Many tumors–especially in the brain, head, and neck–can’t be removed surgically because of their location or because they are so diffuse. In such cases, doctors can only provide chemotherapy and hope the tumor shrinks, monitoring changes in its size as months go by. MIT researchers led by Michael Cima, professor of materials science and engineering, hope to avoid that wait.

Cima’s group has created a device smaller than a BB that could be implanted in inoperable tumors during routine biopsies and monitor cancer treatments in real time. It could show if drugs are reaching tumors within days of administration, verify whether they’re working over the course of weeks, and alert doctors if cancer reappears years later.

The device is a silicone rod about eight microliters in volume filled with sensing nanoparticles designed to clump and be­come visible on an MRI scan in the pres­ence of cancer drugs and cancer markers such as growth hormones. A core of iron oxide, a good contrast agent for MRI scans, is coated with the carbohydrate dextran, to which the researchers attach antibodies for whatever molecule in a tumor they want to detect. Pores in the rod let proteins and other compounds in the blood enter but don’t let the nanoparticles out. When the target molecule enters, several nanoparticles will attach to it; the resulting clump appears as a dark speck on an MRI. A monitor that detects one compound is in preclinical tests in mice. Cima’s group is now developing silicone rods with separate compartments for each compound doctors might want to detect.