Radioactive beams provide a real-time picture of cancer treatment in mice

Rays of cancer-killing particles have been caught red-handed.

Particle beams can deliver a burst of destructive energy directly to tumors – assuming the beam is in the right place. Now, using a radioactive beam, scientists have determined the location of the beam while treating tumors in mice. It is the first successful treatment of tumors with a radioactive beam, the scientists report in a paper submitted Sept. 23 to arXiv.org.

The technique could eventually allow scientists to treat human patients with millimeter precision — especially important when a tumor is located near a sensitive organ such as the spinal cord or brain stem.

Different types of radiation can treat cancer. The most common is X-rays, high-energy light that can destroy the DNA in tumor cells. But X-rays deposit their energy along the beam path, resulting in possible collateral damage to other parts of the body. More precise tumor targeting is possible with particles such as protons or ions – electrically charged atoms – which throw most of their energy into a single spot.

Ionic treatment is currently performed in more than a dozen centers around the world. These treatments use stable, non-radioactive ions – usually carbon-12, a variety of carbon with six protons and six neutrons in its nucleus. Electrons are removed from the particles in the beam, giving them a positive charge.

The tumor is targeted based on calculations of how deep a beam will penetrate, along with previous images of the patient, for example, a CT scan (SN: 12/10/21). But bodies are not rigid, and organs can move between imaging and treatment. Ideally, the position of the beam would be confirmed in real time. This is exactly what the new technique allows.

“If you use a radioactive ion, you can simultaneously kill the tumor and see the beam,” says physicist Marco Durante of the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany.

Durante and colleagues used carbon-11 ions, which have one less neutron in their atomic nuclei than carbon-12 ions, making them radioactive. When carbon-11 decays, it emits a positron—a positively charged antimatter partner of an electron. Scientists can detect that the positron annihilates with an electron in the body through positron emission tomography, or PET (SN: 2/13/14). This identifies where the beam throws its particles.

In the study, scientists used carbon-11 ions to treat mice with tumors near the spine. The scientists were able to check the beam’s position during treatment and confirm that it was in place. Sure enough, the treatment shrank the tumors.

Scientists had already tried to use PET to measure the location of a stable ion beam. Stable ions do not emit positrons, but some of the stable atomic nuclei split as they pass through the material. These fragments can make radioactive ions that emit positrons in their decay. But the technique is difficult since the number of such particles is small.

With radioactive ion beams, many more positrons are emitted. “This allows [you] to get a very clear and beautiful image of where the particle stops,” says radiation physicist Mitra Safavi-Naeini of the Australian Nuclear Science and Technology Organization in Sydney, who was not involved in the research.

The technique could also help scientists understand how radioactive material moves through the body after an ion treatment, says Safavi-Naeini. Radioactive particles are washed out of the bull’s eye of the beam by the blood flowing through the body. This propagates the positron signal over time. The amount of this washout can help scientists understand whether the blood vessels are being destroyed by the treatment, thereby cutting off the tumor’s energy supply. This could help scientists figure out how best to use the particle beam to make cancer disappear.