A few years back, I posted a blog on the “magic” of conventional TRUS-guided prostate biopsy. Presto! It turns the walnut sized gland into a “human pincushion!” That’s because it requires at least 12 needles to submit tissue samples for analysis. In fact, a so-called TRUS saturation biopsy may use as many as 24 needles – ouch! Why so many? Because a biopsy guided by ultrasound is basically blind to prostate cancer (PCa). Standard ultrasound can’t tell the difference between healthy vs. cancerous tissue, so each needle is essentially a random shot in the dark. As a result, TRUS biopsy tends to over-detect insignificant PCa, and under-detect significant PCa. On average, 30-40% of TRUS biopsies are inaccurate.

Common sense suggests that the more needles used (despite the patient’s pain and discomfort), the greater chance of diagnosing the PCa cells with the highest aggression level. This accuracy, however, comes at a cost: risk of infection, hospitalization, post-biopsy blood in urine and semen, even erectile dysfunction. The more needles, the worse the risks. And there’s one more risk: repeat biopsies. It’s not uncommon for a 12-needle biopsy to miss PCa altogether, but since a man’s PSA continues to rise while the cancer is active, biopsies will be repeated until the culprit is found. Hopefully not too late for cure.

In-bore MRI-targeted biopsies

Thankfully, multiparametric MRI (mpMRI) has revolutionized the PCa diagnostic pathway. First and foremost, for men with a high PSA, mpMRI can rule out the need for a biopsy. A landmark 2017 study found that 1 out of 4 men with a high PSA can safely avoid biopsy based on their mpMRI results!

However, 3 out of 4 men will still need to undergo a biopsy—and here’s the second important point: In-bore MRI-targeted biopsies require only a minimum number of needles because the most aggressive tumors are clearly revealed. This makes all the difference between TRUS guidance and MRI guidance.

In PCa tumors, the most dangerous, aggressive cells typically lie at or near the innermost core of the primary or largest tumor, where they originally began spawning more tumor cells. While the tumor was growing outward in size, the oldest central cells have had time to mutate into more dangerous variants. Thus, the likelihood of a biopsy needle capturing the significant cell line that needs to be quickly and lethally treated is greatest when image guidance shows the size, shape and location of the tumor. This is not possible with ultrasound, only with mpMRI. An added benefit is the PI-RADS scoring system, which characterizes the aggression level of the tumor.

How many MRI-targeted needles are enough?

In the hands of an experienced radiologist using a powerful 3T magnet, research shows that in-bore MRI diagnostic results are superior to TRUS as well as “fusion” guidance (often misleadingly marketed as “MRI guidance”, but this belies the fact that fusing previous MRI scans with real-time ultrasound compromises the fidelity of the MRI results).

A brand new published paper (as of this writing) sheds light on the recommended number of needles for an in-bore MRI-targeted biopsy. What do you think it is? 12? 10? 7? 5? Guess again.

According to Seyfried, et al. (2020), “Obtaining only three cores from the primary lesion during [in-bore MRI-targeted biopsy] may be sufficient for high diagnostic yield while reducing patient discomfort and procedure times.”^[i] They derived this conclusion based on a study of 128 men (163 lesions among them) who had this type of biopsy. The first needle core taken was so accurate that only 12.9% of the second cores found any upgrade in aggression, further reduced to 10.7% on the third sample, and 1.9% on the fourth core.

That’s impressive, but it’s even more remarkable when the upgrade represented clinically significant PCa. In this case, the percentages are 7.4% for the second core, 4.1% for the third, and 0% for the fourth! Thus, for this study the very first targeted core tended to be highly accurate, and the fourth sample did not significantly increase the diagnostic yield.

Compare the difference: Anywhere from 12 or more needles for TRUS guidance, vs. only 3 needles for real time, in-bore MRI-targeted biopsy. Of course, the Seyfried study guideline is not intended as a hard rule, since every patient’s prostate anatomy is unique, and every PCa lesion is unique. There will be cases in which additional needles are warranted, as determined by the radiologist upon viewing the mpMRI scans.

The key point is, why go through a TRUS biopsy with its comparatively high number of needles, greater risk of adverse side effects, and high rate of inaccuracies, when an average of three needle cores under mpMRI visualization gives maximum diagnostic results? The answer seems obvious: in-bore MRI biopsy is simply the better choice.

NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you are experiencing pelvic pain, or have any other health concerns or questions of a personal medical nature.

[i] Seyfried N, Mahran A, Panda A, Obmann VC, Buzzy CA, Jiang Y, Wright KL, Nakamoto DA, Patel IJ, Conroy B, Ponsky L, Gulani V. Diagnostic Yield of Incremental Biopsy Cores and Second Lesion Sampling for In-Gantry MRI-Guided Prostate Biopsy. AJR 2020 Dec 21 [published online]. Accepted manuscript. doi:10.2214/AJR.20.24918.