Mayo Clinic study reveals proton beam therapy may shorten breast cancer treatment

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By Lynda De Widt

Mayo Clinic Comprehensive Cancer Center researchers have uncovered evidence that supports a shorter treatment time for people with breast cancer. Their study, published in The Lancet Oncology, compared two separate dosing schedules of pencil-beam scanning proton therapy, the most advanced type of proton therapy, known for its precision in targeting cancer cells while preserving healthy tissue to reduce the risk of side effects.

Survival rates for breast cancer continue to improve due to advances in diagnosis and treatment, leading to increasing emphasis on reducing the long-term toxicity of cancer treatment, including radiotherapy (radiation therapy).

Before this study, all people with breast cancer treated with proton postmastectomy radiotherapy (PMRT), had received a conventional 25- to 30-day course delivered five days per week over five to six weeks. The researchers hoped to demonstrate that condensing the course of proton PMRT would result in a similar side effect profile.

Eighty-two patients, many of whom had prior breast reconstruction, were randomized to receive either conventional fractionation (fractions of radiation dose) administered in 25 days, or a condensed 15-day hypofractionated schedule. With hypofractionation, a larger dose of radiotherapy is delivered with each treatment, allowing all radiotherapy to be completed in just three weeks. The investigators found that both conventional and hypofractionated proton therapy resulted in excellent control of the cancer while sparing surrounding normal tissue. Further, complication rates were comparable between the two study groups.

Robert Mutter, M.D.

"The study provides the first prospective data supporting the use of shorter-course proton PMRT, including in patients with immediate breast reconstruction, and the first mature results of a randomized trial in the field of breast particle therapy," says Robert Mutter, M.D., a radiation oncologist and physician-scientist at Mayo Clinic Comprehensive Cancer Center. "We can now consider the option of 15 days of therapy with patients based on the similar treatment outcomes observed as the longer conventional course. Of note, the short course actually resulted in reduced skin side effects during and after treatment."

Importantly, the researchers noted that the new schedule spares patients additional inconvenience, cost, and other burdens associated with the longer regimen. As there are a limited number of proton therapy facilities in the United States and the world, Dr. Mutter adds that demonstrating the safety and feasibility of the shorter treatment course could result in greater access to proton beam technology for difficult-to-treat breast cancer cases.

The researchers say additional investigation into the optimal PMRT dose and fractionation is warranted. The team plans additional research, investigating the delivery of PMRT in as few as five days.

"Ultimately, our goal is to personalize radiotherapy based on tumor biology," Dr. Mutter says. "We want to identify the best possible radiotherapy schedules or drug-radiotherapy combinations to eliminate cancer while minimizing side effects."

The research was funded by a grant from the National Cancer Institute, Mayo Clinic, and a gift from Lawrence and Marilyn Matteson. For a full list of funding, authors, and conflicts of interest, see the paper.

Mayo Clinic is expanding its proton beam therapy services in Rochester, Minnesota, with a $200 million facility expansion project expected to open in 2027. Mayo Clinic opened its first Minnesota proton facility in 2015. In 2016, Mayo Clinic opened the Southwest’s first proton therapy facility at its Arizona site. Now, construction is underway on a new, 200,000-square-foot integrated oncology building on Mayo Clinic’s campus in Florida where proton beam therapy will be delivered to patients when available in 2026. The facility also will allow Mayo Clinic researchers to study carbon ion therapy in clinical trials as a potential treatment.

Learn more

Learn more about proton therapy and breast cancer. Find a breast cancer clinical trial at Mayo Clinic.

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A version of this story was originally published as a news release on the Mayo Clinic News Network.