The prostate gland is part of the male reproductive system. It is located in front of the rectum and below the bladder, where urine is stored, and surrounds the first part of the urethra, the tube that connects the bladder with the tip of the penis and carries urine and other fluids out of the body.
The prostate helps make the milky fluid called semen that carries sperm out of the body when a man ejaculates. Ultrasound and MRI are the most commonly used techniques to image the prostate gland. The primary indication for MRI of the prostate is the evaluation of prostate cancer.
Post-Prostate cancer is the second leading cause of cancer death in men. Prostate-specific antigen (PSA) testing has led to an over-diagnosis of relatively indolent disease, which has been further compounded by the limitations of traditional diagnosis by transrectal ultrasound-guided biopsy.
Prostate cancer does not normally cause symptoms until the cancer has grown large enough to put pressure on the urethra (the tube that runs from the bladder to the penis).
This normally results in problems associated with urination, such as:
- needing to urinate more frequently, often during the night
- needing to rush to the toilet
- difficulty in starting to pee (hesitancy)
- straining or taking a long time while urinating
- weak flow - feeling that your bladder has not emptied fully
For over 30 years, the only way to diagnose prostate cancer and determine whether or not it needs treating, has been based on the results of a random TRUS biopsy. This involves using a series of needles that randomly sample tissue across the prostate, to see whether or not it contains any cancerous cells.
The problem is that because there are gaps between the needles, sometimes significant cancers can be missed if that section of tissue isn’t sampled. The other problem is that although biopsies are a key part of diagnosis, they are invasive and painful, and can sometimes lead to serious infections – so we only want men to have them if they need them.
Magnetic resonance imaging (MRI) of the prostate uses a powerful magnetic field, radio waves and a computer to produce detailed pictures of the structures within a man's prostate gland. It is primarily used to evaluate the extent of prostate cancer and determine whether it has spread. It also may be used to help diagnose infection, benign prostatic hyperplasia (BPH) or congenital abnormalities.
Improvements in MRI technique and, in particular, functional imaging have enabled the radiologist to play a key role in the risk stratification and management of patients, with a drive towards utilizing MRI early in the diagnostic pathway. However, the technique remains challenging both in the acquisition of images and in their interpretation, highlighting the need for the recent push towards greater standardisation.
Exams may be performed using an endorectal coil – a thin wire covered with a latex balloon – that is inserted a short distance into the rectum. Prostate MRI does not use ionizing radiation, and it provides images that are clearer and more detailed than other imaging methods.
The MRI test is used to evaluate the extent of prostate cancer in order to determine if the cancer is confined to the prostate, or if it has spread outside of the prostate gland.
Occasionally, MRI of the prostate is used to evaluate other prostate problems, including:
- infection (prostatitis) or prostate abscess.
- an enlarged prostate, called benign prostatic hyperplasia (BPH).
- congenital abnormalities.
- complications after pelvic surgery.
Researchers have developed an advanced type of MRI scan, known as a multi-parametric MRI, which as well as looking at the shape and size of the prostate, can also assess other factors such as blood flow through the gland.
The scan was used on more than 500 British men. The results suggest that scanning would safely reduce the number of men needing a biopsy by about a quarter. Moreover, in men with an aggressive prostate cancer the combination of MRI scan followed by a biopsy is almost twice as likely to pick up clinically significant cancer.
Multi-parametric (mp) MRI of the prostate is essentially any functional form of imaging used to supplement standard anatomical T1 and T2-weighted imaging. The functional sequences of choice are dynamic contrast-enhanced (DCE) MRI and diffusion-weighted imaging (DWI), including the calculation of apparent diffusion co-efficient (ADC) maps. Another technique, MR spectroscopy has recently fallen out of favour. To a certain extent the more sequences the better: it has been shown that inclusion of all three of these functional parameters achieves a positive predictive value for cancer of 98%, compared to the detection rate of 68% for T2W MRI alone.
A standard MRI scan creates an image of an internal organ. But unfortunately, it is rarely clear enough to confidently diagnose early prostate cancer. That’s where mpMRI, or multi-parametric magnetic resonance imaging, is different. By combining up to three different types of scan, we can get a clearer picture of what’s going on in the prostate. Also, an injection of a contrast agent means that scan images can be enhanced making it clearer to see if cancer is present or not.
