By: Dan Sperling, MD
Since the late 1980s, magnetic resonance imaging (MRI) has emerged as the leading imaging modality for the purpose of detecting, localizing and characterizing prostate cancer. While transrectal ultrasound is more commonly used due to its portability and economic feasibility, it best reveals only the gland itself (size and shape), the bladder, seminal vesicles and anterior rectal wall. It does not offer definitive information about prostate tissue, nor can it distinguish benign from malignant abnormalities. Computed Tomography (CT) likewise has a limited role, providing general pelvic images that are helpful for evaluating potential metastases in lymph nodes and soft tissue.
The last decade brought significant strides in MRI with the introduction of ways to image tissue for various functional hallmarks such as tissue density, diffusion of water molecules, and identification of metabolites that discriminate between healthy vs. malignant structures. During the first five years of the decade, the most commonly used magnets were 1.5 Tesla (1.5T) with a reduced field strength compared to more powerful 3 Tesla (3T) which have become increasingly more available. Because of the lesser field strength of 1.5 magnets, use of an endorectal coil (ERC) was desirable to gain greater spatial resolution and reduce misleading artifacts, resulting in better quality images. (An ERC is a latex (rubber) balloon with a central tube that contains the coils, and is inserted into the rectum and inflated before the scan to hold the coil in position close to the prostate. It stays in during the scan and is removed when the scan is finished.) However, “use of the endorectal coil adds discomfort, time and cost to the MR exam.”[i]
Although numerous researchers continue to favor use of the ERC, it is beginning to appear that advances in multiparametric MRI done under a 3T magnet without an ERC yield excellent quality scans with a high rate of identifying significant cancer. A recent Swiss study is a case in point.[ii] The research team studied 111 men with cancer of the prostate and/or bladder who had 3T DWI (diffusion-weighted imaging, one of the functional parameters) before radical prostatectomy or cystoprostatectomy (removal of the bladder and the prostate). Three independent readers blinded to clinical and pathological data rated the scans for suspicion of significant prostate cancer, assigning values according to degree of suspicion. (Significance was defined as either the largest diameter of the index lesion being 1 cm or greater, extraprostatic extension, or Gleason > 7 on final pathology.) Their evaluations were compared with the post-surgery pathology results. On final pathology, 93 of the 111 patients had PCa (significant in 80, insignificant in 13) and 18 had no PCa. MRI sensitivity ranged from 89-91% (correct prediction of presence of cancer) and specificity ranged from 77-81% (correct prediction of no cancer. Interreader agreement was good, with a median reading time of 13-18 minutes. The research team concluded that even without an endorectal coil or contrast medium, 3T DWI has a “high probability” of detecting significant PCa with good interreader agreement and a short reading time. They suggest use of this technique be further evaluated as a way to stratify patient risk levels.
Perhaps even more compelling, a Dutch team compared “the diagnostic performance of 3 Tesla, 3-dimensional (3D) magnetic resonance spectroscopic imaging (MRSI) in the localization of prostate cancer (PCa) with and without the use of an endorectal coil (ERC).”[iii] From 2004-2006, 18 men with biopsy-proven PCa who were scheduled for prostatectomy underwent 3D Magnetic Resonance Spectroscopy Imaging (MRSI, another functional parameter) with and without an endorectal coil. The prostate was divided into 14 regions of interest (ROIs). Four readers independently rated (on a 5-point scale) their confidence that cancer was present in each of these ROIs. Among the four, a total of 504 ROIs were identified and ranked. Their evaluations were correlated with whole-mount prostatectomy specimens. The team found that use of the ERC slightly increased the localization performance over the images produced without the ERC. While some clinicians might interpret this as justification for continuing the use of the ERC, only a single functional parameter (spectroscopy) was the subject of the study.
It would be logical to continue to compare 3T MRI scans done with and without an ERC and a combination of functional parameters (T2 weighted, DWI, Direct Contrast Enhanced MRI, and MRI spectroscopy). The Sperling Prostate Center offers 3T Multiparametric MRI without the use of an ERC. It is our experience, as it appears to be for the Swiss team who also studies only one parameter (DWI) that the powerful and advanced technology at our facility produces accurate and precise images for the detection, localization and characterization of prostate cancer.
[i] Hegde J, Mulkern R, Pahych L, Fennessy F et al. Multiparametric MRI of prostate cancer: an update on state-of-the-art techniques and their performance in detecting and localizing prostate cancer. J Magn Reson Imaging. 2013 May;37(5):1035-54.
[ii] Bains LJ, Studer UE, Froehlich JM, Giannarini G et al. Diffusion-weighted magnetic resonance imaging detects significant prostate cancer with high probability: results of a prospective study with final pathology of prostates with and without cancer as the reference standard. J Urol. 2014 Mar 15. doi: 10.1016/j.juro.2014.03.039. [Epub ahead of print]