Multiparametric-magnetic resonance imaging (mpMRI) has shown promising results in diagnosis, localization, risk stratification, and staging of clinically significant prostate cancer.
One of the challenges of prostate cancer evaluation today is the risk of overdiagnosis and overtreatment of prostate cancer using the current diagnostic pathway, which is initiated with an elevated PSA followed by a transrectal US guided biopsy (TRUS).
Under diagnosis occurs because TRUS biopsy is often unable to diagnose cancer in the anterior prostate cancer until they grow to a large size; they also miss cancers near the urethra because biopsies are generally avoided near the urethra for fear of pain and bleeding. TRUS biopsies also often underestimate the grade of cancer in a number of patients. The blind biopsy may go through the edge of the cancer and then misses the main cancer which may be of a different grade.
Over diagnosis occurs because random blind biopsies detect many small inconsequential cancers that do not harm the mann and can be left untreated.It is estimated that 50 to 60% of men in the age group that get prostate cancers have indolent cancers and that 20 to 40 %of these cancers may be detected by random biopsies.
All this results in inaccurate risk assessment of prostate cancer and patients may not get the appropriate therapy for their risk of cancer.
Multiparametric MRI (MP-MRI) has recently emerged as a valuable tool for determining risk of prostate cancer. Directed biopsy of the prostate based on the MRI decreases both overdiagnosis and underdiagnosis of anterior and transition zone cancers.
mpMRI uses three parameters for assessing presence of cancer in the prostate.
Morphological assessment by T2-weighted imaging (T2WI)
Diffusion weighted imaging (DWI)
Dynamic contrast enhanced perfusion imaging (DCE)
T2 imaging is a morphological assessment while the other two are considered functional assessments.
The best equipment today for a prostate MRI is a 3 Tesla magnet although a 1.5 Tesla magnet may be accurate if a endorectal coil is used along with it.
T2 weighted imaging provides high spatial resolution and defines zonal anatomy and also the nerves of erection are outlined in the T2 weighted imaging. However, T2 abnormalities can occur in benign conditions such as scar tissues, fibrosis, prostatitis as well as enlarged prostate.
The DWI is a measure of the movement of water in the cells and in cancer, there are more cells and less water and the density of tissue is increased. The increased number of cancer cells restricts water movement and the diffusion weighting is expressed as an apparent diffusion coefficient or ADC map. Most abnormal areas in the prostate that have an ADC value less than 1000 usually have cancer.
DCE or Dynamic contrast enhanced MRI has the ability to track the enhancement of tissue and therefore provides a measure of angiogenesis, on which tumor growth is dependent. DCE is performed by acquiring fast T1W images, prior to, during, and after contrast injection. DCE has high sensitivity, reported to be in the range of 74–96%, which is useful for the preliminary detection of tumors. It is widely used for staging and in monitoring therapeutic response of prostate cancer.
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MRI in Patients with Recurrence after Radiation
Two thirds of recurrent prostate cancer after External Beam Radiation (EBRT) tends to occur at the site of the primary tumor. Radiation to the prostate induces gland atrophy and fibrosis and causes diffusely decreased T2 signal. Decreased T2 signal after radiation results in loss of visualization of the normal zonal anatomy on T2W images and may impair the ability to detect recurrent tumor, which is typically T2 hypointense. The fibrotic changes and decreased microvasculature associated with atrophic tissue following radiation tend to make the neovascularity of recurrent tumor more conspicuous relative to the remainder background atrophic tissue. Therefore, Dynamic Contrast Enhanced Imaging (DCE) has excellent ability to detect recurrence. The recurrence is seen as an area of contrast enhancement.
It has been found that diffusion weighted imaging(DWI) can detect prostate cancer because diffusion weighting reflects tumor cellullarity and tumor aggressiveness. The tumor aggressiveness is a reflection of rapidly dividing cells and cell density which prevents movement of water molecules within and outside of cells. It has been shown that DW Imaging is better than T2 weighted imaging alone. For detection of recurrence after radiation addition of the DCE imaging to T2W images and ADC images may not be incremental. Donati et al. found that the combination of T2W and DWI achieved the highest diagnostic accuracy and inter-reader agreement in the detection of prostate cancer after radiation therapy. The addition of DCE to T2W and DWI did not significantly improve the detection of recurrent prostate cancer in their study. They concluded that in the evaluation of recurrence after radiation, DCE imaging can be omitted from the protocol without sacrificing diagnostic performance.
Performing a non-contrast prostate MR examination has advantages, including eliminating the risks and costs associated with intravenous administration of gadolinium-based contrast agents. The MR scan time is also reduced. It should be noted that MR with DCE Imaging should not be performed within 3 months of radiation because of inflammation in the prostate which increases blood flow and gives a false result.
Brachytherapy and High Dose Radiation both cause diffuse hypointense T2 signal in the prostate gland and loss of the normal zonal anatomy, which limits morphologic imaging sequences in the detection of recurrence. Following bracytherapy the diffusion weighting may be inaccurate because of artifacts and image distortion due to radiation seeds. A Dynamic contrast enhancement study is more important after brachytherapy (seed placement) than after external beam therapy because DWI is not useful and DCE can detect area of recurrence.
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