In early November the media carried an announcement concerning this trial sponsored by the National Cancer Institute. The cohort consisted of heavy smokers ages 55 to 74 years. The trial compared chest X-rays with CT scans, using death from lung cancer as the primary endpoint. The cohort was randomly divided into two groups, each of which received three scans over two years and were followed for up to 5 years.
The Institute considered the results so important that they provided early results, not in a paper, but via the media which provided widespread coverage. Those who received the standard X-ray screening suffered 442 lung cancer deaths compared to 354 among those who received CT scans. All cause mortality was also lower in those receiving CT scans. The majority of the suspected cancers found by CT proved false, resulting in unnecessary invasive procedures and some deaths.
According to the conventional wisdom, the CT scans with 20 times the radiation exposure of X-rays should increase the risk of cancer. In their press release, the NCI took the position that the benefit of potentially finding more treatable cancers using CT scans appears to outweigh the risk from receiving what they describe as low-dose radiation. They also admit that screening with chest X-rays does not reduce lung cancer mortality.
Screening for lung cancer with the modern low-dose spiral computed tomography which was employed in this study is not new but the results so far have been controversial. Until the results of the NCI trial are actually published in a peer reviewed journal, it is not possible to put them is perspective or compare to earlier non-randomized studies. Two of the most recent non-randomized studies came to diametrically opposed conclusions as to the merits of screening. It appears that this type of screening has variable ability to detect very early stage cancer, some of which is indolent, and this impacts the endpoint and results in over diagnosis and overtreatment.
The November 2008 Newsletter contained a Research Review questioning the conventional wisdom that low-dose ionizing radiation such as from CT scans and X-rays was dangerous and pointing out (a) that there was no acceptable evidence of risk and (b) that there was considerable evidence that low dose ionizing radiation conferred benefits of unproven origin but possible associated with enhanced immune systems. The review contained a lengthy description of the apartment house episode where the occupants were unknowingly exposed to low dose radiation over an extended period due to contaminated building materials. It documented their dramatic decrease in cancer incidence. This is called Hormesis and is the subject of a just published scholarly work (Radiation Hormesis and the Linear-No- Threshold Assumption, by Charles L. Sanders) carrying a price tag consistent with medical textbooks (about $200). In 2008, the Journal of American Physicians and Surgeons published a paper titled CT Scans May Reduce Rather than Increase the Risk of Cancer which reviews the existing hormesis literature in the context of CT scans.
The position implied by the title strongly challenges the conventional wisdom. Hormesis may provide a simple, evidence based explanation for at least part of the NCI study’s observed mortality benefits, given that three CT scans were done over 2 years. The belief that CT scans are dangerous in the context of cancer initiation now occupies the position of a super dogma. Every year there are number of papers that employ the linear no- threshold model to come up with dire predictions about the population impact of diagnostic radiation, presenting the results as evidence based when they are not. Such studies discourage individuals from having, for example, so-called coronary artery calcium scans and are responsible for the caution exhibited in guidelines. It seems also clear that the dogma will have a long lifetime. Most in mainstream medicine have probably never heard of hormesis or even thought to question the models used for predicting radiation induced cancers.