Several MR imaging criteria for ECE have been used. Table 1 presents commonly used criteria for ECE with its specificity and sensitivity. Most frequent used criteria are asymmetry of the neurovascular bundle, obliteration of the rectoprostatic angle, and bulging of the prostate capsule (Fig. 3). SVI is detected by an abnormal, asymmetric, low signal intensity within the lumen on T2-weighted images (Fig. 4) [15]. It should be noted that amyloid deposits, stones or blood could also cause low signal intensity of the seminal vesicles on T2-weighted images [14-17].

In staging, MR imaging should have a high specificity for periprostatic extension, to ensure that only few patients will be deprived of a potentially curative therapy [18]. Sensitivity for periprostatic extension is of minor importance, because even a low sensitivity is an improvement on clinical staging [18]. MR imaging is considered cost-effective if performed in a subgroup of patients with a prior-probability of ECE of at least 30%; that is, a PSA greater than 10 or a Gleason grade greater than 7 [19].

The initial accuracy in 1990 for the staging of prostate cancer with MR imaging was 69% [20]. Since then the most prominent change was the development of an en-dorectal coil (ERC), which resulted in faster imaging and improved spatial resolution. Accuracy for ECE with the ERC has a wide range, between 58-90% [21-24]. Several reasons for this wide range can be given. Firstly, due to the rapidly developing MR imaging technique, different studies used different imaging protocols. Secondly, due to inexperience with this new method, considerable interobserver variation may be present. A third important reason is that different studies use different criteria for ECE (Table 3) resulting in different accuracies. Although this variation remains, the use of an ERC is considered to be an improvement of the conventional MR examination [23-25]. Although major developments have changed the MR imaging technique, it is still not possible to detect microscopic ECE [20, 22, 26, 27]. The detection of SVI is generally not

Table 1. Criteria to predict extracapsular extension of prostate cancer

Criteria for capsular penetration Acc Spec Sens PPV

Asymmetry of neurovascular bundle 70% 95% 38% -

Obliteration of rectoprostatic angle 71% 88% 50% -

Overall impression 71% 72% 68% 32%

Extracapsular tumor 73% 90% 15% 34%

Acc, accuracy; Spec, specificity; Sens, sensitivity; PPV, positive predictive value; -, no data available a ni:

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