Wireless prototype tracks tumors during radiation therapy

Patient movement, even simple respiration, can cause tumors to shift during radiation therapy, sending less radiation to the tumor and more to collateral tissues. Engineers at Purdue University are creating a wireless device that can be implanted in tumors to provide an exact fix on their location and measure precisely the dose administered.

Ultimately, the device will be the size of a grain of rice. The prototype being tested at Purdue's Birck Nanotechnology Center is still about the size of a dime. It is small enough to test the concept, however.

Babak Ziaie, Ph.D., an associate professor in the Purdue School of Electrical and Computer Engineering, expects to have a miniature version completed by the end of summer.

The device, a passive wireless transponder, will have no batteries. Rather, it will be driven by energy transmitted by electrical coils placed next to the patient.

"It will be like a capsule placed into the tumor with a needle," said Ziaie, who is working with colleagues at Purdue and the Indiana University School of Medicine.

Preliminary results were detailed in a paper that appeared earlier this year in proceedings of the 19th Institute of Electrical and Electronics Engineers International Conference on Micro Electro Mechanical Systems. Doctoral student Chulwoo Son coauthored the paper with Ziaie.

Although imaging systems can provide a 3D fix on a tumor's shifting position during therapy, they are not easy to use during radiation therapy. The wireless technology could precisely track a tumor by using three to six coils placed around the body to pinpoint the location of the electronic device, Ziaie said.

The prototype contains a miniature version of the dosimetry devices commonly used in radiology departments to provide up-to-date information about the cumulative dose a tumor is receiving over time.

"It's basically like a very small tuning circuit in your radio," Ziaie said. "This will be a radiation dosimeter plus a tracking device in the same capsule. It will be hermetically sealed so that it will not have to be removed from the body."

The device, an example of a microelectromechanical system, operates on the same principle as electret microphones found in consumer electronics stores, which contain a membrane that vibrates in response to sound waves. Between the membrane and a metal plate is an air gap that serves as a capacitor to store electricity. As the membrane vibrates, the size of the air gap changes slightly, increasing and decreasing the capacitance and altering the flow of electric current through the circuit. This creates a signal that transmits information stored in the dosimeter. Rather than sound waves, however, the device developed at Purdue is sensitive to high-energy radiation.