Proton Therapy Machine

Proton therapy is an invaluable option for treating tumors, including terminal cases. The following content describes what proton therapy machines are, how they work, and whether they are as effective as radiotherapy for those with cancer.

What is a Proton Therapy Machine?

A proton therapy machine is a type of cyclotron or particle accelerator that concentrates protons using electro-magnetism in order to generate a high-intensity beam of energy that can be directed at a target, such as a tumor. It is primarily used to administer proton therapy, which is similar to radiotherapy, to obliterate tumors.

Cyclotrons can concentrate any small particles including x-ray radiation photons, ordinary photons, and electrons, in a beam of energy, similar to a laser. Proton therapy machines specifically concentrate protons, which are positively charged subatomic particles found in the center of an atom. Protons have unique properties that make them better equipped than standard radiotherapy x-ray emissions for targeting difficult-to-reach tumors.

Proton therapy machines may also be referred to as proton synchrotrons, cyclotrons or magnetic proton accelerators. They were originally made for the study of matter and used in physics laboratories. Since then, they have been adapted for medical proton therapy and diagnostic imaging.

How Does a Proton Therapy Machine Work?

Proton therapy machines are adaptations from the original particle accelerators, first created in the early 20^th century. Their construction and development is briefly summarized below, describing the basic principles underscoring how proton therapy machines are able to efficiently obliterate tumors.

Basic Cyclotron Mechanics. The earliest cyclotrons can be described as gigantic cylinders that house two large electrodes, which face one another and divide the internal compartment of the cylinder in half. Between the semi-circular electrodes is a thin gap in which particles are placed for acceleration. A frequency oscillator generates an electric field inside this gap that alternates its polarity. This causes a magnetic field to form around the electrodes, which guides the movement of the charged particles. The particles accelerate in a spiral motion, moving between the electrodes until they reach the periphery, where they are funneled out toward a target. The velocity generated in the particle accelerator increases the energy of the particles in the beam to anywhere between several million to 1 billion electron volts.^[1]

Modified Synchrotrons. Modern cyclotrons, known as synchrotrons, are drastically improved versions of the earliest cyclotrons. They are comprised of complex circuits of magnets that serve to increase the particle acceleration speed, focus the particles more precisely, and steadily amplify the energy of the beam. Protons are first generated, usually through the stripping of electrons from an appropriate medium, such as hydrogen gas. Once made, they are injected into a small cyclotron device, either a linear accelerator or microtron, that begins the process of particle acceleration. These small cyclotrons are similar to the first cyclotrons developed.^[2]

Medical Proton Cyclotrons (Proton Therapy Machines). In proton therapy, the beam emitted from a cyclotron can pass through body tissues and be used to target very specific points, such as tumors. Medical cyclotrons used for proton therapy have been modified for this purpose with a special laser head attachment and a rotating gantry. These two components allow for optimal beam intensity, control and positioning. Prior to use, a CT scan or MRI is required in order to properly position the proton beam. Proton therapy machines are undergoing constant development in order to enhance their accuracy, which includes better imaging technology.^[3]

Medical Applications of Proton Therapy Machines

Proton Therapy for Cancer. Cancer treatment is the main use for proton cyclotrons. Proton therapy aims to obliterate tumors while minimizing damage to non-cancerous tissues. Due to the larger mass of protons, they slow down while penetrating body tissues and, through modification, can be made to stop at specific points in the body, such as the tumor site. The energy of the beam scatters minimally when entering body tissues, allowing for more energy to be focused on the target. Various techniques have been developed that allow for the beam to target the tumor from multiple angles, further increasing the precision and ablative power of the therapy.

Scientific Uses. Over and above proton therapy, cyclotrons are used mostly for scientific imaging in a similar way to x-rays and CT scans, yet with a much higher degree of accuracy. They can concentrate photons and other particles more accurately than other imaging machines and can thus generate more in-depth renditions of structures at much smaller scales. Synchrotrons are used in spectrometry to image molecules, cellular structures and object surfaces in microscopic detail. They are also used in experimental physics laboratories to investigate sub-atomic particles, quantum mechanics and physical properties of matter.

Therapeutic Efficacy of Proton Therapy Machines

Proton therapy machines can technically be used to treat any tumor. However, due to high cost and low availability, they are usually utilized to treat tumors that standard radiotherapy cannot effectively treat. These include aggressive tumors of the brain, cranium, meninges, lungs, eyes, spine, head and neck, as well as those found in difficult-to-reach internal compartments.

At present, there are little more than 30 centers worldwide that offer medical proton therapy^[4], and studies documenting the benefits are highly limited. Based on the observations so far, the benefits of proton therapy machines over conventional radiotherapy counterparts are listed below:

May Be Safer than Radiotherapy. Proton therapy is an improvement to the safety and accuracy of other radio-ablative techniques. The x-ray photons used in radiotherapy are known to pass right through the body, leading to an increase in radiation toxicity in tissues along the radiation path. In proton therapy, tissues near the beam are affected much less due to a lower “spillover” of charged particles near the beam and no continuation beyond the tumor site. Due to advancements in proton technology, the energy of the beam can also be focused more on the final destination, minimizing the impact on tissues it passes through before reaching the tumor. Radiation-induced toxicity is shown to be less in those receiving proton therapy than radiotherapy. However, more studies are required to confirm these findings.^[5]

Preliminary Evidence for Greater or Equal Benefits. Studies have so far shown that proton therapy machines are either safer or as efficacious as photon radiotherapies. Clinical trials observing the effects of proton therapy are very small and limited. A 10-year study revealed that among those who underwent proton therapy for retinoblastoma, none of the participants contracted recurrent tumors. However, 14% of participants who underwent radiotherapy during the study period encountered secondary malignancies.^[6] Other studies have shown that proton therapy can induce secondary tumors in a similar proportion of patients after 3-10 years^[7]. These patients were treated for cranial tumors, and tumor recurrence was most commonly seen along the spinal column. Overviews of current research suggest that proton therapy may be as effective as radiotherapy in treating a wide variety of cancers yet with minimal toxic side effects, particularly with respect to brain tumors.^[8]

Could Be Best for Treating Childhood Cancer. Proton therapy machines are also advocated for the treatment of children with cancer as they tend to be more sensitive to the effects of radiation. Small studies have indicated that, as seen with adults, toxicity in children is less compared to radiotherapy.

Conclusion

Proton therapy machines emit laser-like beams of energy comprised of charged subatomic particles called protons. The latest technology allows for the beam to be focused on a precise target with minimal agitation of surrounding tissues. Cancer patients who undergo treatment with a proton therapy machine tend to have a similar or improved prognosis than those who opt for radiotherapy, with fewer side effects. When treatments become more affordable, proton therapy is expected to supersede radiotherapy for oncology treatment.

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