Multiparametric MRI
and Prostate Cancer
Prostate cancer diagnosis is dependent on a number of tests, and Multiparametric MRI is now one of the most important.
Background
Multiparametric MRI provides detailed anatomical and functional information which is unavailable from traditional ultrasound imaging.
Radiologists can use multiparametric MRI to:
- Identify the location of a tumour
- Measure the extent of a tumour
- Estimate the Gleason score of a tumour
- Determine whether a tumour has spread beyond the prostate gland
What’s special about Multiparametric MRI?
A multiparametric MRI exam consists of three separate imaging techniques (three ‘parameters’) which together give a clear view of the gland.
Multiparametric MRI exams are interpreted according to the Prostate Imaging Reporting and Data System) PI-RADS. This is a classification system that uses a 5-point scale to standardise assessment of exams.
A PI-RADS assessment indicates the likelihood of intermediate – and high-risk cancers based on findings from the three multiparametric MRI sequences.
- Very unlikely that clinically significant cancer is present – PI-RADS 1
- Unlikely that clinically significant cancer is present – PI-RADS 2
- Uncertain whether clinically significant cancer is present – PI-RADS 3
- Likely that clinically significant cancer is present – PI-RADS 4
- Highly likely that clinically significant cancer is present – PI-RADS 5
For results of PI-RADS 4 or 5, prostate biopsy is almost always indicated.
Results of PI-RADS 1 or 2, a recommendation for biopsy is unlikely, but other factors should be considered. PI-RADS 3 results may require a biopsy depending on patient history, local preferences and preferred standard of care.
Advantages of MRI
Some men may be so reassured by a negative MRI that they decide not to have a biopsy at all. We know that a very high quality MRI (ideally at 3T) is, if negative, more reassuring about the absence of tumour than a negative biopsy. They may decide to go no further, and to have both DRE and their PSA checked regularly, and ideally another MRI at an interval. If they do decide on biopsy, and that is negative also, they are very unlikely indeed to have a cancer that will harm them.
Most significant tumours are visible on MRI, so that a biopsy (or biopsies) can be accurately targeted to the suspicious area. This minimises the risk of missing tumours that lie in difficult to reach places (around 10% of significant cancers are completely missed by standard biopsy because of where they lie) and it also helps to be sure that the sample is representative: sometimes random biopsies just shave the edge of a large tumour, underestimating how much there is.
If a small amount of tumour is detected, MRI can check that this is not the edge of a large amount, or that there is a larger tumour in the front of the prostate. Active surveillance is unsuccessful in some men precisely because of such undetected tumours, most of which can be seen with MRI.
The finding of a significant cancer means that staging is required to detect spread outside the prostate. Radiologists who have looked at MRI images both before and after biopsy have no doubt that they are degraded for several months by the effects of bleeding from the biopsy: the best quality staging scan is undoubtedly one done before any of this has occurred – before the biopsy.