Endorectal MRI Forecasts Prostate Cancer Metastasis After Radiation

By Crystal Phend
SAN FRANCISCO, March 25 -- Pretreatment endorectal MRI in seemingly early prostate cancer appears valuable in finding extracapsular extension and predicting the risk that the disease will metastasize after radiation therapy, according to a retrospective study. The extent to which extracapsular spread was seen on MRI was the only independent predictor of subsequent metastasis, reported Fergus V. Coakley, M.D., of the University of California San Francisco, and colleagues, in the April issue of Radiology. Because MRI outperformed "all the other standard findings that would be available to the clinician," Dr. Coakley said, "I think that indicates MRI really should be more widely used, certainly in the population who take radiation treatment."
Endorectal MRI is relatively accurate for evaluating the local extent and aggressiveness of prostate cancer, but hasn't been widely adopted because of concerns about false-positive and false-negative results and interobserver variability, the researchers said.
However, many of the studies in which MRI didn't measure up well used surgical pathology as the reference standard, which may have underestimated the potential of MRI, they said. "Surgical series inevitably include patients with lower-risk tumors as compared with the tumors of patients undergoing radiation therapy."
So the researchers conducted a retrospective study of all 80 men who had an interpretable endorectal MRI at a single institution prior to receiving external-beam radiotherapy for biopsy-proven prostate cancer from March 1998 through December 2003.
The mean age of men in the study was 59 with a mean pretreatment serum PSA of 7.8 ng/mL and a median Gleason score of 7.
Depending on the radiologist reading the MRIs, 50 to 60 patients had T1 disease not visible on imaging or T2 cancer confined to the prostate while 20 to 30 of the men had T3 tumors extending beyond the capsule of the prostate. The mean diameter of extracapsular extension averaged 7 mm according to one radiologist and 4 mm according to the other.
After a mean 43 months of follow-up, 20 patients had biochemical recurrence. Three patients developed bone metastases and one developed metastases in the lung.
Baseline spread of the tumor outside the prostate on MRI predicted development of metastases (P=0.018 and P=0.001 for the two readers).
Baseline PSA level also predicted subsequent risk of metastasis (relative hazard ratio 1.12, 95% confidence interval 1.02 to 1.23, P=0.02), but degree of extracapsular extension was a stronger predictor (relative HR 2.72, 95% CI 1.23 to 5.99, P=0.01).
In the multivariate analysis, only the extent of extracapsular extension remained as a significant predictor of later metastasis (relative HR 2.06, 95% CI 1.22 to 3.48, P=0.007).
"It is unlikely that this finding was due simply to upstaging to T3 disease," the researchers said. It more likely reflected more aggressive tumors or longer presence of untreated tumors that increased the likelihood of microscopic metastatic spread, they said.
Radiation dose fall-off at the edge of the field of standard radiation may have had an impact as well, they said.
"Arguably, MRI results could allow more patient-specific treatment planning," they wrote, "particularly in patients with marked extracapsular extension."
Of the five patients with locally advanced tumors as indicated by more than 5 mm extracapsular extension on MRI before radiation therapy, three developed metastases within 63 months after therapy, "which is quite early from the point of treatment," Dr. Coakley said.
This small subset of high-risk patients "may be candidates for more aggressive therapy such as radiation dose escalation or extended androgen deprivation," the researchers wrote.
However, these strategies may improve only local control without impacting regional or distant metastatic disease and would need to be tested in a larger prospective study because of the relatively short follow-up and single-center retrospective design of the study, they noted.
And, "it comes back to the old problem of cost," Dr. Coakley said.
In addition to the relatively high expense, problems with availability both of the endorectal coil used and of radiologists with the expertise needed to read the images could limit wider adoption of the technology in this setting, he said.
Dr. Coakley reported no conflicts of interest.
Primary source: RadiologySource reference:McKenna DA, et al "Prostate cancer: Role of pretreatment MR in predicting outcome after external-beam radiation therapy -- initial experience" Radiology 2008; 247: 141-6.