What is multiple myeloma? Louis Prevost, Pope Leo’s older brother, also battles cancer; his wife reveals shocking details

Louis Prevost, the older brother of Pope Leo XIV, has been suffering from multiple myeloma, his wife, Deborah Prevost, told CBS News on Tuesday (October 13, 2026). It is a type of cancer that affects plasma cells, according to the Multiple Myeloma Research Foundation. The shocking news about Prevost’s illness comes after the Vatican announced that the pope has been diagnosed with lung cancer.

Pope’s diagnosis has been officially identified as lung adenocarcinoma. It is one of many forms of lung cancer. He had the lower section of his right lung removed after doctors diagnosed him with the lung cancer. Lung cancer is divided into two major groups. One is small cell, and the other is non-small cell lung cancer. According to news agency AP, the non-small cell kind makes up the vast majority of the world’s cases, and adenocarcinoma is its most common subtype.

As far as Prevost’s illness is concerned, his wife, Deborah, did not reveal details regarding the stage of his cancer, but she did say that he is currently in a clinical trial at a local hospital where he's being treated.

What is multiple myeloma?

Multiple myeloma occurs when healthy plasma cells become abnormal cells that multiply and produce abnormal proteins, according to Cleveland Clinic. In this type of cancer, plasma cells are transformed into cancerous cells that grow out of control, crowding out the normal cells that help fight infection, according to the Multiple Myeloma Research Foundation website.

The website further states that these malignant plasma cells then produce an abnormal antibody called M protein. Multiple myeloma can leave a person suffering with medical issues that can affect bones, kidneys, and blood cells.

According to Deborah, Prevost was first diagnosed in 2017. He managed to maintain it at a "very low level" after undergoing treatment in 2019. According to CBS News, the treatment included stem cell treatment and a clinical trial. She further stated that he "fell out of remission" three months ago.

Deborah did not share details regarding the specifics of Pope Leo's condition but said her husband and his brother spoke last week before Louis went in for his clinical trial. "He called to let us know what would be happening," she said, as quoted by CBS News. "We keep praying, keeping spirits up for both of them." Pope Leo’s lung cancer was confirmed after he underwent surgery on Tuesday (October 13, 2026), the Vatican had said.