The prostate gland is not a homogeneous organ. There are different types of tissue that make up the three zones of the prostate (transitional, central and peripheral), and structurally the gland is divided into lobes:
- Two lateral (side-by-side) lobes
- A median lobe
- A posterior lobe (behind the urethra, closest to the rectal wall)
- An anterior lobe (in front of the urethra, furthest from the rectal wall)
The anterior lobe contains no glandular tissue. It is composed of fibromuscular tissue that functions to expel semen during orgasm. Prostate cancer (PCa) that arises in the anterior lobe is difficult to detect by any of the transrectal methods: transrectal ultrasound (TRUS) guided biopsy; digital rectal exam in which the anterior cannot be felt by the examiner’s finger; and even MRI with an endorectal coil can miss anterior prostate cancer. Conventional 12-core TRUS biopsy misses anterior PCa up to 46% of the time.[i] An ongoing problem is not only the undersampling of the gland, but also that the next TRUS biopsy tends to miss the same area, leading to repeat biopsies. This may explain why anterior PCa is often diagnosed as a higher grade cancer; it may have been growing and evolving in aggression during the period in which detection failed. A solution is the use of multiparametric MRI (mpMRI), with an abdominal coil, before any biopsy is done, and especially prior to a repeat biopsy if PCa is still clinically suspected due to abnormal PSA. Imaging thus serves as an essential precursor to MRI-guided targeted biopsy into suspicious areas, especially in anterior locations, seen on mpMRI.
Two recent studies, both published in the summer of 2015, report encouraging data regarding the pre-biopsy detection of anterior PCa. The first study compares the rates of detecting anterior and posterior PCa using T2-weighted MRI and diffusion weighted MRI (DW-MRI).[ii] The research team analyzed the MRI scans of 87 biopsy-naïve patients who then underwent radical prostatectomy (RP). The images were divided into anterior and posterior scans, and were interpreted by radiologists using a 5-point scale to rate the level of PCa. Statistical analysis was used to evaluate the sensitivity and specificity for anterior vs. posterior disease. They also compared the imaging results of healthy anterior fibromuscular tissue vs. anterior prostate cancers. The team found, “The sensitivity for detecting anterior prostate cancer was 90% … when scores of 3-5 were considered as positive for prostate cancer.” This was a significantly better performance than the detection of posterior PCa. The authors concluded that mpMRI was highly useful in detecting anterior PCa that is frequently missed by TRUS biopsy.
The second article goes more deeply into the superiority of mpMRI and MRI-guided targeted biopsy over TRUS biopsy for diagnosing anterior PCa. The study was done by a Dutch team[iii] out of Radboud University Medical Center (Nijmegen, The Netherlands), arguably an international leader in mpMRI of the prostate. For this study, the researchers focused on men who had at least one previous negative TRUS biopsy but who continued to have clinically concerning PSA/DRE. The team hypothesized that the TRUS biopsies had missed anterior PCa. The imaging results were compared with histopathology results (lab analysis) of MRI-guided targeted biopsies into the suspicious areas revealed by mpMRI. The study cohort consisted of 176 patients who met the inclusion criteria (PSA > 4 ng/mL, one or more negative TRUS biopsies, the presence of suspicious lesions on previous mpMRI, and PCa proven by MRI-guided biopsy). Based on the biopsy results, the authors identified the location of “intermediate- or high-risk cancers and cancers with a maximum cancer core length of 6 mm or longer [from the biopsy needles]…” They found that 93% of patients had intermediate- or high-risk PCa, and of that group, 73% had anterior tumors. The team reports, “Cancers with a maximum cancer core length of 6 mm or more were more likely to be located in the anterior part of the prostate than were cancers with a core length of less than 6 mm…” Anterior involvement was statistically even higher in men who had two or more previously negative TRUS biopsies.
These two studies confirm the advantage of undergoing mpMRI before having a prostate biopsy. Ascertaining the location, size, shape and probable aggression level of prostate tumors enables highly accurate results of a subsequent MRI-guided biopsy that can be targeted into the tumor core where the most aggressive cells are likely to be. Based on such a precise diagnosis, clinical decisions can be made that best match the treatment strategy to the individual’s disease.
[i] Rothwax J, George A, Wood B, Pinto P. Multiparametric MRI in Biopsy Guidance for Prostate Cancer: Fusion-Guided. Biomed Res Int. 2014; 2014: 439171. doi: 10.1155/2014/439171
[ii] Shimnoto H, Tamura C, Soga S, Okamura T et al. Anterior Prostate Cancer: Diagnostic Performance of T2-Weighted MRI and an Apparent Diffusion Coefficient Map. AJR Am J Roentgenol. 2015 Aug;205(2):W185-92. doi: 10.2214/AJR.14.13392.
[iii] Schouten MG, Hoeks CM, bomers JG, Hulsbergen-van de Kaa CA et al. Location of prostate pancers determined by multiparametric and MRI-guided biopsy in patients with elevated prostate-specific antigen level and at least one negative transrectal ultrasound-guided biopsy. AJR Am J Roentgenol. 2015 Jul;205(1):57-63.