Magnetic resonance imaging (MRI) is feasible as a primary screening test for prostate cancer, Canadian investigators suggested based on the results of a pilot study.
Researchers led by Robert K. Nam, MD, MSc, head of genitourinary oncology and professor of surgery at Sunnybrook Health Sciences Centre in Toronto, found that MRI performed better than prostate-specific antigen (PSA) testing in predicting not only the presence of prostate cancer, but also the aggressiveness of the disease.
The results of the small study were published in the August issue of The Journal of Urology.
For purposes of the study, men in the greater Toronto area were recruited with the understanding that they were to undergo MRI for prostate cancer screening, followed by prostate biopsy. To ensure that participants would reflect the general population, the requirements for inclusion in the study included (1) age between 50 and 75, (2) having no family history of prostate cancer, and (3) having no prior history of prostate biopsy.
A total of 50 men were selected for the study, 47 of whom agreed to undergo both MRI and prostate biopsy. Of those, 18 (38.3%) were diagnosed with prostate cancer, while 29 (61.7%) had no evidence of cancer.
Nam and his colleagues determined that MRI was a much better predictor of prostate cancer than PSA was, with the adjusted odds ratio (OR) of prostate cancer "significantly higher" for MRI score than PSA level (2.7, 95% CI 1.4-5.4, p=0.0004 versus 1.1, 95% CI 0.9-1.4, p=0.21).
MRI was also much better than PSA in predicting a diagnosis of aggressive prostate cancer -- i.e., tumors with a Gleason score of 7 or higher; the adjusted OR of aggressive cancer was higher for MRI score than PSA level (3.5, 95% CI 1.5-8.3, p=0.003 versus 1.0, 95% CI 0.9-1.2, p=0.58).
Furthermore, when the patients were divided into normal (less than 4.0 ng/ml) and "abnormal" (4.0 ng/ml or greater) PSA groups, MRI score was a strong predictor of cancer.
Among the 30 patients with normal PSA, prostate cancer was diagnosed in 9 (30%). The positive predictive value (PPV) in patients with an MRI score of 4 or more was 66.7% (six of nine patients), while the negative predictive value (NPV) in patients with an MRI score of 3 or less was 85.7% (18 of 21 patients).
"For patients with normal PSA (less than 4.0 ng/ml), MRI significantly improved PPV and NPV for prostate cancer," the researchers wrote.
They noted that the results of the pilot study address two of the major concerns associated with PSA-based prostate cancer screening: the problems of overdiagnosis and the limitations in the negative predictive value of PSA.
"MRI-based prostate cancer screening has the potential to address both concerns," Nam and co-researchers wrote. "In PSA-screened patients without a prior history of prostate biopsy, MRI-guided biopsy strategies increased the diagnosis of clinically significant prostate cancer while decreasing the detection of clinically insignificant prostate cancer."
The researchers also pointed out "obvious downsides" to prostate MRI: high cost and availability.
"However, the potential savings resulting from detecting and treating aggressive prostate cancer at an earlier stage, and avoiding unnecessary prostate biopsy and treatment of indolent prostate cancer could justify the cost of prostate MRI screening," they said.
Three editorial comments accompanied the article. In the first, Xiaosong Meng, MD, PhD, and Samir S. Taneja, MD, both of New York University Langone Medical Center, called the study "provocative" and noted several unanswered questions: "The sensitivity of multiparametric MRI for cancer detection is not reported and a clear threshold for biopsy could not be ascertained," they said, adding that it is possible that the implementation of this kind of screening could actually increase rather than decrease the number of biopsies, as well as increase the risk of over-detection.
In another commentary, Pedro Recabal, MD, of Memorial Sloan Kettering Cancer Center in New York City, wrote "that before embracing MRI as a screening tool, rigorous criteria must be met to demonstrate analytical and clinical validity. Current data are still insufficient to support a role for MRI in biopsy decision making," he said.
In response, Nam and colleagues acknowledged that the small size of the pilot study limited their ability to draw definite conclusions about the accuracy of MRI for prostate cancer screening, but said the data are sufficient to propose that prostate MRI should be further evaluated in a prostate cancer screening study.
And the team is now planning to conduct such a randomized clinical trial comparing the use of MRI with PSA in prostate cancer screening, with the trial registered in clinicaltrials.gov.
Nam and co-authors reported having no "direct or indirect commercial incentives" related to the study.
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