How Physics Could Help Cancer Doctors
Medical physicist Robert Jeraj speaks at the American Physical Society March Meeting about emerging trends in cancer imaging and treatment.
More than 120 years ago, Wilhelm Conrad Roentgen sparked a revolutionary new field of science -- medical physics -- when he accidentally discovered X-rays during a cathode ray experiment. Within a year of Roentgen's breakthrough, physicians were using X-rays clinically to image bone fractures and gunshot wounds.
Since then, several significant discoveries in physics -- such as radioactivity, protons, and nuclear magnetic resonance -- have led to continued progress in medical diagnosis and treatment. But what will the future of medical physics look like, particularly when it comes to tackling cancer?
“A few things are on the horizon that are exciting from the physics perspective,” said Robert Jeraj, a professor of medical physics at the University of Wisconsin-Madison. Jeraj, who works on image-guided therapy for patients with cancer, gave a talk on March 8 at the American Physical Society March Meeting in Los Angeles on advances in modeling, imaging and treating cancer.
One way physics is pushing the boundaries of cancer care is through the development of new imaging techniques. For instance, Jeraj mentioned photoacoustic imaging, a hybrid technique based on the photoacoustic effect. A laser beam heats up tissue, causing it to expand and emit a pressure wave, which is then detected by an ultrasound transducer. Photoacoustic imaging of vascular changes in tissue has shown promise in cancer detection, and many clinical trials with the technology are ongoing.
“Another interesting example is magnetic particle imaging, which is a bit different than magnetic resonance imaging,” he said.
While MRI relies on manipulating the nuclear spins of protons in a patient’s body to create an image, magnetic particle imaging uses superparamagnetic ion oxide nanoparticles as an injectable tracer. Compared to protons, the superparamagnetic ion oxide nanoparticles have a 100 million times higher magnetization and a 10,000 times faster relaxation time, Jeraj said. These enhanced properties result in very high resolution images. Currently, the spatial resolution of magnetic particle imaging is approximately 1 mm (akin to a typical research-quality MRI) but is expected to improve to better than 300 microns in the future and with faster acquisition time. The technique's excellent contrast and sensitivity -- without the harms of ionizing radiation -- has led several research groups to test it for the purpose of early-stage cancer detection.
Jeffrey Siewerdsen, a professor of biomedical engineering at Johns Hopkins University in Baltimore who chaired the session, said that while both photoacoustic imaging and magnetic particle imaging are emerging new technologies that aren't necessarily pervasive in medical imaging today, they demonstrate how interesting physics can break new ground.
“We haven't had a major revolution in medical imaging technology for some time, but new modalities allow us to image at new scales -- finer scales or faster scales -- with new contrast mechanisms,” said Siewerdsen. “[Jeraj is] touching on the kind of technologies that physicists are good at and could bring to medical physics.”
New imaging systems can also improve treatment, Jeraj noted in his talk. The newest radiation therapy systems are adding imaging functionality to take the guesswork out of delivering the dose to the right place.
“Accelerators for radiation therapy are now equipped with imaging equipment, which allows for very accurate imaging of the anatomy, a plan for where to deliver the radiation dose, and actually delivering that dose with often submillimeter or millimeter precision,” Jeraj said.
While CT image-guided systems for radiotherapy had been developed 20 years ago, MR image-guided systems like ViewRay and Elekta represent the latest exciting advance, Jeraj said. Since it doesn’t involve harmful radiation exposure, MRI offers the ability to scan the patient many times throughout treatment with the potential for real-time adaptive planning and delivery.
Broadly speaking, Jeraj said the integration of “big science” physics approaches that leverage the fundamental principles of the field are required to push the boundaries of cancer imaging and treatment.
Siewerdsen agrees with Jeraj about the value of a more basic science approach. Medical physics programs at schools tend to be very applied, he said, and don't necessarily go deep into the basic science and engineering at the heart of innovative research.
“Linear accelerators are a common clinical tool in medical physics, but what do the particle beam physicists have to say about creating better treatment systems?” Siewerdsen said. “I think that's especially valuable to the field of medical physics, to get that injection of hard basic science.”