MRI of the prostate, a useful test between PSA measurement and biopsy

Over the last ten years or so, the detection of prostate cancer using MRI (magnetic resonance imaging) has become much more effective, thanks to the acquisition of more precise images.

DiffusionMRI has made its mark, demonstrating its reliability in locating tumour foci⁽¹⁾. The data from this examination can now be used to guide the biopsy of a suspected lesion. It is therefore now possible to apply the general principle of oncology to the prostate, which applies to every organ in the body: first locate the target, then guide the sampling by imaging.

By Drs. François Cornud and Arnaud Lefevre, Clinique de l'Alma (Paris)

Advances in MRI

The imaging department at the Clinique de l'Alma has acquired a top-of-the-range 1.5 Tesla MRI, thanks to the addition of hypergradients inside the magnet and a 60-element receiving antenna.

This cutting-edge technological configuration produces high-resolution images and high signal power for diffusion MRI, giving image quality approaching that of 3 Tesla magnets.

It also makes it possible, for the first time, to do away with the endorectal antenna which, while maintaining unrivalled image quality, is a source of discomfort that, although moderate, is felt by the majority of patients.

Another factor contributing to progress is the post-processing of diffusion images, which increases the visibility of cancerous tumour foci (fig. 1).

Figure 1 - Diffusion MRI
-The tumour (PIRADS score 4) is in T2 hyposignal (arrow). Its visibility on diffusion MRI increases as the diffusion gradient is increased (value of b), while the signal from the benign prostate gradually fades. The diffusion coefficient (ADC) is low. Tumour with a Gleason score of 3+4 on targeted biopsy.

An MRI score to classify the presumption of malignancy for a detected lesion

Progress in the quality of MRI has led to the development of a diagnostic score on a scale of 1 to 5 (PI-RADS score) to determine the presumption of cancer ⁽²⁾. Scores 1 and 2 correspond to the absence of focal abnormality and are considered benign. Scores 3 to 5 refer to focal abnormalities detected on diffusion images. The presumption of cancer is 90% for a score of 5.

The advantages of MRI for ultrasound-guided biopsy

Ultrasound-guided biopsy based on MRI data dramatically improves the performance of ultrasound guidance by incorporating MRI images during ultrasound ⁽³⁾.

The simplest incorporation of MRI can be done visually. In fact, around 80% of PI-RADS 4 or 5 MRI lesions in the posterior prostate are detectable on ultrasound ⁽³⁾ by a trained operator. With high-quality equipment, ultrasound performed after MRI provides an optimal detection rate. The biopsy is targeted at the lesion detected on ultrasound thanks to the MRI.

If the lesion is not visible on ultrasound, the MRI and ultrasound volumes are superimposed (or merged) using appropriate software (Figure 2). The MRI target is transferred to the ultrasound and the biopsy is guided by image registration. This registration, based on a computer calculation, gives an acceptable accuracy of 3-4 mm on average in relation to the target, although the registration error may reach 5 mm or more in 25% of cases ⁽⁴⁾.

Figure 2 - Biopsy with image fusion - The suspected lesion (arrow), not visible on ultrasound, was propagated on ultrasound (yellow circle, drawn on MRI). Biopsies were guided by ultrasound into the target. Anterior tumour, Gleason score 4+3 on biopsies and radical prostatectomy specimen (small white arrows)

Compared with the 10-12 systematic biopsies spread over the posterior part of the prostate, MRI-targeted ultrasound-guided biopsy has two advantages: it limits the number of samples taken to three (reducing morbidity) and enables aggressive cancers to be diagnosed, avoiding the detection of indolent micro-cancers ⁽⁵⁾.

Biopsy under real-time MRI guidance with robotic assistance

The performance of ultrasound-guided biopsy with image fusion diminishes when the lesion becomes deep and when it is small. In these difficult cases, guiding the biopsy directly under MRI has become an attractive alternative (Figure 3) because the same imaging medium is used to select the lesion and guide the needle ⁽⁶⁾. This avoids the registration errors inherent in image fusion.

Figure 3 - MRI-guided biopsy with robotic assistance - Anterior cancer (arrow) missed by image fusion biopsies. The robot, placed between the patient's legs during the biopsy, aligns the path of the guide and the target. The biopsy is carried out by the doctor: cancer Gleason score 3+4 on both samples.

Robotic assistance considerably simplifies the procedure. The robot is made of MRI-compatible material. Its motors are driven by compressed air delivered by a compressor located outside the MRI room. The adjustment time for aligning the needle guide and the target is calculated by the software that controls the robot. This takes just a few minutes and the sampling time is identical to that for an ultrasound-guided biopsy. Since we have been using the robot to guide the biopsy under MRI, we have also found that the precision of the guidance is optimal, particularly for small lesions located anteriorly (Cornud et al., submitted for publication).

MRI before biopsy: the filter for detecting aggressive tumours

The very high negative predictive value of MRI ⁽⁷ ) can be used to select patients with high PSA levels for biopsy.

- If a PIRADS 4 or 5 lesion is detected, immediate biopsy is recommended to detect aggressive cancers.

- In the case of a PIRADS 1 or 2 score, the detection rate of an aggressive cancer not visible on MRI is less than 10%. It is therefore increasingly suggested that sampling be deferred and that regular PSA monitoring be introduced. In the event of progression, MRI can be repeated to detect the appearance of a suspicious lesion.

This strategy can also be applied to certain equivocal lesions, PIRADS score 3, but judged benign on so-called quantitative diffusion MRI (Pierre Th, F Cornud et al, Eur radiol, 2017, in press), which avoids an immediate biopsy, the risk of which is to detect a micro-cancer for which therapeutic abstention is recommended and which therefore does not need to be diagnosed. In this way, patients are spared the sometimes negative psychological impact of being told they have a micro-cancer that simply needs to be monitored.

References

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