MRI-Invisible Prostate Lesions: Are They Dangerous?

MRI-invisible prostate lesions. It sounds like the stuff of science fiction and fantasy, a creation from the minds of H.G. Wells, who wrote The Invisible Man, or J.K. Rowling, who authored the Harry Potter series.

But MRI-invisible prostate lesions are real. And what these lesions may, or may not, indicate is the subject of intense debate.

MRI plays an increasingly important role in detecting and diagnosing prostate cancer, staging prostate cancer as well as monitoring disease progression. However, on occasion, a puzzling phenomenon arises. Certain prostate lesions that appear when pathologists examine biopsied tissue samples under a microscope are not visible on MRI. The prostate tissue will, instead, appear normal to a radiologist’s eye.

Why are certain lesions invisible with MRI? And is it dangerous for patients if these lesions are not detected?

Some experts believe these MRI-invisible lesions are nothing to worry about.

If the clinician can’t see the cancer on MRI, then it simply isn’t a threat, according to Mark Emberton, MD, a pioneer in prostate MRIs and director of interventional oncology at University College London, London, England.

Laurence Klotz, MD, of the University of Toronto, Toronto, Ontario, Canada, agreed, noting that “invisible cancers are clinically insignificant and don’t require systematic biopsies.”

Emberton and Klotz compared MRI-invisible lesions to grade group 1 prostate cancer (Gleason score ≤ 6) — the least aggressive category that indicates the cancer that is not likely to spread or kill. For patients on active surveillance, those with MRI-invisible cancers do drastically better than those with visible cancers, Klotz explained.

But other experts in the field are skeptical that MRI-invisible lesions are truly innocuous.

Although statistically an MRI-visible prostate lesion indicates a more aggressive tumor, that is not always the case for every individual, said Brian Helfand, MD, PhD, chief of urology at NorthShore University Health System, Evanston, Illinois.

MRIs can lead to false negatives in about 10%-20% of patients who have clinically significant prostate cancer, though estimates vary.

In one analysis, 16% of men with no suspicious lesions on MRI had clinically significant prostate cancer identified after undergoing a systematic biopsy. Another analysis found that about 35% of MRI-invisible prostate cancers identified via biopsy were clinically significant.

Other studies, however, have indicated that negative MRI results accurately indicate patients at low risk of developing clinically significant cancers. A recent JAMA Oncology analysis, for instance, found that only seven of 233 men (3%) with negative MRI results at baseline who completed 3 years of monitoring were diagnosed with clinically significant prostate cancer.

When a patient has an MRI-invisible prostate tumor, there are a couple of reasons the MRI may not be picking it up, said urologic oncologist Alexander Putnam Cole, MD, assistant professor of surgery, Harvard Medical School, Boston. “One is that the cancer is aggressive but just very small,” said Cole.

“Another possibility is that the cancer looks very similar to background prostate tissue, which is something that you might expect if you think about more of a low-grade cancer,” he explained.

The experience level of the radiologist interpreting the MRI can also play into the accuracy of the reading.

But Cole agreed that “in general, MRI visibility is associated with molecular and histologic features of progression and aggressiveness and non-visible cancers are less likely to have aggressive features.”

The genomic profiles of MRI-visible and -invisible cancers bear this out.

According to Todd Morgan, MD, chief of urologic oncology at Michigan Medicine, University of Michigan, Ann Arbor, Michigan, the gene expression in visible disease tends to be linked to more aggressive prostate tumors whereas gene expression in invisible disease does not.

In one analysis, for instance, researchers found that four genes — PHYHD1, CENPF, ALDH2, and GDF15 — associated with worse progression-free survival and metastasis-free survival in prostate cancer also predicted MRI visibility.

“Genes that are associated with visibility are essentially the same genes that are associated with aggressive cancers,” Klotz said.

Next Steps After Negative MRI Result

What do MRI-invisible lesions mean for patient care? If, for instance, a patient has elevated PSA levels but a normal MRI, is a targeted or systematic biopsy warranted?

The overarching message, according to Klotz, is that “you don’t need to find them.” Klotz noted, however, that patients with a negative MRI result should still be followed with periodic repeat imaging.

Several trials support this approach of using MRI to decide who needs a biopsy and delaying a biopsy in men with normal MRIs.

The recent JAMA Oncology analysis found that, among men with negative MRI results, 86% avoided a biopsy over 3 years, with clinically significant prostate cancer detected in only 4% of men across the study period — four in the initial diagnostic phase and seven in the 3-year monitoring phase. However, during the initial diagnostic phase, more than half the men with positive MRI findings had clinically significant prostate cancer detected.

Another recent study found that patients with negative MRI results were much less likely to upgrade to higher Gleason scores over time. Among 522 patients who underwent a systematic and targeted biopsy within 18 months of their grade group 1 designation, 9.2% with negative MRI findings had tumors reclassified as grade group 2 or higher vs 27% with positive MRI findings, and 2.3% with negative MRI findings had tumors reclassified as grade group 3 or higher vs 7.8% with positive MRI findings.

These data suggest that men with grade group 1 cancer and negative MRI result “may be able to avoid confirmatory biopsies until a routine surveillance biopsy in 2-3 years,” according to study author Christian Pavlovich, MD, professor of urologic oncology at the Johns Hopkins University School of Medicine, Baltimore.

Cole used MRI findings to triage who gets a biopsy. When a biopsy is warranted, “I usually recommend adding in some systematic sampling of the other side to assess for nonvisible cancers,” he noted.

Sampling prostate tissue outside the target area “adds maybe 1-2 minutes to the procedure and doesn’t drastically increase the morbidity or risks,” Cole said. It also can help “confirm there is cancer in the MRI target and also confirm there is no cancer in the nonvisible areas.” 

According to Klotz, if imaging demonstrates progression, patients should receive a biopsy — in most cases, a targeted biopsy only. And, Klotz noted, skipping routine prostate biopsies in men with negative MRI results can save thousands of men from these procedures, which carry risks for infections and sepsis.

Looking beyond Gleason scores for risk prediction, MRI “visibility is a very powerful risk stratifier,” he said.

Howard Wolinsky is a Chicago-based freelancer. He just observed his 14th year on active surveillance. In late January, he is moderating a program on MRI-invisible lesions, featuring Emberton, and hosted by the patient group, Active Surveillance Patients International.