Journal of Medicine in the Tropics

REVIEW ARTICLE
Year
: 2021  |  Volume : 23  |  Issue : 1  |  Page : 1--10

Contemporary role of multiparametric magnetic resonance imaging in the management of prostate cancer


Idorenyin C Akpayak1, Kenis S Felangu2, Lemech E Nabasu3,  
1 Division of Urology, Surgery Department, Jos University Teaching Hospital, Jos, Nigeria
2 Radiology Department, Nisa Hospital, Abuja, Nigeria
3 Surgery Department, Nisa Hospital, Abuja, Nigeria

Correspondence Address:
Dr. Idorenyin C Akpayak
Division of Urology, Surgery Department, Jos University Teaching Hospital, Jos
Nigeria

Abstract

Background: In contemporary practice, multiparametric magnetic resonance imaging has become a useful tool to differentiate between prostate cancers of high and low aggressiveness, reduce misdiagnosis, overdiagnosis and therefore overtreatment. This article aims to provide a concise review of the multiparametric magnetic resonance imaging (mpMRI) of the prostate, its interpretation and its role in the current management of prostate cancer. Methods: his was a narrative review of the contemporary role of the mpMRI in the management of prostate cancer. The databases and journals in urology and radiology were searched for relevant and contemporary existing literature on the subject. Results: We reviewed the technical aspects of the mpMRI of the prostate, describing the T-2 weighted imaging, the diffusion weighted imaging and the dynamic contrast enhanced imaging as well as the magnetic resonance spectroscopy of the prostate. We also reviewed the current interpretation and reporting of the mpMRI of the prostate using the PI-RADS; as well as the contemporary role of the mpMRI in prostate cancer management. Conclusion: The mpMRI is technologically robust and fast evolving imaging modality that has become a significant tool in the diagnosis, staging and treatment planning of prostate cancer.



How to cite this article:
Akpayak IC, Felangu KS, Nabasu LE. Contemporary role of multiparametric magnetic resonance imaging in the management of prostate cancer.J Med Trop 2021;23:1-10


How to cite this URL:
Akpayak IC, Felangu KS, Nabasu LE. Contemporary role of multiparametric magnetic resonance imaging in the management of prostate cancer. J Med Trop [serial online] 2021 [cited 2021 May 7 ];23:1-10
Available from: https://www.jmedtropics.org/text.asp?2021/23/1/1/314844


Full Text



 INTRODUCTION



Prostate cancer is the most commonly diagnosed cancer in men globally.[1] It is commoner in blacks and the commonest cancer in Nigerian men.[2],[3] Also, in Nigeria, prostate cancer is the leading cause of cancer death in men.[2]

Traditionally, the diagnosis of prostatic carcinoma is based on clinical presentation of the patient, digital rectal examination (DRE), evaluation of prostate specific antigen (PSA) followed by TRUS-guided prostate biopsy.[4],[5] However, the DRE has very low sensitivity and can only detect tumours with volume over 0.2 ml.[6] Also, the level of PSA rises with prostate cancer, benign prostatic hyperplasia (BPH), prostatitis, prostatic infarct and prostatic injury. Therefore, PSA as a biomarker is organ specific but not disease specific.[7],[8]

The 12-core systematic TRUS-guided prostate biopsy is the current gold standard technique in obtaining specimen for histological diagnosis of prostate cancer.[9] The technique however is affected by sampling errors and fails in occasions to detect clinically significant prostate cancer.[10],[11]

These shortcomings put together may lead to prostate cancer misdiagnosis and inaccurate risk assessment of the cancer. Prostate cancer screening carried out with the hope of making early diagnosis depends heavily on the above traditional diagnostic process and therefore also suffers prostate cancer misdiagnosis and additionally over-diagnosis and therefore overtreatment.[12],[13]

Magnetic resonance imaging (MRI) as an imaging modality demonstrates both the anatomy and the pathological lesions based on the principle that atomic nuclei in a strong magnetic field absorbs pulses of radiofrequency energy and emit them as radiowaves that can be received, then reconstructed into 2D images.[14] The technical improvements and introduction of functional parameters have improved the accuracy of the MRI.[15]

In contemporary practice, multiparametric magnetic resonance imaging (mpMRI) has become a useful tool to differentiate between prostate cancers of high and low aggressiveness, reduce misdiagnosis, overdiagnosis and therefore overtreatment.[16],[17] Currently, MRI has made a tremendous impact in the management of prostate cancer by using a combination of morphologic and functional sequences which allow the analysis of aggressiveness of the cancer.[18] This article aims to provide a concise review of the mpMRI of the prostate, its interpretation and its role in the current management of prostate cancer.

