Dig into the history of nuclear medicine and you’ll discover some surprising facts. For example, did you know that PET scanners look inside the patient's body for matter-antimatter annihilation events? When a positron emitted by a radioactive tracer encounters an electron, the particles literally destroy each other, leaving only a pair of gamma rays flying off in opposite directions. But that’s jumping ahead in the story. It all started when Henri Becquerel discovered radioactivity by accident. Then Ernest Rutherford acquired basic knowledge about the structure of atoms through incredibly simple “table top” experiments—the likes of which we may never see again. And Marie Curie earned Nobel Prizes in two different scientific fields for, among other things, extracting a tenth of a gram of radium chloride from a ton of pitchblende.
Interesting things also happened when nuclear physics met biology. In 1913, George de Hevesy showed that radioactive tracers can be used to track specific molecules absorbed by plants. Hermann Blumgart took that idea a step further in the 1920s, applying radioactive tracers to medical research. By the 1940s, doctors successfully treated thyroid cancer with what they dubbed an “atomic cocktail.”
Nuclear imaging took off with the development of single photon emission computed tomography (SPECT) and positron emission tomography (PET). Despite their formidable names, SPECT and PET are simply two different ways of displaying metabolic activity—useful for detecting cancers (which are glucose gluttons) and assessing the effectiveness of various treatments. Which one is more appropriate depends on the application. A CT scan can show that there is a tumor; a SPECT or PET scan can show what the tumor is doing. Combine SPECT or PET with CT, and you can match the metabolic activity map to the patient’s anatomy with precision.
Radiation therapy is another important application—crucial to treating many types of cancer. A common trick of the trade is to aim multiple, low-level radiation beams coming from different directions at the target. Only tissue at the point of intersection is destroyed by systems such as the Gamma Knife and Cyberknife. Proton therapy adds another twist, taking advantage of the Bragg peak—the tendency of protons to unload most of their energy in the last few millimeters of travel. The major drawback is that proton therapy requires large and very expensive particle accelerators. Again, the best approach depends on the specific application.
Progress in treating HIV suggests that keeping diseases at bay for long periods may sometimes be more practical than hunting for cures. Perhaps radiation therapy can be improved to the point that it enables physicians to contain most cancers. A combination of nanotechnology and radioactivity may be the key, making it possible to track down and kill individual cancer cells.
There is one other exciting application for radioactivity in medicine—if we can muster the courage to embrace it. The energy locked in atoms can be used to power implantable medical devices for ten years—far longer than chemical batteries. Nuclear-powered pacemakers were introduced at one time, but people freaked out over the Chernobyl and Three Mile Island accidents, and the products were discontinued.
All in all, nuclear medicine has experienced its ups and downs, but it has not lost its glow.
Next time: Sensing Health
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