EURURO Vol. 71 No. 4

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review will yield much clearer insight into the preferred
MRI biopsy technique (ref. [43] in the review).
Still unanswered, however, are the more basic questions
of the appropriate setting and role for MRI biopsy in the first
place. Is a standard TRUS biopsy sufficient to make
treatment recommendations? The recent active surveil-
lance guideline endorsement by the American Society of
Clinical Oncology, for example, makes it clear that surveil-
lance should be the preferred initial management strategy
for most men with Gleason score 6 prostate cancer, and
that surveillance regimens should be guided by serial
prostate-specific antigen (PSA) measurement and TRUS
biopsies. Any additional risk stratification tools including
MRI and genomic tests are considered investigational, and
the guideline explicitly states that MRI should not to be a
replacement for systematic TRUS biopsy .
Indeed, in an era in which active surveillance is finally
becoming the standard of care for men with low-risk disease
, overuse of MRI biopsy poses a substantial risk of
disqualifying men who would otherwise be excellent
surveillance candidates on the basis of a greater likelihood
of finding low-volume Gleason 3 + 4 tumors and
especially of the near certainty of finding more biopsy cores
that are positive, an inclusion criterion that persists in the
more stringent surveillance protocols Some biologically
aggressive cancers inappropriate for surveillance and missed
by TRUS biopsy alone will undoubtedly be found with MRI
biopsy, but as with all questions in prostate cancer, this
decrease in underdiagnosis needs to be measured against the
probability of overdiagnosis because of oversampling of
lesions with limited if any metastatic potential.
The next problem is the fact that the vast majority of the
MRI literature is derived from high-volume centers with
subspecialty radiology expertise in prostate MRI. Substantial
interobserver variability in scoring has been documented
even within institutions and at this point there is no more
assurance that generalist radiologists can replicate published
outcomes than that a generalist urologist can expect to
duplicate the outcomes of high-volume subspecialty prosta-
tectomy experts. The Prostate Imaging Reporting and Data
System version 2 (PIRADS 2.0) scoring system
) attempts to standardize interpre-
tation, but comes at its own cost. The rich and complex
molecular signals read by the MRI scanner during a multi-
parametric examination are reduced to grayscale images for
human interpretation; the reader further reduces each
component of the examination to binary or 1–5 scales, and
finally to the 1–5 summary PIRADS score. Worse still is the
practical dichotomization of PIRADS into ‘‘negative’’ or
‘‘positive’’ in many research studies using a threshold of 3
versus 4. The information loss between the magnet and this
yes-or-no answer is staggering, and MRI reporting seems an
area long overdue for more sophisticated approaches driven
by machine-learning algorithms.
In 2016, then, the role for routine MRI biopsy in
community-based care has clearly not yet been established.
In centers of excellence with dedicated, prostate-specialized
radiologists, the technique may be a valuable addition to
TRUS biopsy, especially for patients with negative biopsies
and rising PSA, or those for whom PSA or other features
appear to be of disproportionately high risk compared to
TRUS biopsy findings. Even in these settings, however, the
relative value of MRI biopsy compared to a growing array of
blood- and tissue-based tests has not been proven. Indeed, in
many respects, MRI should be considered a candidate
biomarker and held to the same evidentiary standards as
tissue-based biomarkers Costs of MRI vary dramatically
in different parts of the world, and the relative cost-utility of
imaging versus tissue biomarkers varies accordingly.
In the relatively near future, novel PET tracers —
particularly those targeting prostate-specific membrane
antigen — and next-generation magnetic resonance
spectroscopic techniques based on hyperpolarized [13C]
appear poised to either complement or supplant current MRI
protocols. These next-generation imaging techniques are
likely to be more quantitative and reproducible, and should
help to clarify the optimal role for advanced imaging in
guiding prostate biopsy.
Conflicts of interest: The author has nothing to disclose.
References
[1]
[2] Eisenberg ML, Cowan JE, Carroll PR, Shinohara K. The adjunctive use
of power Doppler imaging in the preoperative assessment of pros-
tate cancer. BJU Int 2010;105:1237–41
.
[3] Chen RC, Rumble RB, Loblaw DA, et al. Active surveillance for the
management of localized prostate cancer (Cancer Care Ontario
Guideline): American Society of Clinical Oncology clinical practice
guideline endorsement. J Clin Oncol 2016;34:2182–90
.
[4] Cooperberg MR, Carroll PR. Trends in management for patients with
localized prostate cancer, 1990-2013. JAMA 2015;314:80–2
[5] Siddiqui MM, Rais-Bahrami S, Turkbey B, et al. Comparison of MR/
ultrasound fusion-guided biopsy with ultrasound-guided biopsy
for the diagnosis of prostate cancer. JAMA 2015;313:390–7
.
[6] Tosoian JJ, Mamawala M, Epstein JI, et al. Intermediate and longer-
term outcomes from a prospective active-surveillance program for
favorable-risk prostate cancer. J Clin Oncol 2015;33:3379–85.
.
[7] Greer MD, Brown AM, Shih JH, et al. Accuracy and agreement of
PIRADSv2 for prostate cancer mpMRI: a multireader study. J Magn
Reson Imaging. In press.
[8] McShane L, Altman D, SauerbreiW, Taube S, GionM, Clark G. Reporting
recommendations for tumor marker prognostic studies. J Clin Oncol
2005;23:9067–72
[9] Hope TA, Aggarwal RR, Westphalen AC, Cooperberg MR, Greene KL.
Targeted PET imaging for prostate-specific membrane antigen in
prostate cancer. Future Oncol 2016;12:2393–6
[10] Wilson DM, Kurhanewicz J. Hyperpolarized 13C MR for molecular
imaging of prostate cancer. J Nucl Med 2014;55:1567–72
.
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