Multiparametric MRI (mpMRI) can help prevent overdiagnosis of prostate cancer. It can potentially replace biopsy in active surveillance, help avoid biopsies in low-risk patients with normal MRI, and reduce the number of cores per biopsy in prostate cancer treatment.
“In the past, we used to do our biopsies in the randomised fashion – gradually get biopsies from the prostate. Nowadays, we can use MRI images and target the lesions directly,” said Prof. Levent Türkeri (TR). These and other benefits were discussed in his lecture “Multiparametric MRI of the prostate: Has a new era begun?” on 23 September at the 12th South Eastern European Meeting (SEEM16).
What is mpMRI?
mpMRI is any functional supplementation to the standard T1 and T2-weighted imaging. These supplementations include dynamic contrast-enhanced MRI (DCE-MRI) and diffusion-weighted imaging (DWI).
DCE-MRI is when an MRI contrast agent is injected then T1-weighted scans are acquired afterwards. DWI is when a diffusion of water molecules is used to generate contrast in MR images so that tissue characteristics can be defined.
Türkeri cited the study “Multiparametric MRI and targeted prostate biopsy: Improvements in cancer detection, localization, and risk assessment” regarding the role of mpMRI in biopsy naïve patients. The findings were that the detection rates were similar between mpMRI and the biopsy; that mpMRI consistently detected more clinically significant prostate cancer; and that majority of the tumours missed are clinically insignificant.
MRI progress through the years
“What has changed is the power of the magnetic field of the MRI. We’re able to see the lesions in the prostate more clearly. This was not the case in the last 15 years when ultrasound imaging was the standard method of choice. Most of the lesions were not clearly seen because they were not differentiable from the surrounding tissue,” said Türkeri.
Due to the standardization of the definition of the lesions seen in the MRI, the lesions that are significant become more identifiable. Thus, preventing overdiagnosis.
Using the PI-RADS score
Türkeri cited the study “Use of the Prostate Imaging Reporting and Data System (PI-RADS) for Prostate Cancer Detection with Multiparametric Magnetic Resonance Imaging: A Diagnostic Meta-analysis” which analysed 14 studies (1,785 patients). Based on the pooled results, PI-RADS showed good diagnostic accuracy in prostate cancer detection.
“Using systems like PI-RADS can help define the significance of the lesions that you see on the MRI so you can decide what to do about them,” said Türkeri. “A biopsy can be avoided unless the patient has a PI-RADS score 4 or 5, where the incidence of a clinically significant disease is quite high.”
Gleason score and lesions
The lower the Gleason score, the bigger the size of the lesion, the more it becomes detectable.
“If a patient has a high-grade disease, a small lesion can already be detected by the MRI. But if a patient has a low-grade disease, the lesion should be bigger in order to be detected. Therefore by using MRI, overdiagnosis can be prevented because the detected lesions are clinically significant in general, ” said Türkeri. “So even if the Gleason score is low but the lesion is large, the tumour can still be clinically significant.”
Added value of DWI and ADC
Apparent diffusion coefficient (ADC) is a measure of the magnitude of diffusion of water molecules within the tissue, and is calculated using MRI with DWI. The aggressiveness of a tumour can be defined through ADC and Gleason scores.
“The higher the ADC score, the farther the water molecules travel in the tissue. But the higher the Gleason score, the water molecules become restricted in their motion. So you’ll have an idea what kind of tumour it is because of the significant inverse correlation between the ADC and Gleason scores. This is important because you can use this information in the active surveillance of your patients,” said Türkeri.
mpMRI can help reduce overdiagnosis of low-risk cancer and improve the sensitivity detection of moderate to high-risk cancer. It can be a more accurate assessment of tumour grade that can reduce the amount of biopsies performed. “There’s also a possibility that it can be implemented in active surveillance protocols as well,” said Türkeri.