 METHODS



This was a narrative review of the contemporary role of the mpMRI in the management of prostate cancer. The database was searched for recent information on the contemporary role of multiparametric MRI of the prostate. The data base searched included MEDLINE, Excerpta Medica Database (EMBASE), google scholar, individual urology and radiology journals. The search terms included, “multiparametric MRI of the prostate”, “prostate cancer”, “PI-RADS score”, “interpretation of mpMRI of the prostate”, role of mpMRI of the prostate”, “indications of mpMRI of the prostate”. We included relevant images from our hospital mpMRI of the prostate data base. The second author made the interpretation of all the images.

The search were limited to English language articles from 1995 to 2020. We included studies and articles that focused on the technical aspects, the interpretation and the role of the mpMRI of the prostate.

 OVERVIEW OF THE TECHNICAL ASPECTS OF MULTIPARAMETRIC MAGNETIC RESONANCE IMAGING OF THE PROSTATE



Imaging the prostate with MRI was first described in the mid-1980s.[19] Low-field magnets and body coil technology used at that time resulted in low spatial resolution of the prostatic images. With consistent technological improvements, the MRI of the prostate has become unique and a go-to imaging modality for the visualization and characterization of lesions of the prostate.[15]

The multiparametric MRI (mpMRI) of the prostate requires the acquisition and interpretation of the prostate images from the pulse sequences of the MRI, including: the T-2 weighted (T2W) imaging, the diffusion-weighted imaging (DWI) and dynamic contrast-enhanced (DCE) imaging, with or without magnetic resonance spectroscopy (MRS).[20] Each of the MRI technique has advantages and draw backs. Therefore, satisfactory interpretation is achieved by combinations of these sequences.

The functional MRI techniques (MRSI+, DWI and DCEI) add value to the anatomic (T2WI) MRI (the workhorse of the mpMRI of the prostate) in the detection, localization, grading and staging of prostate cancer.[21] When T2WI and DWI are of diagnostic quality, DCEI plays a minor role in determining the prostate Imaging-Reporting and Data system (PI-RADS) assessment category of lesions of the prostate.[22]

Both 1.5T and 3T MRI machines are currently used for mpMRI of the prostate. The 3T MRI machine is becoming increasingly available and is generally preferred due to higher signal-to-noise ratio (SNR) with resultant better image resolution.[23] Beyond the strength of the magnetic field, many technical factors have influence on the SNR and spatial resolution of the mpMRI of the prostate. These include the receiver bandwidth, the coil design, and the efficiency of the radio frequency (RF) coil.[24]

The contemporary 1.5T and 3T MR scanners employ external array of coil elements capable of achieving adequate SNR.[25] When integrated with external array coils, the endorectal coil (ERC) has been thought to increase the SNR.[26] However, the drawback of the ERC is that it may increase the cost and time of the examination, deform the prostate gland and introduce artefacts. It may also increase patient discomfort and reduce acceptability of the mpMRI.[27] With current family of 1.5 and 3T MR scanners, ERC has not been found to be essential in achieving satisfactory image resolution of the prostate.[28]

T-2 weighted imaging

The T-2 weighted imaging is the mainstay of the mpMRI of the prostate and it is traditionally carried out in axial, sagittal and coronal planes.[29] With the fast-spine echo (FSE), high SNR allows the T2W sequence to provide high spatial resolution of the zonal anatomy of the prostate gland.[20] With this sequence, it is frequently possible to differentiate the high signal intensity PZ from the TZ [Figure 1]A and 1B.{Figure 1}

