In a recent report in JAMA Oncology by researchers at the University of Texas Southwestern Medical Center in Dallas, approximately 25 percent of Americans 65 and older and 11 percent of younger adults who were previously treated for cancer were subsequently found to have one or more new cancers in a different site. Depending on the type of original cancer and the person’s age, the risk of developing a second unrelated cancer ranged from 3.5 percent to 36.9 percent. The study covered 765,843 new cancer diagnoses made between 2009 and 2013 and recorded in a population-based national registry, the Surveillance, Epidemiology and End Results (SEER) program.

In many cases, the development of a second cancer resulted from the same risk factors that most likely precipitated the first malignancy. These factors include tobacco use, obesity and infection with human papillomavirus (HPV). For example, a smoker who has been successfully treated for lung cancer may later develop bladder cancer, which is also related to smoking, as well as a second lung cancer. An HPV infection, which most often causes cervical cancer, can also cause cancers of the vagina, penis, rectum and throat. And obesity is a known risk factor for at least 13 kinds of cancer, including cancers of the uterus, esophagus, stomach, liver, kidney, colon and pancreas.

Although much less common nowadays than in years past, sometimes the chemotherapy or radiation treatments used to control the first cancer cause genetic or other changes that lead to a new cancer. Examples include leukemia that can be induced by chemotherapy or radiation therapy, or uterine cancer caused by the drug tamoxifen used to treat breast cancer.

The Texas researchers, led by Caitlin C. Murphy, an epidemiologist, undertook the study of new cancers in cancer survivors in hopes of changing the common practice of excluding former cancer patients from clinical trials when they develop another cancer.

“This exclusion is not evidence-based,” Dr. Murphy said in an interview. “Patients with a prior cancer do not necessarily have a worse prognosis than those without a cancer history. They should be allowed to participate in clinical trials, which may be one of their only treatment options. If they’re excluded, a lot of patients are left out from what may be the best available treatment.”