Template mapping biopsy (transperineal template prostate mapping)

A template biopsy takes transperineal core biopsies through a brachytherapy grid, usually two to three cores from each of eight sites, under general anaesthetic. A mapping template biopsy is the exhaustive version: NICE's own glossary defines it as systematic sampling of 20 sites with two or three cores per site, sometimes meaning more than 50 cores from one gland. Sampling the gland on a fixed grid, typically at 5 millimetre intervals, is what makes it a near-complete map rather than a sample, and going through the perineal skin rather than the rectal wall greatly reduces the risk of sepsis that transrectal biopsy carries.

Its role is as a reference standard. PROMIS used it for precisely that: 576 men with prostate-specific antigen up to 15 nanograms per millilitre and no previous biopsy had 1.5 Tesla multiparametric magnetic resonance imaging, then both transrectal ultrasound-guided biopsy and template prostate mapping biopsy, each read blind to the others. Comparing a scan against the standard biopsy it is meant to replace answers nothing, because that biopsy is itself inaccurate; comparing both against an exhaustive mapping biopsy is what produced the numbers that changed the pathway, with the scan 93 percent sensitive and the standard biopsy 48 percent sensitive for clinically significant cancer. Of 576 men completing all three tests, 408 (71 percent) had cancer on mapping biopsy and 230 (40 percent) had clinically significant cancer.

It is not a routine test, and in the United Kingdom it is explicitly not one. NICE NG131 recommendation 1.2.5 says do not offer mapping transperineal template biopsy as part of an initial assessment unless as part of a clinical trial. The reasons are the general anaesthetic, the theatre time, the acute urinary retention that follows a heavily sampled gland, and the overdiagnosis that comes with sampling everything: a test that finds every cancer in a gland finds a great many cancers that were never going to matter. It remains in use in research, in imaging validation, and in selected clinical situations such as planning focal therapy or resolving a persistently raised prostate-specific antigen after repeated negative biopsies. PROMIS itself reported serious adverse events in 44 of 740 enrolled men (5.9 percent), including 8 cases of sepsis, a reminder that the reference standard has harms of its own.