A form of radiation used to treat skin cancer may do more harm than good.

That’s the upshot of new French research into radiotherapy, published in the journal Nature Immunology, that found the treatment suppresses the immune system, Medical Xpress reports.

Radiotherapy is used in half of all skin cancer treatments, noted Guido Kroemer and Laurence Zitvogel of Université Paris, who conducted the new analysis.

They explained that radiotherapy quite often shrinks tumors initially, but then come back even stronger. They suggest the treatment may actually be making it easier for some skin cancer tumors to return and thrive by causing a suppression of the immune system.

The researchers’ findings are based on testing lab mice at Icahn Medical School in New York. The tests showed mice treated with radiotherapy had a reduction in cancer-fighting immune cells.

The study suggests doctors should be more cautious when using radiotherapy to treat skin cancers, particularly when treating large skin areas.