High-energy charged particles: the beginning and end of cancer

The Research Topic, published in Frontiers in Oncology, consists of a collection of scientific articles investigating high-energy charged particles either for cancer treatment or for space radiation protection. 

— By Conn Hastings

Our planet is bombarded by galactic cosmic rays and they are a constant presence in interstellar space. Much of this high-energy radiation is thought to be emitted during colossal explosions called supernovae, which happen during the death of massive stars. Cosmic rays might sound like something from science fiction, but they are a huge hurdle to successful long-haul space flights, such as a manned mission to Mars. Why? Because they can cause cancer and other health complaints in exposed astronauts, and we don’t yet have effective ways to shield spacecraft and their occupants. Ironically, the high-energy charged particles that form a significant portion of cosmic rays are used to treat cancer on Earth. It’s all about context – used in a focused and localized way the particles can destroy tumors, but long-term uncontrolled exposure can cause tumors.

So, what are these particles? They mostly consist of energetic protons and other high-energy charged particles, including carbon ions. In cosmic rays, these particles are flung across space because of the massive energies they are subjected to during processes like supernovae. On Earth, our magnetic field protects us from the worst of this cosmic ray bombardment. However, for astronauts outside the Earth’s protective magnetic bubble, the risk of cancer on long space journeys could be significant. If humans are ever to travel to Mars, we will need to develop ways to protect astronauts from these rays. The amount of shielding we can add to a spacecraft is limited by the weight restrictions and expense involved in launching rockets. In fact, very thick shielding is not actually much better than thin shielding, making massive spacecraft shields not worth the extra weight and expense.

To complicate matters, scientists are unsure of the levels of these particles that astronauts are likely to be exposed to in space, and the risk of cancer in the long-term. This makes it very difficult to assess how safe it is to send humans to Mars, for example. Researchers have been able to learn a little from humans who have been exposed to radiation, including people who have been irradiated during nuclear accidents or the atomic bombings in Japan, and astronauts on the international space station. However, none of these situations are the same as an interplanetary voyage, making comparisons difficult. Estimating the risks of cosmic ray exposure and developing methods to protect astronauts are active and important areas of research.

Meanwhile, many of these same particles can be created by and emitted from specialized machines on Earth and are used to treat cancer. You might wonder how this came about. Well, using radiation to treat cancer is nothing new. Typically, doctors use X-rays to irradiate tumors and kill cancerous cells. However, charged particles have several advantages over X-rays, including an increased cell killing effect in the tumor and decreased damage to surrounding healthy tissues. Charged particles are also more effective against tumors that are resistant to conventional radiotherapy.

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Particle therapy for cancer, including that using protons or carbon ions, is now available in several centers across the world. As particle therapy is less harmful to healthy tissue, it has been approved for a large range of tumors in children, who are more sensitive to the effects of radiotherapy. However, particle therapy is not without risks and side-effects. It can occasionally cause new tumors to grow, which is perhaps unsurprising, given the potential carcinogenic effects of these particles in astronauts.

Scientists are in the process of establishing new ways to make particle therapy as safe and effective as possible. An area of significant overlap between space and cancer research is working out how charged particles cause cancer and tissue damage. The Frontiers Research Topic “Charged Particles in Oncology”, published in Frontiers in Oncology, consists of an extensive collection of scientific articles exploring high-energy charged particles in space travel and cancer treatment. “These two fields, ostensibly very different, share many research questions on the biological effects of charged particles. Astronauts and cancer patients will both benefit from research in these fields,” says Topic Editor Marco Durante, of The Trento Institute for Fundamental Physics and Applications.

One of the studies featured in the Research Topic deals with creating a magnetic field around a spacecraft by using electromagnets in different configurations. The idea is that this field could deflect harmful cosmic rays, in the same way as the Earth’s magnetic field, protecting astronauts on long-haul spaceflights. Another article discusses the hurdles to simulating cosmic rays on Earth using particle accelerators. Scientists want accurately to recreate cosmic rays, to measure their biological effects and develop new shielding techniques. A review article discusses radiation detectors on board spacecraft, such as the international space station, and ways that researchers can collect and analyze the radiation data. It is also important to work out the risk to individual astronauts before they travel. People have different susceptibilities in terms of the health impacts of radiation. One article discusses different ways to measure radiation susceptibility in individuals, to ensure that the least vulnerable people are selected for long-haul space missions.

Other studies in the Research Topic focus on investigating how effective particle therapy is at destroying tumors. In one study, researchers compared carbon ion radiotherapy with conventional X-ray radiotherapy, and found that the carbon ions were better at killing prostate and colon cancer cells. A review summarizes the advantages of particle radiotherapy compared with conventional radiotherapy, including an improved effect against radiotherapy-resistant cells, while another study found that particle therapy was more effective against chemotherapy-resistant cells.

However, many of the studies featured in the Research Topic are relevant to both cancer therapy and space travel, as they investigate the effect of high-energy charged particles on cells and tissues. Understanding how these particles affect living cells could help scientists to develop safer particle therapies and techniques or treatments to reduce cancer risk during interplanetary travel. In one study, scientists showed that titanium ions caused inflammation and changes in DNA in the lungs and testicles of mice, suggesting that these particles could damage both normal and reproductive organs. A review summarizes the effects of radiation on bone marrow, where it could damage stem cells involved in the normal growth and maturation of blood cells, leading to hematological disorders.

DNA damage, including double-strand breaks where both DNA strands are broken, is the most direct way that charged particles can damage cells and cause cancer. In one review, scientists discuss ways to mathematically model this type of damage to better understand and prevent it. However, some researchers think that the indirect effects of high-energy charged particles, including changes in cell signaling pathways and changes to the environment surrounding the cell, can also be carcinogenic. In one article, scientists discuss how this phenomenon has been demonstrated in a mouse model of breast cancer.

Learning more about high-energy charged particles will help us to improve cancer treatment and make it safer. It may also help us to develop strategies to keep astronauts safe on long-distance missions. Our health, on Earth and in space, and our capacity to explore and colonize new worlds hinge on this type of research.

Durante and his fellow Topic Editors have been shortlisted for the Frontiers Spotlight Award, where the winners are granted US$100,000 to host their own conference themed around their Research Topic.

Take a look at the other finalists here.