An acknowledged concern about transrectal ultrasound guided (TRUS) prostate biopsy is the possibility of false negative findings, that is, the biopsy misses cancer that is present in the gland. Estimates of false negative rates vary from 25-30%.[i] This situation puts patients at risk in two ways: 1) undetected insignificant (low grade) prostate cancer has the potential to migrate to a higher grade, depending on the cell line, and 2) the patient is more likely to undergo future repeat biopsies that create tissue artifacts which can confound future imaging.
A new study from a Scottish research team was designed to test the future use of MRI to help make transperineal prostate biopsies more targeted. Their study objective was to correlate MRI-revealed areas of suspicion with actual locations of transperineal biopsy-demonstrated cancer.[ii]
Transperineal biopsy differs from TRUS biopsy in two major ways:
- TRUS biopsy needles are directed into the prostate gland through the wall of the rectum, posing a risk of introducing colon bacteria into the pelvic cavity. Transperineal biopsy uses a fixed grid as a mapping template to guide needles through the outer skin between the scrotum and anus.
- TRUS uses a spring-loaded apparatus that “fires” needles into the gland. Transperineal biopsy needles are inserted and placed by hand.
- TRUS biopsy is done in an office setting, often with a numbing agent and/or prostate nerve block; patients report a range of discomfort from none to great pain. Transperineal biopsy is performed under general anesthesia in an operating room setting, so a greater number of needle samples can be withdrawn. It is well-tolerated with patients reporting no pain.
Traditionally, real-time ultrasound is used to guide both types of biopsy. It accurately depicts the size and shape of the gland, and the placement of needles is clearly seen. However, ultrasound cannot differentiate prostate cancer from normal gland tissue. Thus, despite following a systematic placement scheme both biopsy procedures are essentially blind, much like groping in a dark room.
The Scottish team from the NHS Greater Glasgow and Clyde health board explored whether information gained from MRI imaging, which can depict tissue differences that define suspicious areas, would match up with the tissue results from transperineal template mapping biopsy (TTMB). Their goal was to learn if advanced MRI imaging could benefit TTMB by increasing its ability to target regions of suspicion in a way not possible with ultrasound. They enrolled a cohort of 44 consecutive patients who each had a TTMB. All had previously had negative TRUS biopsies; 34 had undergone one to two TRUS biopsies, and 10 had had three or more TRUS biopsies. Radiology readers scored the MRI images on a scale from 1 to 5, and in 28 patients the score was 3 or greater.
The results of the TTMB were as follows:
12 patients were diagnosed with high grade PIN (prostatic intraepithelial neoplasia or abnormal cells that are thought to be possible precursors of cancer)
- 13 patients were found to have Gleason less than 7
- 10 patients had Gleason 7
- 8 patients had Gleason greater than 7.
(One patient was not reported on.)
When the TTMB findings were compared with the suspicious areas rated 3 or higher, “Magnetic resonance imaging correctly localised transperineal template mapping biopsies-detected prostate cancer in a hemi-gland approach, particularly in a right to left manner (79% positive prediction rate)…” Based on this data, they concluded that “lesions revealed by magnetic resonance imaging are likely useful for targeted biopsies.” Their work boosted the confidence of the radiology/urology team that MRI will enhance the accurate targeting of transperineal biopsies.
[i] Barqawi A, Rove K, Gholizadeh S, O’Donnell C et al. The role of 3-dimensional mapping biopsy in decision making for treatment of apparent early stage prostate cancer. J Urol. 2011;186(1):8085.
[ii] Mukherjee A, Morton S, Fraser S, Salmond J et al. Magnetic resonance imaging-directed transperineal limited-mapping prostatic biopsies to diagnose prostate cancer: a Scottish experience. Scott Med J. 2014 Oct 14. pii: 0036933014556197. [Epub ahead of print]