In the T2WI, the prostate cancer appears as area of low signal intensity within the high intensity signal of the peripheral zone (PZ).[30] Also, it has been found that the higher the Gleason score in histological evaluation, the lower the signal intensity on the T2WI of the mpMRI.[31]

The limitation of the T2WI is the lack of specificity for areas of low signal intensity in the PZ of the prostate as this may represent chronic prostatitis, post-prostatitis scars, atrophy and post-biopsy haemorrhage.[32] These conditions may be correctly identified as benign on the basis of their wedged-shaped or diffuse appearance on T2WI. Yet, their presence makes assessment of PZ on T2WI difficult. Therefore where possible, mpMRI of patients suspected of having prostate cancer should be delayed 8 weeks after prostate biopsy or should be repeated after six month of a suspected prostatitis to allow reduction of artefacts due to post-biopsy haemorrhage or prostate inflammation.[33],[34] The use of T2WI is also limited in the transition zone (TZ) of the prostate where benign prostatic hyperplasia appearance is similar to that of prostate cancer focus.[35]

Diffusion weighted imaging

Diffusion weighted Imaging (DWI) is an essential sequence for the detection and prediction of prostate cancer aggressiveness.[36] The Diffusion weighted images are produced by taking advantage of the diffusion properties of proton in water to produce image contrast.[22] The apparent diffusion coefficient (ADC) is obtained from DWI and signifies the movement of water molecules within the interpluse time.[37]

The normal prostatic tissue, including PZ is rich in water and tubular structures which allows extensive diffusion of water molecules within the gland. Consequently, the ADC in the normal prostate gland is high.[38] When prostate cancer develops, the normal tubular structure is replaced and the gland becomes higher in cellular density. The ADC then becomes lower at the tumour focus compared to the normal surrounding tissues. With lower ADC compared to the surrounding healthy prostatic tissue, the prostate cancer foci appear hypointense on ADC maps but hyperintense on the diffusion weighted DW), at high b-value image [Figure 2]A and 2B.[39] The limitation of the DWI is that it has poor spatial resolution, it is susceptible to motion artefacts as well as magnetic inhomogeneity, and therefore not be used for local staging.[40] As a result, DW images need to be correlated with T2W images.{Figure 2}

Dynamic contrast-enhanced imaging

Dynamic contrast-enhanced imaging (DCEI) consists of series of T1-weigheted sequences of the entire prostate gland before and after rapid bolus injection of 3-4 ml/s of gadolinium contrast agent.[41],[42] It asses signal intensity changes to detect areas with pharmacokinetic features seen in cancer angiogenesis. Where there is prostate cancer, the focus enhances faster and to a greater extent compared to the normal prostatic tissues. The focus also shows earlier contrast washout [Figure 3]A and 3B.[41],[42]{Figure 3}

In radiologic practice, post-processing of DCEI to generate colour overlay maps on T2WI are used to depict signal intensity. The colour overlay images are semi-quantitative data that represents the concentration of gadolinium in tissue with time (the gadolinium concentration-time curve).[43] Despite these efforts, the heterogeneity in the enhancement characteristics of prostate cancer adds to poor image interpretations.

The drawback of DCEI is that the visual and subjective comparison of preconstrast and postcontrast images in the background of hypervascularised tissues of the prostate can be challenging.[44] Also, in DCE imaging, the features of BPH, chronic prostatitis and prostate cancer can also overlap.[44] Therefore, currently there is little evidence to support its routine use.

Magnetic resonance spectroscopy

This is a functional technique in which the spectral profiles reflecting resonance frequencies that are unique for protons in different metabolites (choline, citrate and creatinine) are measured. The choline and creatinine signals are increased in prostate cancer compared to the normal tissues of the prostate, while those of citrate decrease in prostate cancer compared to the normal prostatic tissue.[45] A (choline + creatinine)/citrate ratio >0.86 or citrate/(choline + creatinine) ratio of <1.4 was found to be specific marker for prostate cancer.[45]

Currently, magnetic resonance spectroscopy (MRS) is not routinely included in mpMRI of the prostate because of the long time required to acquire the data, as well as the experience and the expertise required to interpret the images. With new technological advancements, the robustness of MRS will further increase and it would likely become a routine part of mpMRI of the prostate.[46]

