St. Louis, — Physicians at Washington University School of Medicine and the University’s Alvin J. Siteman Cancer Center in St. Louis have found evidence that positron emission tomography (PET) is more accurate than the current standard, computed tomography (CT), in determining whether cervical cancer has spread to other areas of the body. Their results are published in the Sept. 1 issue of the Journal of Clinical Oncology.
“Our study shows that PET is the most accurate imaging method presently available for identifying secondary tumors in patients with cervical cancer,” says Perry Grigsby, M.D., professor of radiation oncology at the School’s Mallinckrodt Institute of Radiology and first author of the study.
Knowing whether a tumor has spread to the lymph nodes is essential for determining the most appropriate treatment for a patient. Tumors confined to the wall of the uterus are treated by surgically removing the uterus and the woman has a 90 percent chance of being cancer-free five years later. Once the tumor has spread, however, radiation therapy without surgery is administered and the odds of survival drop to 45 percent.
At present, physicians use CT to assess the extent of cervical cancer. But CT scans are only moderately accurate. They often suggest that a cervical tumor has not spread to lymph nodes when in fact it has. Grigsby, along with colleagues Barry Siegel and Farrokh Dehdashti of the Division of Nuclear Medicine, compared CT scans with PET scans in 101 women with cervical tumors detected during a physical examination. They took images of the cervical tumor itself (the primary tumor) and of lymph nodes in three areas of the body: the pelvis, the abdomen around the aorta (para-aortic lymph nodes) and the base of the neck above the collar bone (supraclavicular lymph nodes). These areas follow the path taken by cervical cancer as it advances.
PET scans confirmed the presence of a cervical tumor in 100 of the 101 women, while CT scans identified only 77. In the lymph nodes, PET revealed abnormal pelvic nodes in 67 of the women, while CT found 20; PET revealed abnormal abdominal nodes in 21 women, whereas CT found them in 7 women; PET identified abnormal supraclavicular nodes in 8 women, whereas CT found none. All abnormalities detected by CT also were detected by PET. “We were surprised to find that these patients had more traces of tumor in their lymph nodes than we previously suspected,” says Grigsby. “These findings are so significant that we have stopped using CT scans for tumor evaluation in patients with cervical cancer.”
In fact, he says, “PET findings altered treatment for about half the women in the study.” It helped some avoid unnecessary surgery by revealing that their cancer was more advanced than first thought. In others, PET helped better define the radiation dose, the size of the area requiring irradiation, or both. “Based on our results,” says Grigsby, “we believe that clinical trials comparing treatments for cervical cancer should use PET to more accurately determine what treatment each patient should receive.”
PET’s superiority in detecting the spread of cervical cancer is due to a fundamental difference in the two technologies. CT scans produce an x-ray image of a thin cross-section of the body. To diagnose the spread of cervical cancer, radiologists study the images for abnormally enlarged lymph nodes, those greater than 1 centimeter in diameter. Lymph nodes are likely to be enlarged because they contain cancer cells that have spread from the primary tumor. “The problem is that lymph nodes that are not enlarged also can have tumor cells,” says Grigsby, “and these nodes will go undetected in CT scans.”
PET scans, on the other hand, reveal a metabolic—rather than anatomical—difference between cells and structures. In this study, PET detected the difference in the way cells consume glucose, a simple sugar required by cells to live and grow. Cervical cancer cells grow at a faster rate and therefore have a higher rate of glucose consumption than the normal cells making up the lymph node. Thus, PET can often identify the presence of cancer cells in normal-sized lymph nodes.
PET, however, does have a limit of detection, and it will miss tumor-containing lymph nodes if the number of cancer cells is too low. PET scans also provide images of the entire body, says Grigsby. “A patient’s PET scan shows up on a computer monitor. It shows an anatomical outline of the patient in 3-D, and the image is slowly rotating. I can see the cancer that I found during the pelvic exam, and I can see the abnormal lymph nodes that I wouldn’t have known existed without the PET scan. And that means I can more accurately, more effectively, deliver that patient’s treatment.”
Commenting on the study, David Mutch, M.D., the Ira and Judy Gall Professor of Gynecologic Oncology and head of the Division of Gynecologic Oncology, said: “This gives us another tool to evaluate whether a patient has metastatic disease before we operate, and it can help us avoid surgery if it is not appropriate. It’s still not 100 percent accurate, but it’s the best tool available to help in determining which patients need surgery and which patients need radiation therapy.”
Journal
Journal of Clinical Oncology