Minibeams may minimize damage in cancer treatment

The goal of radiation therapy is to deliver a fatal dose to a tumor while sparing the surrounding tissue. One advantage of using high-energy x rays is that they spare the first couple of centimeters of tissue they pass through, a region that is often clinically significant but radiosensitive. But then the dose—the energy deposited per unit mass—rapidly builds, and it can damage healthy regions both upstream and downstream of the target. Proton beams, in contrast, deliver most of their dose in a confined region, at a depth that depends on the beam energy (see the article by Michael Goitein, Tony Lomax, and Eros Pedroni, Physics Today, September 2002, page 45 ). Attaining full coverage of the tumor, though, can cause excess exposure of shallow tissues. That damage is of particular concern for pediatric brain tumors, since it can affect neurological and cognitive development. Now Avraham Dilmanian (Stony Brook University), John Eley (now at the University of Maryland), and Sunil Krishnan (MD Anderson Cancer Center) show that using a collimator to break up a particle beam into multiple parallel planar or pencil-shaped “minibeams,” only 0.3 mm in size, can significantly spare shallow tissue: Because of the small irradiated volume, the tissue can both tolerate high doses and begin repairs promptly. As the minibeams penetrate, they gradually broaden and reunite, as seen in this simulated dose map. Through simulation and experiments on tissue surrogates, the researchers find that proton minibeams can stay safely small to depths of about 25 mm; helium and lithium minibeams, even further. Those depths suffice to spare much of the cerebral cortex. Moreover, say the researchers, the collimation is straightforward to implement in current treatment facilities. (F. A. Dilmanian, J. G. Eley, S. Krishnan, Int. J. Radiation Oncol. Biol. Phys., in press, doi:10.1016/j.ijrobp.2015.01.018 .)