Prostate radiation only slightly increases the risk of developing another cancer, Stanford researchers find

It has been known for decades that radiation therapy can treat cancer but can also raise the risk of developing another type of cancer years later. Now Stanford Medicine researchers have found that, for prostate cancer, the increased risk is negligible.

This study quantifies the risks associated with radiation therapy and shows that the risks aren’t equal between radiation and surgery. The findings can help providers better inform their patients about the relative risks of treatment options, while addressing any concerns about radiation exposure.

In a study of about 145,000 men with prostate cancer, the team found that the rate of developing a later cancer is 0.5% higher for those who received radiation treatment than for those who did not. Among men who received radiation, 3% developed another cancer, while among those who were treated without radiation, 2.5% developed another cancer.

The study was published July 28 in the journal JAMA Network Open Oncology. It is one of the largest studies exploring the risk of later cancers — known as second primary cancers — after prostate radiation. The findings, according to the study, are specific to prostate cancer and should not be used to guide treatment decisions for other cancers.

“We’ve known for a long time that radiation can cause cancer,” said the study’s senior author, Arden Morris, MD, a professor of surgery. “But for prostate cancer, the difference is shown to be relatively small. Because there is risk every year of developing cancer and it increases as patients age, radiation becomes more important as a treatment option for older men who are less worried about long-term effects.”

Understanding options

More than 3 million men in the United States live with prostate cancer. The most common treatments are radiation therapy and surgery, but other options exist, including hormone therapy, cryotherapy and chemotherapy.

Through radiation, providers target cancer cells with high-energy beams of particles and dismantle their replicating machinery, making it a good treatment option for many cancers including persistent, drug-resistant cancer. But radiation can also damage a patient’s normal DNA: The high energy beams infiltrate and disband the tiny molecular bonds that keep healthy cells running.

The question is, which is the better option: surgery or radiation? Side effects of surgery include urinary incontinence and erectile dysfunction, but radiation is more likely to cause diarrhea, among other symptoms. There are other trade-offs as well: Surgery requires a stay in the hospital and a few weeks to recover, while radiation treatments are outpatient procedures that can extend over several weeks. That, said Morris, is why it’s so important to have as much data as possible to back one’s decision.

Dispelling myths

Using de-identified data from the Veterans Health Administration, the researchers were able to retrospectively follow prostate cancer patients for up to 20 years after their radiation treatments.

In a span of five years post-radiation, only 1 in 333 patients developed second primary cancer, according to the study. Fifteen to 20 years after treatment, that number rose to 40. Morris noted that it’s important to consider the patient’s age when deciding if radiation is the best treatment.

Generally, the earlier one receives radiation in their life, the more years they have left to live. But with every passing year, a person’s risk for second primary cancer increases, as deleterious DNA mutations accumulate. Most patients who develop second primary cancer do so six or more years after their initial diagnosis.

“If you’re in your 80s and you have a handful of years left to live, developing a second primary cancer 15 to 20 years later isn’t as much of a concern,” Morris said. “Compared to 40-year-olds with decades left to live, someone in their 80s might be more likely to pursue radiation.”

The researchers also explored assumptions about common types of second primary cancer. Previously, because of its proximity to the prostate, cancer of the rectum was thought to be the most common second primary cancer of prostate cancer. But the most common cancers to occur post-radiation, the researchers found, were bladder cancer, followed by leukemia, lymphoma and rectal cancer, among others.

Personalized care

Doctors and patients consider a variety of factors when deciding the type of treatment a patient will receive that will least interfere with patient’s quality of life, including how the treatment will impact urination, sexual function, age and other aspects of health.

Morris said the study shows that patients should not feel dissuaded from choosing radiation for fear of causing more cancer. She also suggested that prostate cancer patients talk to a surgeon and radiation oncologist as they weigh both options.

The study gives providers clearer information about the risks of radiation so they can better counsel patients, said Morris. “Although radiation patients technically have a higher risk of a second malignancy, the absolute risk is actually quite low,” she said. “I'm hoping that this helps dispel the myth that radiation is likely to cause cancer.”

Stanford’s Department of Radiation Oncology, Department of Surgery, Department of Urology, the Stanford Surgery Policy Improvement Research and Education Center and the VA Palo Alto Health Care System supported the work.

This work was funded by a Palo Alto VAMC Ci2i Supplement Grant (AMM).