetic field on the particle path is explained by the Lorenz Force equation:

where ‘r’ is the radius of the trajectory of the particle, ‘m’ is its mass, ‘v’ is its speed, ‘q’ is the charge of the particle and ‘B’ is the magnetic field.

Thanks to these two principles, particle accelerators can create high-energy particle beams and drive them with high precision to impact with a target or another beam. In fundamental research, particle colliders represent an incredibly powerful tool thanks to Einstein’s equation, that establishes the relationship between mass and energy:

where ‘E’ is the energy, ‘m’ is the mass of the particle and ‘c’ is the speed of light.

Simply put, to discover new particles physicists make high-energy beams collide together and the total energy of the collision is transformed in mass that can produce a variety of particles, whose decay products reach the detectors. This principle is behind the discovery of the famous Higgs Boson at the LHC (Large Hadron Collider) at CERN that confirmed the Standard Model of Particle Physics (the elementary physics equivalent of the Periodic Table).

(William A. Barletta) (U. Amaldi et al, 2010)

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Ref: Felix Stollberg . https://commons.wikimedia.org/wiki/File:Low_Energy_Ion_Ring_(LEIR).jpg
Description: The LEIR (Low Energy Ion Ring) particle accelerator at CERN.

But what has particle physics in common with cancer therapy?

High-energy particle beams, on other energy ranges (hundreds of MeV versus the tens of TeV of LHC), can cause ionization in matter. When a charged particle, like a proton, impacts an object, like organic tissue, it begins to lose energy because its electric field interacts with the electron shells of atoms in the object. While the proton loses kinetic energy and slows down, the electrons in the matter can be dragged out of atoms, creating ions and disrupting the reactivity of molecules, possibly destroying them. If this process involves DNA molecules contained inside a cell, and the damage exceeds the DNA self-repairing capabilities, the cell dies. This is the reason why all forms of ionising radiation (X-rays, gamma-rays, accelerated charged particles) are dangerous for human beings.

The common radiotherapy employs X-rays to destroy cancer cells by shaping the beam with high precision on the tumour. The treatment is repeated from different directions to minimise the radiation dose received by the surrounding healthy tissue. Nowadays this technology is widely used and the small electron accelerators required, called “LINAC” (LINear ACcelerator), have become relatively cheap and compact. But another technology is beginning to spread thanks to the reduced dimensions and reliability reached by bigger proton accelerators like cyclotrons and synchrotrons: hadrontherapy. This treatment involves “hadrons”, a family of particles that includes protons and, consequently, all the atomic nuclei. When protons or carbon ions interact with matter, they lose energy following a completely different profile in comparison to electrons or photons. In particular, they don’t lose energy along the entire path, but instead their energy loss is focused in the last part of the path, creating a ‘Bragg peak’ in the “dose vs depth” graph.

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Ref: Mark Filipak – https://commons.wikimedia.org/wiki/File:Comparison_of_dose_profiles_for_proton_v._x-ray_radiotherapy.png
Description: How different particle beams used in cancer therapy ionize the human tissues.

Aligning this ionisation peak with a tumour, you can destroy the cancer cells releasing a very low amount of energy to the surrounding healthy tissues. This therapy requires more complex particle accelerators because hadrons are heavier than electrons and so the magnets have to be bigger (see the Lorentz force equation above). However, this treatment has been shown to give exceptional results for tumours near vital organs or in paediatric patients. (S.B. Harrabi et al, 2016) (U. Amaldi et al, 2010)

In conclusion, particle therapy represents an outstanding example of how fundamental research in fields apparently far from practical applications can lead to life-changing innovations.

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Ref: Llorenzi – https://commons.wikimedia.org/wiki/File:Protonterapia_Trento_3.jpg 
Description: Protontherapy gantry system at the Italian Trento Protontherapy Facility.

Bibliography

1. B. Harrabi, N. Bougatf, A. Mohr, T. Haberer, K. Herfarth, S.E. Combs, J. Debus,S. Adeberg. 2016. “Dosimetric advantages of proton therapy over conventional radiotherapy with photons in young patients and adults with low-grade glioma.” Strahlentherapie Und Onkologie (Springer) 759-769.
2. Amaldi, R. Bonomi, S. Braccini, M. Crescenti, A. Degiovanni, M. Garlasché, A. Garonna, G. Magrin, C. Mellace, P. Pearce, G. Pittà, P. Puggioni, E. Rosso, S. Verdù Andrés, R. Wegner, M. Weiss, R. Zennaro. 2010. “Accelerators for hadrontherapy: From Lawrence cyclotrons to linacs.” Nuclear Instruments and Methods in Physics Research A (Elsevier) 620 (2-3): 563-577.
3. William A. Barletta – Director, United States Particle Accelerator School. n.d. “Introduction to Accelerators.” of Physics, MIT. (READ MORE ARTICLES AT THIS LINK