The Way I See It: A burning question

The Way I See It
A burning question

BY JERRY VERA

Have you ever used a microwave to cook a TV dinner, only to discover the outside was blazing hot and the inside was still frozen?

As odd as it sounds, scientists encounter a similar problem when using lasers to heat and destroy tumors in the body in a new form of cancer therapy called “nanoparticle-assisted hyperthermia.”

	JEFF FITLOW
	Yildiz Bayazitoglu, left, Rice’s H.S. Cameron Professor in Mechanical Engineering, and doctoral student Jerry Vera have published a paper on their efforts to refine the use of lasers and nanoparticles to kill cancer by heating them.

But the heat required to kill a tumor is not quite as high as that required to cook a Salisbury steak. Just a few degrees above body temperature will cause cell death, and nanoparticles and lasers are able to do just that.

To understand how this works, you have to understand how electromagnetic waves interact with human tissue.

Whether we notice it or not, every single tissue in a human body, to some degree, allows electromagnetic radiation to pass through. Radio waves flow through the walls in our houses and pass through our bodies every second.

X-rays pass through to produce images on a film, but unlike radio waves, the electromagnetic radiation from X-rays gets absorbed by bones and doesn’t reach the film. That’s why bones appear as dark shadows to reveal a skeletal structure on the film.

The radiation is absorbed because bones have a higher density than most organs. In fact, every organ in the body is slightly different, and as a result, each exhibits unique optical properties depending on the type of electromagnetic radiation it is exposed to. Most of the light from a near-infrared laser is allowed to pass unabsorbed through most organs.

This is where nanoparticles become handy. Various kinds of nanoparticles, such as nanoshells, can be specially engineered to absorb light of a particular wavelength. If the light is intense enough, absorption will generate heat. If nanoparticles designed to absorb near-infrared light are dispersed in a tumor, and subsequently the tumor is exposed to a near-infrared laser, one can generate localized heat around the nanoshell-embedded region and kill the tumor. Because most of the body allows near-infrared light to pass without absorption, this technique is preferable to conventional radiation therapy, which harms all the cells in its path.

But there’s a problem. Like that TV dinner in the microwave, raising the temperature of a tumor with a laser doesn’t necessarily produce an evenly heated tumor.

Think about it this way: On a very foggy night, a driver can see only just so far no matter how good the car’s headlights are. That’s because the millions of small water droplets in the air absorb and scatter the light, deflecting the beams from the headlights before they can reflect off of whatever’s on the road ahead.

Nanoparticles dispersed within a tumor do exactly the same thing. They’re very good at absorbing laser light and generating heat, but within particularly thick tumors, that same quality prevents a lot of the light from reaching deeper into the tissue.

This phenomenon is called “extinction.” It’s highly undesirable because one needs to create a uniform temperature profile of at least 60 degrees Celsius to kill the whole tumor. Raising the temperature on one end but not the other will simply allow the tumor to regrow, and that doesn’t solve the problem — or cure the patient.

Other Rice engineers and I are tackling this issue through computer simulations by using optics equations once employed to study starlight passing through clouds in space. Our group, headed by Yildiz Bayazitoglu, Rice’s H.S. Cameron Professor in Mechanical Engineering, published a paper titled ”Gold Nanoshell Density Variation with Laser Power for Induced Hyperthermia” in the January issue of the International Journal of Heat and Mass Transfer.

We’re working on simulations to accurately predict the intensity of a laser beam passing across a tissue laden with millions of nanoshells, mimicking possible therapies. Because tissues differ slightly, the models can be customized, depending on where in the body the tumor is located. The models also allow researchers to study the effect of adding different concentrations of nanoparticles to the mix. If too many are added, the absorption on one end may be too high. Too few, and the treatment may be ineffective.

Our group is also studying the effects of using stronger or weaker lasers. Contrary to reason, stronger lasers are not necessarily better at heating thick tumors. Simulations show that turning up the heat does not always produce a ”well-done” tumor. It might simply overheat the tumor’s surface and little beyond.

A better option is to use a slightly weaker laser and run it longer to let the heat pass into the tissue through conduction. This delicate balancing act between laser power and particle concentration depends on both the size of the tumor and the target tissue’s ability to conduct heat.

For tumors as large as one centimeter, an option may be using two opposing lasers, surgically inserted via fiber optics in a minimally invasive procedure. Simulations have shown this produces the most uniform temperature profile in every case.

With so many tissue types and the great variety of cancers people face, the importance of accurate simulations cannot be overemphasized. The ability to calculate scenarios allows us and, ultimately, doctors in the clinic to find the best laser therapy to produce the perfect heating environment. We hope nanoparticle-assisted therapy will become a standard procedure available to us all.

Then, maybe, we’ll work on that TV dinner problem.

— Jerry Vera is a doctoral student studying at Rice on a grant from the Alliances for Graduate Education and the Professoriate via the National Science Foundation.