What has changed in MRI imaging to make it so accurate?
MRI has advanced recently so that not only can we look at tissue, we can also look at how tightly cells are packed (diffusion-weighted imaging), how blood flows in tissues (dynamic contrast-enhanced MRI) at the chemical makeup of tissue (spectroscopy). These techniques are the basis of multiparametric MRI. Experienced radiologists are now able to much better separate cancerous tissue from benign tissue.
The benefits of multiparametric MRI
Early detection of prostate cancer.
PSA, digital rectal examination and blind transrectal biopsies of the prostate have until now been the gold standard of prostate cancer diagnosis. Whilst they have been shown to save lives, they have also been shown to detect relatively harmless, insignificant cancers. Furthermore, they sometimes miss aggressive tumours. Multiparametric MRI offers the possibility of reducing the number of unnecessary biopsies whilst detecting aggressive tumours in high-risk individuals such as those with family histories at an earlier stage. This, coupled with the appropriate use of transperineal grid-directed biopsy, should aid in the appropriate early detection of significant prostate cancer.
Improving the accuracy of biopsy.
Multiparametric MRI offers the option of more accurate targeting of tumours in the prostate. If an MRI detects an obvious tumour, then, with sophisticated fusion technology overlaying the MRI image with the ultrasound image, it is now possible to accurately biopsy the area in question. This not only detects more significant cancers, which are those picked up by MRI, but it also avoids biopsying unnecessary, insignificant cancers, thus leading to less over-detection. MRI is not perfect in detection at this stage, particularly in certain sections of the prostate called the transition zone. Fortunately, this is the least common site for aggressive prostate cancers.
Planning surgery and radiotherapy.
Multiparametric MRI, by improving the accuracy of assessing the extent of cancer of the prostate, helps us to know whether the cancer is through the capsule, whether it is has eaten into the seminal vesicles or whether it has spread into the lymph glands. This can then help us target with appropriate surgery and radiotherapy the most appropriate form of treatment and dosage of radiotherapy. It is also helping us preserve the erectile nerves in surgery by giving us further confidence that the nerves are not infiltrated with cancerous tissue. Finally, it is even helpful in predicting the likelihood of incontinence after surgery by measuring the length of the urethra.
Monitoring patients on active surveillance.
An increasing number of patients with low-risk cancers have their cancer simply monitored. This has traditionally involved regular biopsies and PSA readings. MRI has added an extra dimension to the monitoring of these cancers and clearly is much less invasive than biopsy. Furthermore, it helps by excluding cancers which may have been missed by the initial biopsy. It is very likely that MRI will have an increasing role and biopsies will have a decreasing role in the monitoring of these patients on active surveillance after initial diagnosis.
MRI and transperineal grid-directed biopsy have allowed more accurate imaging and sampling of the prostate, allowing the possibility of 'lumpectomy' or focal therapy to be a real option in prostate cancer patients. New energy sources, including high-intensity focused ultrasound, NanoKnife (electroporation), focal brachytherapy and focal laser therapy have allowed this to emerge as a possible treatment. Focal therapy would not be possible without accurate localisation and multiparametric MRI has allowed this to occur. Focal therapy at this stage is in its early formative stage but is certainly going to have an increasing role in the management of patients who have earlier and more localised prostate cancer.