IASLC: Sputum Test Promising for Preliminary Lung Cancer Screening

SEOUL, South Korea, Sept. 6 -- Sputum testing for genetic methylation abnormalities may detect half of lung cancers, suggesting it could serve as an inexpensive and noninvasive adjunct for lung cancer screening or diagnosis.

When the two-genetic-marker test was pitted against blinded sputum samples from known lung cancer patients and controls, it yielded 50% sensitivity and 93% specificity in a study reported here at the International Association for the Study of Lung Cancer's world conference.

Using only a single methylation marker called RASSF1A, the test had 42% sensitivity and 95% specificity, Erik Thunnissen, M.D., Ph.D., of VUMC Pathology in Amsterdam, and colleagues found.

The test's high specificity suggests it could allow some patients to avoid bronchoscopy for diagnosis, Dr. Thunnissen said.

For screening, computed tomography (CT) imaging has shown better sensitivity and is more widely available, he noted. In the International Early Lung Cancer Action Project (I-ELCAP) study, CT screening was shown to have 85% specificity for lung cancer while in another study it showed 90% specificity.

CT screening is still in development with reports from large trials expected in the next year or two, "but we're always looking for a quick method or test to come in before you have to have a CT scan," commented James F. Bishop, M.D., of the University of Sydney in Sydney, Australia, and a moderator at the session in which the study was presented.

So, despite the sputum methylation test's relatively low sensitivity, "it might be a quick method that is noninvasive, doesn't require tissue, and, therefore, what we are looking for as an effective adjunct to screening tests now," Dr. Bishop said.

"Sputum's a very good thing to work with because it might be able to screen large populations, particularly in developing countries that can't afford to do a CT scan on everybody," he added.

Methylation has been found to be important in some genetic changes that lead to lung cancer, Dr. Bishop noted. "The difficulty is really just to get the right methylation markers."

Sputum testing was used to detect lung cancer in the past, but fell out of favor as more sensitive tests came along, Dr. Thunnissen said.

"They used to use it for cytology -- only looking at cells," he said. "The new way is looking at genetics."

Dr. Thunnissen's group retrospectively evaluated genetic methylation markers in prospectively collected sputum samples from 102 patients within six months of lung cancer diagnosis and 102 controls who did not have lung cancer seen at the Canisius Wilhelmina Hospital in Nijmegen. Most of the controls had COPD greater than GOLD stage 0.

Samples were randomized and blinded then tested by quantitative methylation specific polymerase chain reaction for the following markers: APC, cytoglobin, MGMT, 3-OST-2, p16, RASSF1A, and TCF21. Dithiothreitol (DTT) was used to dissolve the mucus and make DNA easier to extract.

The marker with the best area under the curve for sensitivity and specificity was RASSF1A, a gene related to apoptosis and cell cycle arrest. The sensitivity in correctly identifying lung cancer patients was 36% when 100% specificity was chosen but 42% when specificity was set at 95%.

With RASSF1A plus any other marker, the sensitivity increased to 50% while specificity decreased slightly to 93%.

Sputum cytology was positive in 19% of lung cancer cases. So while methylation was more sensitive than cytology, together they were 60% sensitive for lung cancer.

Methylation was seen in sputum samples for patients with stage I and II lung cancer. Samples from some controls were also positive for methylation, which Dr. Thunnissen said might have been because of inflammation-related halogenation cytosines.

Dr. Thunnissen said RASSF1A "is a late marker in carcinogenesis compared to other methylation markers," which makes it better for diagnosis of lung cancer than for evaluation of lung cancer risk.

"If you want to make a diagnostic test, go for late markers," he said.

Further studies of the sputum methylation test are needed to see how it measures up in the population at risk, namely heavy smokers, Dr. Thunnissen concluded.