Radiation Sensor Fine-Tunes Cancer Treatments

An implantable detector could monitor changes in tumors

A new implantable device promises to offer doctors more precision in treating patients with cancerous tumors. Babak Ziaie, a professor of biomedical engineering at Purdue University, in West Lafayette, Indiana, has created a wireless radiation detector that measures from within a tumor itself how much therapeutic radiation the tumor is getting.

Ziaie designed the tiny dosimeter, which is about 2 centimeters long, to fit inside a hypodermic needle, making it easy to inject the device into the body. The detector is simple--a modified capacitor attached to an inductor, both of which are encased in a glass capsule. His breakthrough was to use micromachining techniques to shrink the device.

About half of all cancer patients in the developed world receive radiation therapy, according to the International Agency for Research on Cancer, in Lyon, France. In one common type of therapy, a cluster of individually sculpted beams of high-energy photons are shot at a prone patient. Before a series of treatments begins, a computer creates models of the patient's body to tailor the paths of the thin beams to match the shape of the cancerous target.

The challenge is knowing exactly where the tumor is, down to a fraction of a millimeter, on any given day. ”When a patient comes in for radiation day in and day out, his prostate may be off by 5 or 10 millimeters from where it was the day before,” says Allan Pantuck, a urologic oncologist at the University of California, Los Angeles. ”So the question is, are you really hitting the target you think you're hitting?”

If an implanted dosimeter indicates that a tumor received less radiation than the treatment plan had specified, the hospital staff could revise the patient's therapy, Ziaie says. As a result, the tumor would receive a corrected dosage, and the surrounding tissue--which would presumably absorb the missing radiation--could be spared more damage.

The radiation-sensing capacitor consists of two plates separated by an air gap. The top plate is mounted on a flexible membrane that allows it to move. The bottom plate is fixed in place and consists of a layer of Teflon on top of glass. The Teflon plate forms an electret. Often referred to as the electric equivalents of magnets, electrets are materials that are able to maintain an electric field, sometimes for longer than a decade. The electret's electric field pulls the top capacitor plate toward it.

During a treatment, the radiation ionizes the air between the electrode plates, and the resulting charged particles weaken the electret's electric field. That in turn causes the top plate to move away from the bottom one.

To read the dosimeter, an external antenna delivers an oscillating electric field to the implanted device. The inductor in the device picks up this field and resonates at a frequency proportional to the distance between the plates in the capacitor. Detecting this resonance frequency gives a measure of how much radiation the device has absorbed.

One company, Sicel Technologies, in Morrisville, N.C., has also developed an implantable dosimeter, but it contains a microchip and other circuitry. Ziaie anticipates that his device will be significantly cheaper and easier to manufacture, on the order of US $5 each.

However, not all doctors are convinced that patients need an implanted dosimeter. Generally, doctors treat their patients according to the initial computer simulations rather than measuring the dose each patient is actually receiving. So they don't necessarily know that there may be a problem to fix. Niko Papanikolaou, the director of medical physics at the University of Texas Health Science Center, in San Antonio, tested Sicel's dosimeters in several of his patients. ”All we discovered was that we were doing things right,” he says.

But not everyone does things right. In April 2008, Ottawa Hospital Cancer Centre, in Canada, revealed that over the course of three years 326 cancer patients had received almost 20 percent less radiation than they were prescribed, due to an incorrectly programmed machine.

Whether or not doctors warm up to real-time radiation monitoring with implanted dosimeters remains to be seen. To improve its odds, Ziaie hopes to shrink his device even further, in time to start testing it in pigs by the end of 2008.