When pharmacologists develop anticancer drugs, they need a way of seeing whether the drugs really do shrink and kill tumors. Magnetic resonance imaging can do the job without harming human patients or lab animals, but it's expensive, especially if you need high spatial resolution. Biopsies are cheaper, but they risk interfering with the growth—or, one hopes, the shrinkage—of the tumors under investigation.
Yesterday at SPIE Photonics West, I learned about an optical method for tracking tumors: diffusive optical tomography. In DOT beams of near-IR light are sent through tissue at various angles and detected when they emerge, much depleted by both absorption and scattering. Wavelengths are chosen to reveal differences in concentration of various molecules, including hemoglobin.
Forming a three-dimensional image from those diffuse signals is doubly challenging. First, detecting the signals requires a sensitive instrument. You can't arbitrarily increase the incident intensity to boost the output signal lest tissue be damaged. Second, untangling the paths taken by the light as it makes its way through a scattering medium is a formidable mathematical problem. Those and other challenges have been met. Diffuse optical tomography is now a cheap, fast, and effective imaging modality for use in the lab and the hospital.
In one talk I heard yesterday, Molly Flexman of Columbia University in New York City described how her group is using DOT to monitor the efficacy of bevacizumab, a drug designed to kill tumors by cutting off their ability to grow blood vessels and therefore their ability to obtain nutrients.
Bevacizumab is controversial. Marketed by its maker Genentech under the name Avastin, the drug appears to shrink some tumors, but not others. That known, variable performance suits Flexman because it gives her the opportunity to test whether DOT can indeed monitor the efficacy of any anticancer drug. Moreover, whether or not the drug targets a tumor's blood vessels, DOT can readily detect them thanks to its sensitivity to hemoglobin.
Flexman and her colleagues looked at two types of cancer: Ewing's sarcoma, for which bevacizumab has some beneficial effect, and neuroblastoma, for which bevacizumab is less effective. It turned out that DOT could reveal the difference in how the two cancers responded to the drug.
For her study, Flexman used lab mice. In the introduction to her talk, she explained why she and her colleagues chose to focus on Ewing's sarcoma and neuroblastoma. Both cancers afflict children. Because children are still growing, therapies that attack cells' ability to divide, such as ionizing radiation, have worse side effects for children than for adults.
