Older adults, especially those over the age of 75, are under-represented in clinical trials of new cancer therapies, a new FDA analysis has found.

By 2030, 67 percent of the cancer population will be patients 65 and older, according to the FDA. But participants within the same age bracket are in the minority for clinical trials testing new cancer drugs. For example, 37 percent of lung cancer patients are 75 or older, but just nine percent of people within that age range are represented in lung cancer-related clinical trials. For breast cancer, older adults make up 19 percent of incidence rates, but four percent of clinical trial participants are 75 or older.

The study involved retrospective analyses of demographic data of cancer patients enrolled in trials from 2005 to 2015. The data included 224,766 cancer patients and 105 drug applications.

The researchers categorized the ages of clinical trial participants into the following groups: younger than 64-years-old; 65-69; 70-74; 75-79 and older than 80. The enrollment rates were compared with corresponding rates in the U.S. cancer population, according to the study authors.

They found that the majority of clinical trial participants (60 percent) during the study time frame were younger than 65-years-old. Among the remaining 40 percent, 17 percent were between 65-69; 12 percent were 70-74; eight percent were 75-79; and four percent were 80 or older.

The findings were presented in June at the annual meeting of the American Society of Clinical Oncology (ASCO).

Older adults are also the fastest growing segment of the U.S. population, according to ASCO. Relying on results from trials with younger participants does not accurately portray the risk of toxicity in older patients. This is one of the reasons ASCO published recommendations on how to improve the evidence pool for treating older adults with cancer. Citing a “critical need” to expand the evidence base, ASCO released the five recommendations in November 2015.

A few factors were identified as possible causes of the under-representation of older adults in trials.

Older adults are more likely to suffer from multiple diseases or conditions that could skew results, or they may be prescribed medications that would interfere with the drug candidates being tested in the studies. Transportation to and from the clinical trial site may also prevent elderly adults from participating.