Mayo Clinic is receiving a $100 million gift to expand its Rochester facility for patients who need proton beam radiation treatment.

The gift from the Fred C. and Katherine B. Andersen Foundation of Bayport, Minn., will allow Mayo to nearly double appointments for the treatment, which delivers precise radiotherapy to cancerous tumors.

Mayo doctors use the technology in cases where tumors are near or within vital organs, as well as for young people whose organs are still developing, because the treatment can minimize radiation of surrounding healthy tissue.

"This gift marks a significant milestone in Mayo Clinic's decades-long relationship with Fred and Katherine Andersen and the foundation that executes on their vision for healthy, strong communities," Dr. Gianrico Farrugia, the chief executive at Mayo Clinic, said in a statement.

Mayo started offering the treatment in 2015 with the construction of the Jacobson Building, which was named for Iowa philanthropist Richard Jacobson who also provided the clinic with a $100 million gift. The new Fred C. and Katherine B. Andersen Building will be connected to the existing center.

Mayo says its proton beam therapy program routinely approaches its appointment capacity of 1,200 patients per year. The clinic anticipates treating 900 additional patients annually with the expansion, thereby meeting annual estimated demand of 2,000 patients by 2025.

The expansion will include two floors below ground, with construction to begin next year. The goal is to start treating patients in the facility in 2026; services in the existing center will continue during construction.

"Mayo is the only center offering proton beam therapy in Minnesota and surrounding states," the clinic said in a news release. "The nearest centers are in Illinois and Missouri."

Fred and Katherine Andersen, who are both deceased, were long-time benefactors of Mayo Clinic.

Fred was the president and chair from 1914 until 1972 of Andersen Corp., the Bayport-based manufacturer of windows and doors. Katherine served on the company's board of directors for 50 years. They created the foundation in 1959.

"This gift amplifies the quiet, yet significant philanthropy the Andersens and the foundation have provided to Mayo Clinic over many years and exemplifies the Andersens' 'All Together' spirit by matching the foundation's resources with Mayo's innovative medical care," said Peter Clements, president of the Fred C. and Katherine B. Andersen Foundation.

Over the years, the foundation has supported professorships at Mayo in the names of both Fred and Katherine. It has also financially supported the clinic's capital improvement, women's cancer care and translational cancer research.

Through the foundation, Fred and Katherine are listed among the foundation benefactors of what's now called the Mayo Clinic Alix School of Medicine. The medical school was named for Jay Alix, the founder of a corporate consulting firm who in 2018 gave Mayo its largest-ever gift of $200 million.