CT multitasks in the lungs, serving as a tool for cancer screening, disease diagnosis, lesion characterization, and lung cancer treatment response. In a talk at the 2008 Stanford International Symposium on Multidetector-Row CT in Las Vegas, Michael McNitt-Gray, Ph.D., posited that CT can be used more effectively to assess treatment response in lung cancer patients, but clinicians must look beyond current response parameters.

The Response Evaluation Criteria in Solid Tumors (RECIST) is the standard for treatment evaluation, said McNitt-Gray, director of the Biomedical Physics Graduate Program at the Thoracic Imaging Research Group, based at the University of California, Los Angeles.

RECIST relies on changes to the interdimensional diameter of a lesion. Measurements can be taken on five lesions at a time and done directly on the film. McNitt-Gray pointed out several issues, however, that compromise RECIST's reliability to offer the most accurate information.

"It's difficult to estimate the diameter of irregular lesions. How do we know that the (diameter) change we're seeing is real change and not just measurement variability?" he said. "When a patient comes back for follow-up, he or she isn't always imaged in the same scan plane. There is also intra- and interobserver variability. How long do we have to wait for a response— a few weeks, a month?"

While tumor size is important, McNitt-Gray suggested other parameters for measuring response. Changes in tumor volume can be assessed with segmentation on thin-slice CT, he said. PET/CT has value for measuring tumor function.

At his institution, McNitt-Gray performs 4D mapping of contrast enhancement through the nodule. These contrast uptake patterns can be measured at multiple time points with low-dose CT, offering rapid information on nodule perfusion and permeability, he said.

"Ultimately, what is the minimum amount of change that we can detect?" McNitt-Gray said. "And how long would it take to see this minimally detectable change?"