Therefore, multi-parametric MRI (mpMRI) vastly improves upon ultrasound prostate imaging by combining several MR sequences to improve tissue evaluation and differentiation, leading to improved cancer detection and tumour localisation within the prostate. As mpMRI gains an increasing role as a triage test for disease detection it has been shown to be a useful tool for identifying men that may need a biopsy of the prostate.
The ability of mpMRI to reliably identify clinically significant disease has enabled selection of candidates for focal therapy. The primary goal of a focal therapy treatment paradigm is to achieve cancer control through targeted tissue destruction while simultaneously limiting deleterious effects on peri-prostatic structures.
Focal therapy approaches are employed in several oncologic treatment protocols, and have been shown to provide equivalent cancer control for malignancies such as breast cancer and renal cell carcinoma. Efforts to develop a focal therapy approach for prostate cancer have been challenged by several concepts including the multifocal nature of the disease and limited capability of prostate ultrasound and systematic biopsy to reliably localize the site(s) and aggressiveness of disease.
Focal therapy success relies upon accurate tumour localisation, tumour boundary definition, effective ablation targeting with adequate margin control and accurate follow-up protocols to assess oncological control.
Multi-parametric MRI (mpMRI) of the prostate has significantly improved disease localisation, spatial demarcation and risk stratification of cancer detected within the prostate. The accuracy of this imaging modality has further enabled the urologist to improve biopsy approaches using targeted biopsy via MRI-ultrasound fusion. From this foundation, an improved delineation of the location of disease has become possible, providing a critical foundation to the development of a focal therapy strategy.
While mpMRI provides a critical, first step in developing a strategy for focal therapy, considerable questions remain regarding the relationship between MR identified tumour volume and pathologic tumour volume, the accuracy and utility of mpMRI for treatment surveillance, and the optimal role and timing of follow-up mpMRI.
The current data reviewed illustrates the capability of mpMRI to provide information valuable for each of these requirements. The use of mpMRI thus sets the necessary foundation to begin exploring focal therapy strategies. Future studies will serve to strengthen these data and provide further definition on the exact role of this imaging in disease management.
Finding a way to improve the number of aggressive prostate cancers that get caught in time, whilst reducing the number of men who have biopsies unnecessarily, is really important. Multi-parametric MRI can do both of these things as well as ensuring that the biopsies that do happen are able to be much more targeted to where the cancer actually is in the prostate.
High-quality mpMRI scans rely on MRI scanners being configured in a specific way. With several MRI makes, Tesla strengths and models, it can be challenging to know which protocols to apply to get the highest quality image. Also, the complexity of interpreting and reporting mpMRI before biopsy scans can mean that some centres continue to biopsy every man.
mpMRI before biopsy offers the potential for a step-change in prostate cancer diagnosis. Results of the PROMIS trial have shown that mpMRI before biopsy can:
- Be significantly better at identifying clinically significant prostate cancer compared to TRUS (trans-rectal ultrasound) biopsy
- Reduce the number of men having biopsies unnecessarily by a quarter (27%) because the scan will only pick up cancers which could cause men harm and need further tests.
- Within 2 years, every man with suspected prostate cancer should get access to mpMRI scan before a biopsy.
Multi-parametric MRI incorporating functional imaging has led to a paradigm shift in how prostate cancer is diagnosed and increasingly in how it is followed-up. MRI is moving early in the pathway and with lesion detection now key, optimisation of MR protocols and specialist reporting has become essential. Some Health Boards have already (2018) put mpMRI before biopsy into the prostate cancer diagnostic pathway. mpMRI detects the index lesion accurately in 9 of 10 patients. MRI scanners are expensive and regularly need to be replaced. 51% of NHS MRI scanners either need to be updated or replaced by 2026.
Sources:
https://prostatecanceruk.org/about-us/what-we-think-and-do/mpmri
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5503978/
http://www.europeanurology.com/article/S0302-2838(16)00010-5/fulltext
http://interventional-radiology.imedpub.com/what-is-multiparametricmriof-the-prostate-and-why-do-weneed-it.php?aid=8216
http://www.get-prostate-healthy.com
http://www.radiologyinfo.org/en/info.cfm?pg=mr_prostate
http://www.nhs.uk/news/2017/01January/Pages/MRI-scans-could-spare-25-per-cent-of-men-from-prostate-biopsies.aspx
http://www.openaccessjournals.com/articles/what-is-multiparametricmri-of-the-prostate-and-why-do-we-need-it.pdf
John Sandham
March 2018