(HealthDay News) -- The introduction of filtered and low-tar cigarettes in the 1950s coincided with a steady rise in the incidence of a once-rare type of lung cancer that's now the most common form of the disease, a new study finds.
Decades ago, squamous cell carcinoma was the most common form of lung cancer. But between 1950 and 2007, adenocarcinoma became the most frequently diagnosed lung malignancy, as the market share of filtered cigarettes soared from just 1 percent to almost 100 percent, the study authors said.
Described as a "correlation of evidence," the apparent link was uncovered by study author Dr. Gary M. Strauss, medical director of the lung cancer program at Tufts-New England Medical Center in Boston. He presented the findings Wednesday at the 12th World Conference on Lung Cancer, in Seoul, South Korea.
Strauss and his colleagues suggest that the impact of filtered cigarettes on adenocarcinoma rates is due to the introduction of filter vents in filtered cigarettes, making it easier to draw in smoke. These vents allow smokers to take bigger and deeper puffs, thereby inhaling carcinogens further into the bronchial passages and lungs.
"The rise of adenocarcinoma is consistent with changes in cigarette design and composition -- which the cigarette industry indicated were safer -- that they introduced in response to mounting evidence that smoking causes other forms of lung cancer," Strauss said.
"And so the point is that the tobacco industry, through how they changed the cigarette over time and deceived the public for decades about its safety, has created an epidemic," he added.
Philip Morris USA's media affairs manager, David Sutton, said he could not comment on the findings. "We cannot comment on a study we have not had a chance to review. Smoking is addictive and causes serious diseases. There is no such thing as a safe cigarette," he said.
To explain the dramatic rise in diagnoses of adenocarcinoma, Strauss and his team of U.S, researchers first analyzed data concerning cancer rates that had been collected between 1975 and 2003 through the National Cancer Institute's "Surveillance Epidemiology and End Results" (SEER) program.
The study authors focused on information covering more than 307,000 black and white lung cancer patients, 75 percent of whom were 60 or older at the time of their diagnosis. And they focused on six time periods: 1975-1979, 1980-84, 1985-89, 1990-94, 1995-99 and 2000-03.
Statistics on four major types of lung cancer -- adenocarcinoma, squamous cell carcinoma, large cell carcinoma, and small cell carcinoma -- were tallied to reveal how common each disease had been at the six different time periods. The first three cancers fall into the "non-small cell" category of lung disease that accounts for about 85 percent of all lung cancers in the United States, according to the American Cancer Society.
The SEER figures showed that by the years 2000 to 2003, 47 percent of all lung cancers were adenocarcinoma, Strauss and his team observed.
While American Cancer Society numbers currently place adenocarcinoma at 40 percent of all cases, both ACS and SEER data confirm that adenocarcinoma is by far the most prevalent form of lung cancer today --- regardless of race, age and gender.
In 1950, adenocarcinoma constituted just 5 percent of all lung cancer cases, and a diagnosis of the disease was not typically considered to be due to cigarette smoking. Back in the mid-20th century, most lung cancer cases were squamous cell, the researchers said.
But the SEER data illustrate a sharp rise in adenocarcinoma cases beginning in the 1960s. And from the 1975-79 period to the 1995-99 period, adenocarcinoma cases skyrocketed 62 percent.
Adenocarcinoma surpassed squamous cell carcinoma as the most common form of lung cancer among women in the 1975-79 period and among men during the 1980-84 period.
Because the SEER database did not collect information on smoking demographics, the researchers said they subsequently sifted through a wide range of additional data covering 50 years of U.S. cigarette production and consumer habits in search of an underlying explanation.
Strauss and his colleagues said they found that the wide-scale adoption of filtered and low-tar cigarettes closely tracked the jump in adenocarcinoma rates.
Filtered cigarettes went from 1 percent of the U.S. market in 1950 to 64 percent by 1964. By 1986, filtered cigarettes had captured 95 percent of the market; by 2007 that figure was 98 percent.
"And while adenocarcinoma of the lung has always existed, it is now the most common form of lung cancer, and probably the second most common cause of cancer death," said Strauss. "Probably more people die specifically of smoking-related adenocarcinoma today than die of colon cancer."
"So while nothing is really new here, we're putting it all together," he said. And what emerges, he added, is the story of a tobacco industry that years back actively changed its product to minimize its known connection to certain types of cancers, thereby giving birth to a whole new carcinogenic threat and an even bigger lung cancer killer.
"And so now I'm hoping that there will be a recognition that the tobacco industry actually created this deadly epidemic of smoking-related adenocarcinoma through decades of deception," Strauss said.
The results of several other international studies were also presented this week at the South Korea conference, including a Norwegian finding that hand-rolled cigarettes are more carcinogenic than pre-packaged cigarettes, resulting in a higher risk for lung cancer.
Another study, out of Japan, showed that people with a family history of lung cancer are more likely to develop the disease -- particularly squamous cell disease -- later in life. However, a general family history of cancer was not associated with an increased risk for lung cancer.
For more on lung cancer, visit the American Cancer Society.