Evaluating lung cancer response to therapy: Thinking beyond RECIST

LAS VEGAS—CT multitasks in the lungs, serving as a tool for cancer screening, disease diagnosis, lesion characterization, and lung cancer treatment response. CT can be used more effectively to assess treatment response in lung cancer patients, but clinicians must look beyond current response parameters, Michael McNitt-Gray, PhD, said at the 2008 Stanford International Symposium on Multidetector-Row CT.

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

RECIST, which relies on changes to the unidimensional diameter of a lesion, does offer some benefits. It’s easy to implement and can be used to measure up to five lesions at a time. Measurements can be done directly on the film. Dr. 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?”

Other parameters important

While tumor size is important, Dr. McNitt-Gray suggested other parameters for measuring response: mass, density, shape, and function. 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, Dr. 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?” Dr. McNitt-Gray asked. “And how long would it take to see this minimally detectable change?”

In a separate interview, Claus Peter Heussel, MD, agreed with Dr. McNitt-Gray that RECIST does not tell the whole story. Dr. Heussel, of the Department of Diagnostic and Interventional Radiology, Thoraxklinik am Universitätsklinikum, Heidelberg, Germany, told Oncology News International that tumor size alone may have limited value and suggested also looking at volume and perfusion.

Dr. Heussel led a study that compared RECIST and World Health Organization response criteria to CT-generated volumetry data in liver malignoma. The researchers found that the estimation of the relative tumor development was different based on the measurement criteria used.

“Relatively reliable tumor response evaluation can only take place by means of thin-section [CT] investigation and volumetric analysis,” Dr. Heussel’s group wrote (RöFo 179:958-964, 2007).