Researchers at the Advanced Laser Light Source Laboratory at the Institut national de recherche scientifique (INRS) demonstrated that ultrafast laser technology can potentially be used in radiation treatment of cancer. The INRS team collaborated with researchers at McGill University Health Centre to show that electrons accelerated in ambient air can provide a high enough dose rate for radiation therapy applications. (Left to right) Steve MacLean, CTO at Infinite Potential Laboratories, Sylvain Fourmaux, research associate at INRS, François Fillion-Gourdeau, research associate at Infinite Potential Laboratories, Stéphane Payeur, research officer at INRS, Simon Vallières, postdoctoral researcher at INRS, and professor François Légaré, director of the Énergie Matériaux Télécommunications Research Centre. Courtesy of INRS. “For the first time, we showed that, under certain conditions, a laser beam tightly focused in ambient air can accelerate electrons reaching energies in the MeV (megaelectronvolt) range, the same order of magnitude as some irradiators used in radiation therapy for cancer,” said François Légaré, professor at INRS and scientific head of the Advanced Light Source Laboratory (ALLS). By tightly focusing a few-cycle, millijoule (mJ)-class, femtosecond, IR laser, the researchers generated relativistic electron beams in ambient air and achieved a high dose rate of up to 0.15 Gray per second (Gy/s). They reached laser intensities of up to 1 × 1019 W/sq cm at atmospheric pressure. The team measured the electron beam that was generated and found that it had a maximum energy of up to 1.4 MeV. The team showed how the tight focusing, long-wavelength, and few-cycle pulse duration of the laser combined to limit the effect of the B-integral on the focused laser beam. The high density of air molecules in the focal volume available for ionization was sufficient to form a near-critical density plasma, which provided a high conversion efficiency from the laser to the electrons. Through 3D particle-in-cell simulations, the researchers confirmed that the acceleration mechanism was based on a relativistic, ponderomotive force and showed theoretical agreement with the measured electron energies and divergence. An experimental setup. An ultrashort, IR laser pulse is tightly focused in ambient air, generating high ionizing radiation doses. Courtesy of INRS. The researchers believe that the strength of this laser-driven electron source stems from its simplicity. A single focusing optic in ambient air can produce an electron beam capable of delivering a yearly radiation dose in Advancements in laser technology have enabled laser wakefield acceleration — a process that accelerates electrons to high energies in a very short length by creating a plasma — to operate with mJ-class systems in the MIR to produce a high particle flux of MeV electrons that can be used for radiobiological studies. However, these sources of high-energy, laser-driven electrons require complex, bulky setups contained within a vacuum chamber, which limits close access to the beam. A laser-driven MeV electron source could enable new approaches in cancer therapy, such as FLASH radiotherapy, a method of treating tumors that are resistant to conventional radiation therapy. Using FLASH therapy, high doses of radiation are delivered in microseconds rather than in minutes. The speed of delivery helps to protect the healthy tissue around the tumor from the effects of radiation. Although the effects of FLASH are not fully understood, scientists believe that FLASH may cause rapid deoxygenation of healthy tissues, reducing the tissues’ sensitivity to radiation. Measured radiation dose rate (in log scale), as a function of distance to the focal spot, for three different laser pulse energies. Courtesy of INRS. “No study has been able to explain the nature of the FLASH effect,” researcher Simon Vallières said. “However, the electron sources used in FLASH radiotherapy have similar characteristics to the one we produced by focusing our laser strongly in ambient air. Once the radiation source is better controlled, further research will allow us to investigate what causes the FLASH effect and to ultimately offer better radiation treatments to cancer patients.” The researchers believe that the scalability of their method will increase with the continuing development of mJ-class, high-average-power lasers. The fast pace of laser source development, with the goal of increasing the available pulse energy and repetition rate, could enable the INRS technique to be scaled to higher electron energies and larger dose rates. The researchers also emphasized the importance of safety when handling laser beams that are tightly focused in ambient air. When taking measurements near the source, the team observed a radiation dose rate of electrons three to four times greater than that used in conventional radiation therapy. “The electron energies observed (MeV) allow them to travel more than 3 m in air, or several millimeters under the skin,” Vallières said. “This poses a radiation exposure risk for users of the laser source. Uncovering this radiation hazard is an opportunity to implement safer practices in laboratories.” The research was published in Laser & Photonics Reviews (www.doi.org/10.1002/lpor.202300078).