A breakthrough in laser plasma accelerators could enhance dose delivery in animal tests

In a major interdisciplinary achievement.
Loukia Papadopoulos
A short laser pulse sends particles toward a strand of DNA in this conceptual rendering. HZDR / Juniks

It's been more than three decades since proton therapy first became available to cancer patients in a clinical setting. While this technologically advanced treatment option has saved lives, "everything turned out to be more complex than expected," says physicist Florian Kroll.

One reason is that radiation therapy can cause serious — and unexpected — side effects that biomedical researchers need to study. Another is bridging areas as different as particle physics and oncology research requires serious interdisciplinary muscles. 

That's why it's big news that Kroll and several colleagues have developed a new platform that makes it far easier for researchers to create a tiny, clean beam of particles using a short laser pulse, according to a report by PhysicsWorld published Thursday.

A novel LPA method

Laser plasma acceleration (LPA) has greatly evolved over the years. However, one challenge has remained with the process and that is that laser plasma accelerators particle are notoriously difficult to stabilize and control.

The new LPA method improves reliability by making use of a high-power laser, tightly focused onto a thin (220 nm) plastic target. “The intensity of the laser is so enormous that, when it impinges on the target, it immediately ionizes the target material, turning it into a plasma,” explains Kroll, first author on the new study.

The method manages to strip electrons from their atomic cores and push them through the plasma by the laser. The ones that can't escape the now positively charged target, form a “sheath” on the back side of the target. This results in the engineering of a quasi-static electric field that “pulls” on the target ions allowing the ions to enter the mega-electronvolt (MeV) range.

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Clinical applicability

The researchers also irradiated human tumors on mouse ears although the prime focus of the research was to demonstrate the feasibility of animal studies and to test the limits of dose delivery.

“We don’t want to speculate about the clinical applicability of laser-driven proton beams,” says Kroll. “In the early days of laser acceleration, many claims with respect to revolutionary, compact and cheap laser-driven therapy machines were made. In the end, everything turned out to be more complex than expected. Nevertheless, LPA machines have always been and will always be an interesting complementary accelerator technique to cyclotrons, synchrotrons and more.”

The study was published in the journal Nature Physics.

Study abstract: 

Recent oncological studies identified beneficial properties of radiation applied at ultrahigh dose rates, several orders of magnitude higher than the clinical standard of the order of Gy min–1. Sources capable of providing these ultrahigh dose rates are under investigation. Here we show that a stable, compact laser-driven proton source with energies greater than 60 MeV enables radiobiological in vivo studies. We performed a pilot irradiation study on human tumors in a mouse model, showing the concerted preparation of mice and laser accelerator, dose-controlled, tumor-conform irradiation using a laser-driven as well as a clinical reference proton source, and the radiobiological evaluation of irradiated and unirradiated mice for radiation-induced tumor growth delay. The prescribed homogeneous dose of 4 Gy was precisely delivered at the laser-driven source. The results demonstrate a complete laser-driven proton research platform for diverse user-specific small animal models, able to deliver tunable single-shot doses up to around 20 Gy to millimeter-scale volumes on nanosecond timescales, equivalent to around 109 Gy s–1, spatially homogenized and tailored to the sample. The platform provides a unique infrastructure for translational research with protons at ultrahigh dose rates.