Flash, microLinac, CT


FLASH-RT was intended as a way to freeze the motion of radiotherapy patients who can never quite be still, like a flash-cube might freeze a subject in photography. Interesting as this was, the real excitement comes from the serendipitous discovery that healthy tissues are resistant to radiation, when it arrives quickly, in a ‘flash’.

A FLASH radiotherapy treatment would need to be very fast, requiring a radiation beam about 100 times more powerful than clinically available today. But, given time, we humans are very good at making machines more powerful; and when biology identifies a need, technology has a tendency to meet it.

Because of its radio-biological advantage, when FLASH-RT becomes available to the clinic, it must quickly become the standard of care. Every facility in the country will be shopping for a new particle accelerator. Whoever brings this to market first will be selling them faster than they can make them. And, then Varian will introduce a knock-off.

Electrical things tend to move faster than mechanical things and an accelerator can be electronically switched more quickly than it can be spun around on a rotating gantry. Fast electronic switching is central to the van-mobile FLASH-RT concept demonstrated by SLAC, who hopes to achieve the necessary intensity.

A set of lunch-box-sized mini-linacs mounted on a fixed-ring could be switched on and off quite quickly. This Stanford concept is called the PHASER as the linacs would operate out of phase with each other.

More recently, SLAC announced the creation of a tiny micro-linac “chip”, which is powered by red light instead of microwaves.

micro linac chip

Switching a toothpick-sized micro-linac should be faster still than switching a lunchbox-sized mini-linac. But also, a solid state accelerator, driven by a laser-diode, should produce much less heat. In the way that the solid-state transistor is more efficient and less expensive than a vacuum tube, which made computers more powerful, the micro-linac might play a crucial role in making medical accelerators more powerful.

SLAC’s micro-linac can accelerate electrons to 1-keV, but researchers intend to achieve 1-MeV by the end of 2020. This is nearly the energy of Cobalt-60. Perhaps the toothpick linac will become the basis for the clinical FLASH-RT technology that it seems clear is coming, whatever its form, in the not-too-distant future.

While, it is unclear what has been the impact of clinical ultrasound becoming a cell-phone app available from Amazon, when the x-ray tube shrunk from pounds to grams, it replaced radioisotope brachytherapy. And, this was probably a good thing.

Will there be an intracavitary linac?

Maybe. But it seems unavoidable that, on the path from 1 keV to 1 MeV, will be a device that makes about 100 keV, a diagnostic energy. If it becomes possible to build a solid-state accelerator, operating at diagnostic energies, with a small focal spot; then the diagnostic x-ray tube becomes obsolete.

The Tomotherapy concept was borrowed by radiotherapy from diagnostic radiology. Tomo is essentially a megavoltage CT, in which the dose is the goal and the image is a side-effect. It echoes the kilovoltage CT, in which the image is the goal and the dose is a side-effect.

Perhaps, next time around the inspiration will go the other way, by extending Stanford’s non-rotating treatment concept to a fixed CT imaging device. Electronically switching, in a flash, among micro-sources, fixed around a gantry, interspersed among detectors, and arranged around a full circle could add a whole new dimension to available image data, making possible a much better image for much less dose.

The future could bring interesting times – but, in a good way - for both imaging and treatment.