(Nov. 06, 2010) CT scans may not just detect cancer, they may actually prevent it.
‘This is the first time that we have seen clear evidence of a significant reduction in lung cancer mortality with a screening test,” said Dr. Christine Berg of the U.S. National Cancer Institute, referring to the CT scans that led to a 20% decline in cancer deaths in the largest-ever study of its kind.
“Really stunning,” said David Naidich, professor of radiology and medicine at NYU-Langone Medical Center, a member of the study’s executive oversight committee.
“Huge,” said Regina Vidaver, executive director of the National Lung Cancer Partnership, an advocacy organization. “To me this is a game changer.”
In fact, the stunning results could be even huger than these medical experts realize. CT scans (also known as computed tomography or CAT scans) not only catch tumours early, allowing for early treatment; CT scans may prevent tumours as well.
The $250-million study, jointly conducted by the National Cancer Institute and the American College of Radiology Imaging Network, involved more than 53,000 participants between the ages of 55 and 74 who had smoked a minimum of 30 “pack-years” (say, one pack a day for 30 years or two packs a day for 15 years). The study included ex-smokers who had quit within the previous 15 years and excluded all those who had had cancer in the previous five years, or who currently had any trace of cancer (except some skin cancers).
In the study, called the National Lung Screening Trial, over the course of two years half of the participants received three standard chest X-rays and the other half received three low-dose CT scans to detect tumours. The participants were then followed for up to five years.
The result? Those who received standard chest X-rays suffered 442 lung cancer deaths compared with 354 lung cancer deaths among those who received CT scans. This 20% difference is so robust — both in the number of lives that can potentially be saved in future and in the statistical likelihood that the results are valid — that the researchers stopped the study in order to publicize the results.
Why are CT scans so effective at reducing lung cancers, the biggest cancer killer of them all, with some 1.2 million deaths worldwide, 175,000 of them in North America? The study’s researchers don’t know. Adding to the mystery was their expectation that the low-dose CT scans, which nevertheless deliver a radiation dose 20 times greater than that from chest X-rays — could cause additional cancers. More mystery: The vast majority of the suspected cancers caught by the CT screens proved to be false, leading to unnecessary surgery and some deaths. Yet despite these additional deaths that stemmed from the inaccurate CT scans, more lives were ultimately saved than lost. More mystery still: The lives saved by CT scans extended beyond lung cancers. Over the scant few years that the study was in force, the death toll from all causes was 7% lower. Through what mechanism could looking for cancer tumours in the lung lead to fewer cardiovascular or other deaths?
The researchers will now turn their talents to investigating where and why CT scans proved so effective at saving lives, despite the many knocks against CT scans and radiation. They will investigate whether CT scans especially benefit heavier smokers, or past smokers, whether it will be cost-effective for the medicare system to pay for preventative CT scans, whether the deaths and discomfort caused by unneeded surgery can be minimized, whether the cumulative risk of exposing a body to too much radiation from CT scans poses a problem. One thing they do not plan to investigate — but should — is the possibility that the very radiation from the CT scans themselves was a lifesaver, that radiation has a prophylactic effect.
“Numerous papers have been published related to low-dose radiation stimulating immunity against cancer cells,” state Bobby R. Scott et al. in “CT scans may reduce rather than increase the risk of cancer,” a peer-reviewed paper published two years ago in the Journal of American Physicians and Surgeons. Among other analyses, the paper included a description of a study of 1,877 rats exposed to radiation at levels thought to be cancer-inducing. Contrary to expectation, none of the 1,877 rats developed lung cancer.
In concluding their paper, which was written to counter an earlier study predicting that as many as 2% of all cancer deaths would soon arise from CT scans, Scott et al. state “There is no credible evidence to support the contention that current routine usage of CT scans in clinical settings in the United States will cause future cancers. Rather, the available data indicate that occasional exposure to diagnostic X-rays could possibly reduce the risk of future cancers among irradiated adults.”
In fact, Scott et al. say, no proof exists that CT scans cause harm. The belief among many in the medical community that radiation from CT scans causes cancer stems entirely from an unproven assumption that there is no safe level of radiation. Not just unproven but unprovable, says this school of thought as well as its critics, because the risks from low levels of radiation are too low to measure statistically. Since these risks can’t be measured, this school concludes, the only prudent course is to assume harm from CT scans and other sources of low-level radiation.
But although the potential risks are too small to be measured, the potential benefits can easily be measured, and have been over the decades — studies around the world provide impressive indications that low levels of radiation can be healthful. This competing school of thought, called radiation hormesis, holds that radiation and other substances that kill in high doses can cure at low dose. Although unknown to the general public, the hormesis theory has rapidly been gaining credibility in scientific circles — hundreds of peer-reviewed papers on hormesis are published each year, and many universities now offer courses that deal with hormesis.
The National Cancer Institute study offers a striking opportunity to investigate whether CT scans saved lives that standard chest X-rays did not because X-rays don’t deliver as much of a life-saving dose of radiation. If the researchers do investigate, their findings could truly be a game changer.