SDT- Advanced

Sonodynamic therapy for the treatment of cancer appears to be both art and science. The art is that ultrasound, used to activated a sonodynamic “sensitizer” does not possess enough energy to “excite” electrons in a molecule to a new, higher energy state, where they are free to do “work.”

For example, visible light does have sufficient energy to promote an electron to an excited state where it can (potentially) form free radicals and other reactive species. Look upon this process is lifting a stone off the ground and placing it on a higher platform. The stone can then be used to do work when it returns to the ground in any number of ways.

Now picture that, when you lift the stone, you can only place it on steps. That is, there are no levels in between the steps upon which you can place the stone. Well, in the realm of energy, ultrasound does not have enough energy to move an electron (analogous to the stone) to even the first step whereas light can lift it to the first, or maybe even higher steps.

How does sonodynamic sensitizers produce a free radical (an electron on a higher step)? The answer most likely is the result of two very special and unique things:

1. Ultrasound can create cavitation – a rapidly expanding then collapsing bubble. This localized phenomenon creates high temperatures and pressures that is very short lived. However, during the fraction of a second that the cavitation occurs, a photon is emitted.

2. The sonosensitizer had an abnormally high sensitivity for photons (little packets of light), particularly the light emitted through the cavitation process.

Thus the sensitizer absorbs the photon produced by the ultrasound cavitation process (the stone is raised up a step). The result is an energized electron that is able to do work, like, for example, destroy a cancer cell membrane to which it has adhered.

Maybe SDT is more science than art after all, but we still do not know for sure.

For those interested in more details about sonodynamic therapy for cancer, please read the attached article produced by a bunch of brilliant scientists at the National Institutes of Health who published a detailed paper on sonodynamic therapy.