Legitimate screening tests exist for just five types of cancer (and these tests are all very far from ideal). That’s one reason I’m very impressed by this cancer blood test study by Anne Marie Lennon and colleagues.
Their assay looks for blood-borne fragments of DNA with cancer mutations and proteins expressed more abundantly in cancerous tissue. When tests came back positive, the researchers followed up with PET-CT imaging for confirmation and to find the site and extent of suspected tumors.
A total of 9,911 people volunteered to have their blood tested for the cancer markers. During the next 12 months, 96 of these volunteers were diagnosed with cancer and 26 of the cancers were first detected by the blood test. Several were cancers for which there is no standard screening test, including six ovarian cancers, two uterine cancers, and single tumors of the appendix, kidney and thyroid.
The investigators answered some essential questions too often ignored in past cancer screening efforts:
- Can the test be performed safely, without incurring a large number of futile, invasive follow-up tests?
- Can it be worked into clinical care alongside standard screening?
It’s worth highlighting that nearly half (46/96) of the cancers that emerged were not detected by the blood test or by standard screening. In most of these 46 cases, the cancer was diagnosed because the patient experienced symptoms and sought follow up.
The promotion of cancer screening makes it easy to assume screening tests are the way most of the targeted cancers are found. But that’s never been true. Two out of three women and men with colorectal cancer, for example, were diagnosed because they become symptomatic in one multi-center cohort study of 2,450 patients in Germany. In a study of 361 women breast cancer survivors, only 43% said their cancer was detected by mammography screening.
It’s also worth highlighting the problem of overdiagnosis. That’s when screening tests prompt the diagnoses of cancers destined to never cause harm. Lennon and colleagues point out that they can’t yet be sure if their blood test helped anyone in the study. “It is possible that none of the cancers first detected by this test would ever have caused symptoms or led to death,” they note. The same is true for participants whose cancers were first detected by standard screening; the advantage of a blood test is it’s potential to detect many different types of cancer at once, which could eliminate the need to subject populations to ongoing, multiple screening tests, including the more invasive colonoscopy.
“All that we can confidently conclude at present is that a minimally invasive blood test can be safely used to detect several types of cancers in patients not previously known to have cancer, enabling treatment with intent to cure in at least a subset of individuals,” they said. “This advance will facilitate future randomized, interventional trials to assess the ability of minimally invasive blood tests to improve the effectiveness of cancer screening.”