ALLS improves cancer therapy via femtosecond laser

19 Dec 2023

Tightly focused source generates high dose rate electron beams for use in treatment.

Creating beams of accelerated electrons from the action of focused lasers is a promising route to using the beams in multiple applications, although the experimental set-ups needed have been complex and bulky.

Medical applications of electron beams, including their ability to kill cancer cells and malignancies, would benefit from more convenient ways to generate them, and a project at Canada's Advanced Laser Light Source (ALLS) laboratory has now demonstrated one such route.

Published in Laser & Photonics Review, the research "presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few-cycle, mJ-class femtosecond infrared laser," noted the project in its paper.

Focusing a laser pulse of high enough intensity in ambient air can generate plasma at the focal point, with the plasma then acting as a source of electrons that can be accelerated to energies up to a few keV. But reaching higher energies in ambient air has remained challenging.

ALLS, attached to Quebec's INRS research institute, investigated how a combination of tight focusing, long wavelength and few-cycle pulse duration can combine to limit the effect of the B-integral, a non-linear parameter relating to the phase shift and stability of the laser.

"A single focusing optic in ambient air produces an electron beam capable of delivering a yearly radiation dose in less than one second to a person standing one meter away," noted the project in its paper. "The lack of a complex setup or vacuum chamber improves its usefulness in many irradiation applications."

Understanding FLASH radiotherapy

François Légaré of INRS commented that “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 range, the same order of magnitude as some irradiators used in radiation therapy for cancer."

The project anticipates that the rapid pace of laser source development will soon make increased pulse energies and repetition rates available, allowing the technique to be scaled to higher electron energies and larger dose rates. The only proviso will be that the air volume in the vicinity of the focus has time to replace between pulses.

For cancer treatments, the technique could provide new routes to FLASH radiotherapy, in which ultra-high dose rate radiation is delivered to a target. FLASH therapy offers improved treatment efficacy, but the high radiation doses can bring their own complications, and the therapeutic process is not fully understood.

"No study has been able to explain the nature of the FLASH effect, noted Simon Vallières of INRS. "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 offer better radiation treatments to cancer patients."

The ultrafast nature of the laser-driven electron acceleration mechanism developed by ALLS makes it a very promising candidate for characterizing the potential of the FLASH effect for medical applications, said the project.