 INTERPRETATION AND REPORTING OF MULTIPARAMETRIC MAGNETIC RESONANCE IMAGING (PI-RADS CATEGORY)



The reporting system used for mpMRI of the prostate has changed overtime in the bid to reduce inter-reader errors and disagreement as well as decrease the gap between differently skilled radiologists and improve the communication between radiologists and urologists. To achieve this purpose, the Prostate Imaging-Reporting and Data System version1 (PI-RADS vs 1) was developed in 2012 by the European Society of Urological Radiology (ESUR).[47] This was the first attempt to standardize prostate mpMRI reporting. The PI-RADS v1 was complex and a time consuming scoring system and therefore had poor reproducibility.[48]

PI-RADS v 2 was introduced in 2014 in an attempt to overcome the limitations of PI-RADS v 1 by the joint steering committee, formed by the ESUR, the American College of Radiology (ACR) and the AdMeTech foundation.[22] The PI-RADS vs2 assessment uses a 5-point scale [Table 1] and [Table 2][29] based on the likelihood (probability) that a combination of mpMRI of the prostate findings on T2WI, DWI and the DCEI correlates with presence of a clinically significant prostate cancer for each location in the gland. A detected focus is scored separately by the PI-RADS assessment on the different MRI techniques. Then, they are combined to give overall assessment category score to quantify the likelihood of the focus being clinically significant prostate cancer.{Table 1}{Table 2}

More recently, an updated version of PI-RADS v 2, the PI-RADS version 2.1 aimed to further simplify the assessment and reporting, as well as to reduce interpretation variability of the mpMRI of the prostate.[49] The PI-RADS v 2.1 emphasizes the dominant role of DWI as the parameter for any suspicious lesion(s) found in the PZ and T2WI, in the TZ. For instance, if DWI score is say 4 in PZ, and T2WI score is 2, then the overall PI-RADS assessment category should be 4. On the other hand, in TZ, T2WI is the primary (dominant) sequence. So, if T2WI score is 4 in the TZ lesion, and DWI PI-RADS score is say 2, then the PI-RADS assessment category should be 4.[49]

DCEI scores are a binary assessment, and serves to reduce the number of equivocal lesions (PI-RADS 3 lesions). Its role is limited to upgrading DCEI-positive lesion(s) in the PZ from PI-RADS category 3 to 4.[48] In PI-RADS v 2.1 assessment system, a “positive” DCE (DCE +) MRI lesion is one that there is focal enhancement, earlier or contemporaneous; that corresponds to a finding on TW2 and/or DWI. A “negative” DCE (DCE −) MRI lesion is one that either does not enhance early compared to surrounding prostate or enhances diffusely so that margins of enhancing areas do not correspond to findings on T2WI and/or DWI.[49],[50] Notably, many authors believe that the added value of DCE over and above the combination of T2WI and DWI is modest. Therefore, the role of DCE in determination of PI-RADS vs 2.1 assessment category is secondary to T2WI and DWI.[51]

The role of multiparameteric MRI of the prostate

The different clinical scenarios in which mpMRI of the prostate play important roles are:

1) To detect prostate cancer and guide patient selection for prostate biopsy:

The most important function of mpMRI of the prostate is the localization of the prostate cancer.[52],[53],[54] Multiparametric MRI is currently used to detect clinically significant prostate cancer.[18] In PI-RADS v 2.1, clinically significant cancer is defined on histology/pathology as cancer with Gleason score ≥7, and/or volume ≥0.5ml, and/or extraprostatic extension (EPE).[22] The PI-RADS vs 2.1 does not however, make recommendations for the management as that takes into account other factors like clinical history, patient PSA and clinical examination findings.[22]

The mpMRI also facilitates MRI guided biopsy (MRGB) of the clinically significant prostate cancer foci. All MRGB strategies (cognitive MRI-ultrasound fusion, MRI-TRUS fusion and in-bore MRGB) requires that information from mpMRI is analysed and used to target suspicious focus.

