According to new reports, obese men age 75 and older should not be screened for prostate cancer because they are likely to have a life expectancy of less than 10 years. The screening test may not be accurate and the treatment is less likely to be effective for them.

“Obese men are more likely to be diagnosed with an aggressive form of the disease,” said Dr. Stephen Freedland, co-author of the studies. The study suggests that higher blood volumes cause lower concentrations of prostate-specific antigen, PSA, in obese prostate cancer patients, which leads to the conclusion that screening with PSA tests might miss some cancers in obese men.

“Our assumption is that these men have more blood volume, so PSA gets diluted,” Freedland said. “By the time obese men get to elevated levels, the cancer is more advanced.”

The study conducted by Freedland and his colleagues at the Duke University Prostate Cancer reached to this conclusion by analyzing data of nearly 3,400 men who had PSA tests. They compared men who had their cancers detected by PSA to those who had a digital rectal exam, an older method of screening for prostate cancer. The researchers found that the risk of an aggressive cancer was doubled in obese men because of high PSA levels.

But for those who were diagnosed by a digital rectal examination, the results were similar regardless of their weight.

It appears that the ability to accurately detect prostate cancer can be compromised by any factor that decreases PSA concentrations. Patients with a body mass index, or BMI, of 35 or greater had PSA concentrations that were 11% to 21% lower than normal-weight patients. A 5-foot-8 man who weighs 230 pounds has a BMI of 35, which is considered moderately obese. A BMI of less than 25 is considered normal.

A second study showed that that excess weight influenced the outcome of surgery for prostate cancer. One of the problems is the difficulty of operating a man with a BMI of 35 or higher, said study author Dr. Jayakrishnan Jayachandran, a urology oncology fellow at the Duke Prostate Cancer Center. “The prostate is a narrow thing to operate on, and when there is a big wad of fat in your way, if the abdominal wall is thick, it becomes a technical issue.”

Another report suggested that screening for cancer may do more harm than good for obese patients older than 75 because they are likely to die of some other cause before they begin feeling any symptoms of the disease. The prostate cancer process develops very slowly and such unnecessary test could make it develop faster than it should. PSA tests aren’t precisely enough to diagnose the cancer and they may lead to useless surgical biopsies which could determine other complications.

Moreover, the tumour may remain unchanged for years and “watchful waiting” (which is often used when an early stage, slow-growing prostate cancer is found in an older man) could bring unnecessary worries for an elderly patient.

More than 218,000 men in the US are diagnosed with prostate cancer, 28,000 of them die.