Swelling after shots may cause cancer false alarm

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 anormal 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 last week.

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.