By: Dan Sperling, MD
For patients whose elevated PSA and/or positive DRE raises the suspicion of prostate cancer, there is great clinical value in the use of 3T multiparametric MRI (3TmpMRI) before undergoing a biopsy. I have previously written about the ability of prostate MRI to identify the area(s) most likely to be significant tumors, and how this information can help target biopsy needles under MRI guidance for the most accurate diagnosis.
There is another important reason for obtaining the imaging before, not after a prostate biopsy. It has to do with what is called “MRI artifact” or “hemorrhage” artifact if a biopsy is done prior to the imaging scan, leaving blood behind from the needle punctures.
The confusion between biopsy-related wounds and tumor can result from the ways that a certain MRI parameter registers different types of tissue and its water content. In this case, what are called T2-weighted (T2W) images result in different signals translated into images on a computer monitor. (T2W signals should not be confused with the strength of the magnetic equipment as measured in Tesla, i.e. 3 Tesla or 3T MRI is more powerful that 1.5 Tesla MRI. These are different terms than “weighting” as a way to analyze what the MRI is picking up.) T2W images are useful in revealing the zonal anatomy of an individual’s prostate gland.
On T2W images, fat produces very little signal and shows up as dark, but water appears bright on T2W images. The peripheral, or outer, zone has high water content and shows up as light, but since many tumors arise in the peripheral zone, they will show up as dark areas (more fat content than water) and will therefore be considered suspicious.
If a biopsy is done before an MRI scan, the small patches of blood produce low signal strength, similar to fat content. They can be mistaken for tumors. In fact, according to a journal article out of University College (London), “Artifact can lead to overestimation of cancer burden in 20% of cases…For instance, accuracy of local staging decreased from 83% to 46% in one study as a result of MRI artifact.”[i]
There are three ways to correct for this error, but none is completely satisfactory. One would be to include T1-weighted imaging (T1W) among the MRI parameters used during the scan. T1W reverses the way signals are converted to images such that water shows up as dark while fat shows up as light. If high signals show up in the same area on T1W, it is more likely that it is hemorrhage artifact and not tumor. This is not reliable and is prone to reader error if the person is less experienced with prostate mpMRI. The second way is to correlate the images with MRI spectroscopy (MRI-S) which can help characterize the low signal areas as hemorrhage. Again, there is room for error. The third is to wait for the biopsy bleeding to completely heal, but this takes 3-4 weeks to resolve. Meanwhile, there is a risk of cancer progression, which is anxiety provoking for the patient.
The obvious solution to the dilemma is to conduct the MRI scan before the biopsy. The article I cited lays out a clear rationale for doing so, and addresses objections of time and cost. The authors concede that it sounds expensive to send every man with an elevated PSA for an MRI as a way to screen for cancer. “However, the potential benefits go beyond simply eliminating artifact and increasing detection. These benefits include the potential to reduce the number of patients undergoing biopsy, and greater precision in determining cancer grade and burden for those who have cancer. Prebiopsy MRI might lead, therefore, to a refined diagnostic pathway differentiating those having significant disease, which warrants treatment, from those who have insignificant or no cancer, which does not require detection.”[ii]
Considering that as many as 30% of men whose conventional TRUS biopsy missed cancer, especially in hard-to-sample areas of the gland, it is clear that a noninvasive mpMRI, done before a biopsy, can rule necessity in or out for that procedure. More to the point, it gives patients the option of an alternative to TRUS biopsy: a targeted, MRI-guided biopsy which reduces discomfort and the risk of infection or other side effects while yielding more accurate diagnostic information for better treatment planning. If a TRUS biopsy is positive for cancer, but has missed the more aggressive cell lines, the result is false confidence for the patient and a possible therapeutic approach that is inadequate to the disease.
In fact, when MRI images and subsequent biopsies are in agreement, the patient and his doctor are at the greatest advantage for decision-making. The process from suspected to cancer to imaging to biopsy is efficient; thanks to real-time imaging, a targeted biopsy can be performed immediately following image generation on the computer. There is no need to miss a treatment window, as might happen if a biopsy is done first, then weeks or months pass waiting for blood artifacts to heal.
Patients and clinicians alike will breathe easier when 3TmpMRI before biopsy becomes the standard of care.