For decades, breast-cancer patients have been told to stop using antiperspirant while undergoing radiation treatments. The theory was that the thin layer of material somehow magnified the amount of radiation delivered to the skin, causing irritation and swelling.

But in reality? Don't sweat it.

That is the finding of a new study led by University of Pennsylvania researchers.

The authors applied a generous layer of antiperspirant to a two-by-two inch square of paper, then aimed a high-energy radiation beam at it from four different angles. They did the same with an uncoated piece of paper, and measured the amounts of radiation exposure on each. No difference.

The authors got the same result when they repeated the experiment with extra-strength antiperspirant, they wrote in the journal Radiotherapy and Oncology.

Lead author Brian C. Baumann said women could certainly ask their doctors for guidance, but that the results of the study were clear.

"We think that women undergoing breast radiation therapy can liberalize their use of antiperspirant without fear of side effects," said Baumann, who recently completed a residency in radiation oncology at Penn.

A relatively minor issue when someone is battling a life-threatening disease? Perhaps, but patient comfort is an important quality-of-life issue, said Baumann, who is starting an assistant professorship later this year at Washington University in St. Louis.

Lynn Folkman Auspitz, a Philadelphia resident who underwent radiation treatment for breast cancer in 2009, agreed. Her doctor told her to swear off antiperspirant for a six-week course of treatment that started in early September that year, when the weather was still hot.

"When you're walking, or when you're exercising, it's not the most pleasant," said Folkman Auspitz, manager for community engagement at Living Beyond Breast Cancer.

In addition to measuring radiation doses on the paper squares, Baumann and his colleagues also surveyed patients and providers, and found that the no-antiperspirant advice remains commonplace.

In the past, clinical trials have found that breast-cancer patients who used antiperspirant suffered no more skin side effects than those who did not use it, but the no-antiperspirant advice has persisted — perhaps because there are limitations with the way this skin "toxicity" is measured, and physicians may have felt it was better to err on the side of caution, Baumann said

The origin of this conventional wisdom dates back at least to the 1960s, when physicians treated patients with lower-voltage doses of radiation that were more likely to cause skin irritation and swelling, Baumann said. One theory was that if a patient wore antiperspirant, which contains an aluminum-based compound, it might cause this lower-energy radiation to scatter.

But for several decades, oncologists have been using higher-energy machines, which emit radiation that penetrates deeper into the tissue, reaching its maximum impact at the tumor site.

"The lower-energy one almost sort of pancakes on impact, as opposed to continuing on deeper into the tissue," Baumann said.

Antiperspirants also have been blamed in some quarters for causing breast cancer, but there is no scientific evidence for that claim, according to the National Cancer Institute.

Women still are advised to forgo antiperspirant in another medical setting: while getting a mammogram.

According to the Mayo Clinic, the underarm-drying substances may interfere with getting a clear image.