Recently, researchers from the University of Quebec conducted a successful experiment at the Advanced Laser Light Source Laboratory at the National Research Council of Canada (INRS), demonstrating the promising use of ultrafast laser technology for cancer radiation therapy.
"We have demonstrated for the first time that under certain conditions, a laser beam tightly focused on ambient air can accelerate electrons to the MeV (mega-electron volt) energy range, which is the same energy as some of the radiators used in cancer radiation therapy." said Franois Légaré, INRS professor and scientific leader of the Advanced Light Sources Laboratory (ALLS).
By tightly focusing several cycles of a millijoule (mJ)-level, femtosecond (fs), infrared (IR) laser, the researchers generate relativistic electron beams in ambient air and achieve high dose rates of up to 0.15 Gray per second (Gy/s). At atmospheric pressure, their laser intensity reached 1 × 1019 watts per square centimeter (W/cm-2). The team measured the resulting electron beam and found that it had a maximum energy of up to 1.4 MeV.
The team showed how the laser's tight focus, long wavelength, and short-cycle pulse duration combine to limit the effect of b-integration on the focused laser beam. The high density of air molecules in the ionizable focal volume is sufficient to form a plasma close to the critical density, which provides a high conversion efficiency from lasers to electrons. Through three-dimensional particle-in-cell simulations, the researchers confirmed that the acceleration mechanism is relativistically based, has a mass-motion potential, and is theoretically consistent with measured electron energies and scatter.
Schematic of the experimental setup: pulses of ultrashort infrared laser light are tightly focused on the surrounding air, producing a high dose of ionizing radiation.
The researchers believe that the strength of this laser-driven electron source stems from its simplicity. A single focused optic in the surrounding air can produce an electron beam that delivers a year's worth of radiation dose to a person standing one meter away in less than a second. No complicated setups or vacuum chambers are required, making this method suitable for many irradiation applications by reducing the requirements for producing ultrafast MeV electron sources.
Advances in laser technology have enabled laser wake field acceleration - a process that accelerates electrons to high energies in a very short period of time by generating plasma - to work in the mid-infrared with mJ-class systems to produce high particle fluxes of MeV electrons that can be be used in radiobiology research. However, these high-energy laser-driven electron sources require complex and bulky installations in vacuum chambers, which limit access to the beam.
Laser-driven MeV electron sources could provide new approaches to cancer treatment, such as FLASH radiation therapy, a method of treating tumors that are resistant to conventional radiation therapy. With FLASH therapy, high doses of radiation can be delivered in microseconds instead of minutes. This speed of delivery helps protect the healthy tissue surrounding 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 tissue, reducing the tissue's sensitivity to radiation.
Measured radiation dose rate (logarithmic scale) as a function of distance from the focal point for three different laser pulse energies.
"No study has yet been able to explain the nature of the flash effect," said researcher Simon Vallières, "However, the electron source used in FLASH radiation therapy has similar characteristics to the one we generate by focusing the laser intensely on ambient air. Once the radiation sources are better controlled, further studies will allow us to investigate the causes of the flash effect and ultimately provide better radiation therapy for cancer patients."
The researchers believe that the scalability of their approach will increase with the continued development of high average power lasers in the mJ class. The rapid development of laser sources, targeting increased available pulse energies and repetition rates, could allow the INRS technique to be extended to higher electron energies and larger dose rates.
The researchers also emphasized the importance of safety when dealing with laser beams tightly focused on the surrounding air. When measurements were taken in the vicinity of the radiation source, the team observed radiation dose rates from electrons that were three to four times higher than those used in conventional radiation therapy.
"The observed energy of the electrons (MeV) allows them to move more than 3 meters in the air or a few millimeters under the skin," said Vallières, "which poses a risk of radiation exposure to users of the laser light source. Discovering this radiation hazard is an opportunity to implement safer practices in the laboratory."
