MRI of the prostate has become a common test ordered to look for cancer of the prostate. The test is read by a radiology doctor who assigns each lesion they see a score from 1 to 5. 1 being very low risk and 5 being highly suspicious for cancer.
MRI is also used to assist with biopsy planning since those lesions marked as suspicious by the radiologist can be targeted during the biopsy. This a helps the urology doctor who does the biopsy make a diagnosis of prostate cancer. The urology doctor will also target other unmarked areas of the prostate since not every cancer is shown in the MRI.
MRI does not show every cancer
MRI of the prostate is best for clinically significant or higher grade cancers which threaten the health and life of the patient. It will at time not show cancers of the prostate however. Those are frequently lower grade cancers but can be higher grade as well. MRI of the prostate does enhance the diagnosis of prostate cancer.
Without the prostate MRI, the urology doctor will blindly biopsy the gland. This will lead to areas which are not sampled. Cancer is therefore more likely to be missed. Prostate MRI will give additional suspicious targets to the urology doctor to biopsy. This will increase the diagnosis of prostate cancer.
Prostate cancer can be hidden or normal gland can be mistaken for cancer
Prostate MRI is not a perfect test. There are mimics of prostate cancer from normal anatomy. There are also parts of the gland such as the transitional zone which is can be difficult to diagnose cancer. The normal gland in this area of an older man can hide or even mimic cancer. An inflamed gland called prostatitis can sometimes mimic cancer or even hide it.
Prostate MRI may therefore lead to false positive or false negative diagnosis of prostate cancer. Additionally, the biopsy may miss a lesion or result in sampling error of the lesion. The prostate MRI and biopsy may lead to an under-staging of the cancer leading to less aggressive treatment . That is, the grade of the cancer may come back lower then it is.
The experience of the radiology doctor reading the study is also important. The test has many pitfalls and experience certainly helps in a more accurate reading. The quality of the test is also important. Some higher volume centers will have more experience with doing these tests leading to a higher quality of exam.
Prostate MRI improves on the diagnosis of prostate cancer. It is not a perfect test and not every cancer will be diagnosed. Many clinically significant cancers will be diagnosed however. Especially those that threaten health and life. The test will also help target the biopsy to the most suspicious areas.