One of my chief tasks as a primary care physician is preventing illness. Today we can thankfully prevent or at least postpone some of the most common causes of premature death and disability. By identifying and controlling risk factors such as high blood pressure and high cholesterol, we can largely prevent heart attacks and strokes. By controlling diabetes, we can prevent the most common cause of kidney disease. Through vaccines we can lower the risk of suffering hospitalization and death from infectious diseases like flu, covid, and bacterial pneumonia. We can screen for colon, prostate, breast, lung, and cervical cancer and thereby often provide curative treatment. But there are still many conditions we cannot screen for and prevent. These include several cancers such as pancreatic, bladder, kidney, bile duct, and thyroid.

Enter a blood test called Galleri that screens for 50 types of cancer. It does so by looking for cell-free DNA (cfDNA) that cells shed into the bloodstream. Galleri uses genetic sequencing technology to scan for changes in cfDNA that come from cancer cells. In a study of people already diagnosed with cancer, Galleri accurately detected cancer in 51.5% of people. It predicted the location of the tumor 89% of the time. 

Since this study was published this June 2021, I’ve been intrigued by the potential benefits of Galleri and had several patients ask about it. But further analysis shows it isn’t ready for widespread use. One limitation is that the test only found cancer in 16.8% of the patients with stage I cancer. It was much better at finding cancer at later stages in which treatment options are limited. In addition, I question the value of screening the general population for many of the cancers it seeks to detect. Some of the cancers it screens for are very rare. These include cancers of the small intestine, ampulla of vater, and adrenal gland. For some of the other cancers in the panel, including cancers of the cervix, colon, prostate, and breast, we already have screening tests. Other cancers included in the screening occur almost exclusively in people with certain behaviors or risk factors. For instance, mesothelioma usually presents in people with a large amount of exposure to asbestos. Galleri includes leukemia as one of the cancers it detects. But, if present, leukemia shows up on a complete blood cell count (CBC) that is part of an annual physical. Galleri screens for testicular cancer and yet in the population in which Galleri is seeking an indication—50 years and older—testicular cancer is very rare. 80% of cases occur in men between 20-34 years old. I could go on, but you get the point. The bottom line is that many of the cancers Galleri screens for are quite rare and it’s not very good at detecting them at an early enough stage to effectively impact treatment. 

Nonetheless, the technology is promising. There is clearly a need for screening tests for pancreatic, ovarian, kidney, and lung cancer (in non-smokers). Right now these malignancies are often detected at too late a stage to allow for curative treatment. I’d encourage researchers to focus on more sensitively identifying these more common cancers. Ongoing studies will shed further light on Galleri’s strengths and limitations. Over time, I think it’s likely innovations will enable Galleri and other similar screening tests to detect cancers at an earlier stage. When that occurs,  hopefully in the not too distant future, such testing will be a valuable preventative tool.