How do you track lung tumor motion? A critical question with competing answers.

Martin Fuss, M.D. , remembers the old days when radiologists treated a large area to get at a small tumor. Back then, the bone-encased male prostate seemed like an immovable object, recalled Fuss, who directs the imageguided radiation therapy department at the Oregon Health Sciences University in Portland. But advanced imaging technology now makes it possible to spot the slightest movement, he said. “We’ve found that even the prostate can move.” These slight movements are critical to radiation therapists who can now take precise aim at tumors, sparing surrounding tissues. “Because we are now able to zero in very tightly on a tumor,” Fuss said, “motion is so much more important.” And nowhere more important than in the lung, where every breath can move a tumor. In radiation therapy, tracking lung tumor motion has become a hot area of research. Implanted devices are one way to track motion, but they require surgery and have been associated with a 20% – 50% greater risk of pneumothorax, or air in the pleural cavity, which can collapse or lead to infection of an already embattled lung. A better way, according to some experts, is to use anatomic surrogates — organs or structures, usually near the tumor, whose movements closely track tumor motion. Tracking tumor motion during radiation therapy is “one of the most important things we do,” said Stephen Feigenberg, M.D., a radiation oncologist at Fox Chase Cancer Center in Philadelphia. “Everyone is trying to fi gure out how to do it without implanted tracking devices. It would be wonderful to come up with a noninvasive approach, and that’s the appeal of anatomic surrogates.” Recently, two groups proposed different anatomic surrogates for lung tumor motion — the diaphragm and the carina, a cartilaginous ridge that divides the trachea into the primary bronchi. Their latest studies have ignited debate over which surrogate works best. In June, Steve Jiang, Ph.D., Laura Servino, Ph.D., and fellow researchers from the Center for Advanced Radiotherapy Technologies at the University of California, San Diego, published the results of a 2-year study that favors the diaphragm as a lung tumor motion surrogate. Their fi ndings appeared in the journal Physics in Medicine and Biology . In July 2008, radiation oncologists Suresh Senan, Ph.D., and Lineke van der Weide, heading a seven-person team from Amsterdam’s VU University Medical Center, published a study supporting the carina as anatomic surrogate in the International Journal of Radiation Oncology and Biological Physics . They confi rmed and extended those results in the same journal this June and presented their fi ndings, with some new caveats, at a meeting in early September.