Cancer Found Accidentally (Incidentalomas)

An incidentaloma is a mass found by accident. It turns up on a scan done for stomach pain, on a chest CT after a car crash, on an MRI for headaches. The scan answers the question it was asked. Then it reports something else as well. The usual sites are the adrenal glands, kidneys, liver, thyroid, lungs, pituitary, ovaries and pancreas.

They are common. The USPSTF reviewed lung cancer screening trials. The share of participants with incidental findings judged significant, or needing more checking, ranged from 4.4 percent to 40.7 percent. The figure depended on the trial and on how findings were counted. Those findings included coronary artery calcification, aortic aneurysms, emphysema and masses in various organs. The range is wide because there is no agreement on which findings deserve a mention. The headline point stands. Scan enough of anything and you will find something.

The great majority of incidental findings are benign, meaning not cancer. In the National Lung Screening Trial, cancers of the kidney, thyroid or liver were eventually diagnosed in 0.39 percent of participants. Of the many incidental findings reported, then, only a very small share turned out to be cancer. Small adrenal nodules, simple kidney cysts, hepatic haemangiomas (harmless tangles of blood vessels in the liver) and small thyroid nodules are so common that they are better thought of as normal variation than as disease.

This is why the wording of radiology reports is so unnerving, and so unhelpful on its own. "Cannot exclude malignancy" is a statement about the limits of the image. It is not an estimate of your risk.

Guidelines exist so that a chance finding does not set off an open-ended investigation. The American College of Radiology's Incidental Findings Committee publishes step-by-step white papers, built by panels from several specialties. The ACR frames the problem in both directions: detecting important conditions early, while a lack of guidance leaves room for over-testing and over-treatment of findings that carry very low risk. Topics published and in development cover organ-specific findings, including adrenal masses, fatty liver disease, parathyroid lesions and pineal cysts.

In practice the ladder runs roughly like this.

1. Characterise what already exists. Size, density, margins, enhancement. And, often decisive, comparison with any older scan. A lesion that has not changed over several years is almost always benign. So the single most useful next step is often to find an old image, rather than order a new one.
2. Dedicated protocol imaging. A scan designed for the specific question. Adrenal washout CT, multiphase liver MRI, thyroid ultrasound, contrast-enhanced kidney imaging.
3. Laboratory tests, where the organ makes hormones. Adrenal incidentalomas, for example, are routinely checked for hormone excess as well as for cancer. A benign tumor that is making hormones can matter more than one that is not.
4. Interval imaging. Repeat the scan at a set point to test for growth. Stability is evidence.
5. Biopsy or resection. Last, and only where the earlier steps leave real doubt, or the risk is high enough to justify it.

Investigation is not free. The cost is paid mostly by people who did not have cancer.

In NLST, 24.2 percent of low-dose CT screens were positive across three rounds. The USPSTF review reported false-positive rates of 26.3 percent at baseline. Working up those false positives led to invasive procedures in 1.7 percent of screened participants, with complications in 0.1 percent. In the wider review, complications from needle biopsy for false-positive results ran 0.03 to 0.07 percent of all those screened, and surgical procedures for false positives followed in 0.5 to 1.3 percent.

Repeated CT adds radiation and contrast exposure. Findings written into your record can affect insurance. And the anxiety of an unresolved finding under watch is a real, lasting harm. It does not show up in complication tables.

NCI's screening guidance names the same problems. False positives are "usually followed by more tests and procedures, which also have risks". There are complications from procedures. And there is overdiagnosis — cancers found "that don't matter because they would have gone away on their own or never caused any symptoms". For any one person, there is no way to know whether treatment helped.

The useful question is not "could this be cancer". Almost anything could. The useful question is this: what does the guideline for this specific finding, at this specific size, in someone with my risk factors, actually recommend? That question has a documented answer for most common incidentalomas. Asking it turns an open-ended worry into a defined plan with an end point.

• USPSTF — Lung Cancer Screening recommendation (harms, incidental findings)
• NCI PDQ — Cancer Screening Overview (harms of screening)
• NCI — National Lung Screening Trial