Getting a mammogram or other cancer check soon after a COVID-19 vaccination? Be sure to tell the doctor about the shot to avoid false alarm over a temporary side effect.

That's the advice from cancer experts and radiologists. Sometimes lymph nodes, especially in the armpit, swell after the vaccinations. It's a normal reaction by the immune system but one that might be mistaken for cancer if it shows up on a mammogram or other scan.

"We need to get the word out," said Dr. Melissa Chen, a radiologist at the MD Anderson Cancer Center in Houston who recently had to reassure a frightened patient who sought cancer testing because of an enlarged lymph node.

An expert panel from three cancer centers—MD Anderson, New York's Memorial Sloan Kettering and Boston's Dana-Farber—published recommendations in the journal Radiology last week on how to handle scans complicated by the side effect.

The main message: "This should not prevent patients from getting the vaccine," stressed Chen, one of the coauthors.

Lymph nodes are part of the immune system where infection-fighting white blood cells gather, spots usually too small to feel. But they can swell during illness and after other types of vaccines. And with the anticipated jump in COVID-19 vaccinations, doctors should "prepare to see large volumes" of imaging exams—including chest CTs, PET scans and mammograms—that show swollen lymph nodes, according to similar recommendations in the Journal of the American College of Radiology this week.

The nodes most commonly affected are in the armpit and near the collarbone, on the same side as the vaccination, Chen said.

The Food and Drug Administration lists the swelling along with other injection-related reactions commonly reported in studies of the Pfizer and Moderna vaccines, although not for the Johnson & Johnson vaccine.

It's not clear how often it happens. The FDA found 16% of participants in the Moderna study reported some underarm swelling after their second dose. But if the lymph nodes are only slightly enlarged, they may show up on a medical scan without people noticing any bumps.

The consumer advice still is evolving. Where experts agree: If you've recently been vaccinated, tell the radiologist before any scan. That will help them evaluate if an enlarged lymph node is probably vaccine-related and can simply be monitored, or if it's worrisome enough for a biopsy or other test.

And try to schedule an upcoming screening or other cancer-related scan ahead of vaccination if it's possible without losing your place in the vaccine line, the Radiology panel said.

People with active cancer that's on one side of the body can choose vaccination on the opposite side to minimize confusion.

Don't delay any urgent exams, radiologists stress. But there's some disagreement about non-urgent scans. The Radiology panel said to consider scheduling purely routine screenings six weeks after vaccination. In contrast, recommendations from Massachusetts General Hospital urge handling the side effect with good communication rather than delayed screening.

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—Lauran Neergaard