Tracking Tumor Changes With mpMRI Before Biopsy
By: Dan Sperling, MD
Since the development of the PSA (prostate specific antigen) blood test to screen for prostate cancer, the conventional diagnostic pathway has consisted of
- A warning PSA (elevated or rising) with or without an abnormal digital rectal exam (DRE)
- A transrectal ultrasound (TRUS) guided biopsy, usually 10-14 needle samples
- A Gleason score determined by lab examination of the biopsy specimens.
The merits of the PSA are now in question because its sensitivity is not exclusive to prostate cancer. An elevated PSA may point to a number of benign prostate conditions, including infection and the normal gland enlargement that occurs with aging. Yet nearly one million biopsies are conducted annually in the U.S., and those that return a positive result can lead to overtreatment and reduced quality of life of thousands of patients per year. In an effort to correct this situation, PSA results are now often reported in conjunction with free PSA or PSA density as these improve the blood serum biomarkers for prostate cancer. Since even this method is not foolproof, many experts recommend imaging with MRI before biopsy.
Multiparametric MRI (mpMRI or functional MRI) combines three or four different types of information derived from “tuning in” to qualities that distinguish healthy tissue vs. tumor:
- T 2 weighted imaging (T2WI) utilizes tissue signal characteristics
- Diffusion weighted imaging (DWI) provides information on the movement of water molecules in tissue
- MRI spectroscopy (MRI-S) gives metabolic and biologic tissue information
- Dynamic contrast enhanced (DCE) MRI reveals abnormal blood flow around a tumor.
Together, they depict with high accuracy the presence of a tumor, its size, shape, location and clues to its aggression level. While not diagnostic in the same way as analyzing tissue samples in a lab, it can help determine whether the next diagnostic step (a biopsy) is indicated. For patients on active surveillance (AS), this would offer a tremendous advantage over current practice consisting of repeat TRUS biopsies at specified intervals—usually 1-2 years—or a TRUS biopsy triggered by a rise in PSA.
A new study from New York University demonstrates that mpMRI compares favorably with PSA velocity as a way to predict biopsy outcome.[i] The study involved 55 men who had two prostate MRI scans >6 months apart, followed by targeted biopsy (selective MRI-guided sampling of the lesion shown on the images). The objective was to assess any changes or progression in the dominant lesion shown on MRI as a way of predicting biopsy results. Two radiologists evaluated the dominant lesions seen on MRI for an increase in size or suspicion score. Their assessments were compared with how well PSA velocity predicted a positive biopsy. The results were as follows:
| Progression on MRI | PSA Velocity | |
| Sensitivity (proportion of actual positives identified as such) | 23.5%-35.3% | 70.6% |
| Specificity (proportion of actual negatives identified as such) | 76.2%-90.5% | 52.4% |
| Greatest total accuracy | 65.5%-72.7% | 63.6% |
The authors concluded, “Findings support lesion progression on MRI serving as a basis for performing subsequent targeted biopsy.”
[i] Rosenkrantz A, Rice S, Wehrli N, et al. Association between changes in suspicious prostate lesions on serial MRI examinations and follow-up biopsy results. Clin Imaging. 2014 Oct 16. pii: S0899-7071(14)00230-7. doi: 10.1016/j.clinimag.2014.08.008. [Epub ahead of print]