Many studies have demonstrated that MRI-targeted prostate biopsy is better than the systematic biopsy in the diagnosis of clinically significant prostate cancer.[55],[56],[57] Consensus on selecting patients for biopsy, has not been fully agreed upon. For instance, the EAU guideline on prostate cancer suggests that mpMRI could be used in the following clinical scenarios:To increase the rate of detection of prostate cancer by addition of targeted biopsy to systematic biopsy in patients in whom mpMRI detects a clinically significant prostate cancer. b) To guide when to proceed on systematic prostate biopsy where mpMRI of the prostate is negative and PSA is high. c) To guide when not to proceed on systematic prostate biopsy where the mpMRI is negative.To facilitate only targeted prostate biopsy.[46]

Ahoot et al.[58] colleagues in a more recent study: MRI-targeted, systematic and combined biopsy for the prostate cancer diagnosis took a critical look at what prostate biopsy technique could yield the best outcome. They found that combine (systematic and targeted) biopsy leads to an increase in the number of cancer diagnosis and improves likelihood that the biopsy findings are predictive of the true pathologic nature of the patient’s disease.

In instances where patients had previous negative systematic prostate biopsy, mpMRI is carried out to identify the clinically significant prostate cancer foci that might have been missed and proceed to target biopsy of the foci.

2) To guide patients selection for active surveillance:

Up to 40% of all men over 50 have been found to have lower risk prostate cancer.[59] Over the last decade, active surveillance (AS) has emerged as a treatment option for men with early-stage disease, low-risk prostate cancer.[60] However, the choice of AS is riddled with many uncertainties including optimal patient selection and strategic triggers for intervention.[61]

Epstein criteria has been the most popular tool to identify patients with clinically insignificant prostate cancer and thus eligible for AS.[62] The criteria specify that men should have no more than 2 cores involved, with less than 50% of any one core involved, and a PSA density of less than 0.15ng/ml.

Recruiting prostate cancer patients that are suitable for AS is recently made possible by mpMRI due to its significant ability to discern clinically significant prostate cancer from the low risk ones. Almeida et al.[63] in assessing the role of mpMRI and PI-RADS score in patients with prostate cancer concluded that mpMRI in addition to clinical criteria helps in the selection of patients for AS.

3) To guide staging and treatment of prostate cancer:

The local staging of prostate cancer has been made possible by T2WI.[64] Typically, the PZ tumour appears as hypointense foci [Figure 4]. Once such lesion is detected, evaluation of the tumour is required. The most important overall assessment is whether the tumour is confined to the gland (T≤2) or has extended beyond the gland (≥T3). This could detect the extraprostatic extension, extension to seminal vesicle and the bladder neck. The feature of seminal vesicle (SV) invasion include focal or diffuse low T2W signal intensity [Figure 5], abnormal contrast enhancement within the SV, restricted diffusion on DWI/ADC map and obliteration of the angle between the base of the prostate and the SV.[15]{Figure 4}{Figure 5}

In contemporary practice, multiparametric magnetic resonance imaging has become a useful tool to differentiate between prostate cancers of high and low aggressiveness, reduce misdiagnosis, overdiagnosis and therefore overtreatment. This article aims to provide a concise review of the multiparametric magnetic resonance imaging (mpMRI) of the prostate, its interpretation and its role in the current management of prostate cancer.

The determination of microscopic extraprostatic extension is however more difficult with mpMRI. Nodal staging is possible since the regional lymph nodes are in the field of view (FOV) of the examination. It is also possible to see metastases to the pelvic bone or head of femur on routine mpMRI of the prostate images [Figure 6].{Figure 6}

Multiparametric MRI of the prostate has also found its usefulness in the pre-operation choice of the nerve sparing radical prostatectomy. The use of mpMRI to locate the tumour, determine its clinical aggressiveness and the clinical stage, has made it useful in treatment planning for both external beam radiotherapy and intensity-modulated radio-therapy (IMRT).

 CONCLUSION



The mpMRI is fascinating and fast evolving imaging modality that has become a significant tool in the diagnosis, staging and treatment planning of prostate cancer.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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