Rates of prostate cancer are not increasing, researchers say, but the incidence of existing cases that are aggressive are on the rise.

That’s according to a new study by researchers at Northwestern University in the journal Prostate Cancer and Prostatic Diseases. Their results show that men ages 55 to 69 are now twice as likely to get metastatic prostate cancer now than they were in 2004—meaning the incidence of aggressive prostate cancers has doubled in those 12 years.

According to NBCNews, the study’s authors have a few hypotheses, including that since prostate cancer screening has seen a recent decline, more men have a further-progressed disease when it is finally detected through PSA tests or diagnostic imaging.

But that hypothesis might not be the whole story—check out NBCNews to see why the American Cancer society is skeptical of the results of this study.