Conventional CT Models Not Adequate for Modeling Lung Tumors During Breathing

Conventional CT scans do not adequately take into account the way lung tumors move as patients breathe, according to a new study presented October 8, 2002, at the American Society for Therapeutic Radiology and Oncology's Annual Meeting in New Orleans.

The accuracy of CT scans of lung tumors is important for physicians planning radiation therapy for cancerous tumors. These physicians use data from the CT scans to determine what dose of radiation different areas will receive, in an attempt to maximize the dose to the tumor and minimize the dose to surrounding normal tissue.

Researchers evaluated 14 patients treated for primary lung cancer. Twelve of the 14 had non-small cell lung cancer. All patients had CT scans at inhale, exhale and free breathing on a fast helical scanner. A physician contoured tumors on each of the scans consistently. A composite tumor volume was created to combine the inhale and exhale tumors. Two methods of expansion were used to compare this volume to the free-breathing tumor. First, the free-breathing data set was expanded uniformly by one centimeter. Next, a non-uniform expansion was generated in all six directions to ensure complete coverage.

The amount of excess normal lung tissue treated with each of these two expansions was measured, and researchers found that there was a significant amount of normal tissue treated with the uniform one centimeter expansion as well as with the non-uniform expansion. In addition, there were also cases where the technique led to marginal miss of the tumor, including one case where 34 percent of the overall tumor was missed with the one centimeter expansion.

"Breathing causes movement and potential shape change in lung tumors that is not fully appreciated using conventional, free-breathing CT models. Traditional models of expanding the tumor volume by one centimeter are less than ideal, and can lead to parts of the tumor being missed and healthy tissue receiving radiation," said Aaron M. Allen, M.D., of the Department of Radiation Oncology at the University of Michigan in Ann Arbor, Mich., and lead author of the study. "New techniques need to be developed that aid radiation oncologists in accurately planning radiation treatments for lung cancer patients."