INTRODUCTION
Magnetic resonance imaging (MRI) is a precise modality for detecting and staging prostate cancer. Therefore, MRI has become increasingly available for men with elevated prostate-specific antigen levels to detect prostate cancer based on the Prostate Imaging and Reporting and Data System (PI-RADS). Currently, PI-RADS version 2.1 has been released to improve the risk stratification of prostate cancer [1-3]. Accordingly, radiologists and urologists need to learn MRI features to understand the PI-RADS. However, focal lesions are still categorized based on MRI findings alone. Besides, there is a wide variation in PI-RADS categorization in terms of interpretation experience, education, and MRI quality [4-7].
Recently, high-resolution transrectal ultrasonography (TRUS) has the potential to provide more detailed imaging features of prostate cancer [8-10]. Some imaging features are not visible on MRI, but are visible on TRUS alone [8-10]. TRUS cannot replace MRI completely in characterizing prostate cancer, but there is an increasing need for radiologists and urologists to use TRUS for cancer detection and biopsy. To the best of our knowledge, only a few reports have shown the added value of TRUS in the preoperative staging of prostate cancer. However, MRI has some limitations in the preoperative staging work-up or the differentiation of insignificant and significant prostate cancers. This review aimed to assess the added value of TRUS to preoperative MRI for cancer detection, characterization, and staging.
TUMOR LOCATION: MRI VS. TRUS
Before localizing a lesion on TRUS, we need to know the difference between MRI and TRUS in terms of lesion location. An MRI axis is usually perpendicular to the prostate urethra, whereas that of TRUS is oblique, coronal to the prostate [11]. Therefore, when a lesion is closer to the posterior capsule on MRI, it is detected more supperiorly on TRUS (Fig. 1A) [12,13]. Radiologist and urologist can be surprised to see a lesion in the base although it is located around the posterior capsule of mid-gland [14,15]. In contrast, when a lesion is closer to the anterior capsule on MRI, it is detected more inferiorly on TRUS (Fig. 1B) [12,13]. On TRUS, they should find a lesion at the apex when it is near the anterior capsule of mid-gland on MRI [14,15]. Also, the lesion size and shape can differ between MRI and TRUS. It is not surprising to see that a tumor size was greater or less than 1.5 cm on TRUS, even though it was a PIRADS 4 or 5 tumor. From this point of view, radiologists and urologists need to be familiar with the difference in lesion location between MRI and TRUS to precisely detect it. This is the first step in local tumor staging using TRUS. Unless radiologists and urologists understand this difference, they cannot detect or assess a focal lesion on TRUS, even though they are categorized as PI-RADS 4 or 5 on MRI [11-13,16,17]. For the same reason, the size and shape of a lesion is different between MRI and TRUS.
PERIPHERAL CANCER: SIGNIFICANT VS. INSIGNIFICANT
Generally, peripheral cancer appears as a hypointense tumor on T2-weighted imaging (T2WI) and hyperintense tumor on diffusion-weighted imaging (DWI). As the Gleason score (GS) increases, a lesion tends to be more hypointense on T2WI and more hyperintense on DWI (Table 1). Tumor size is useful only for differentiating between PI-RADS 4 and 5. On TRUS, peripheral cancer is usually observed as a hypoechoic tumor. As the GS increases, a lesion tends to be more hypoechoic (Table 1). Additionally, the contour of a peripheral cancer becomes irregular and infiltrative when it becomes significant (Table 1). These morphological changes are well observed on TRUS, but are not clearly defined on MRI (Figs. 2, 3). Therefore, PIRADS version 2.1 does not use these findings in lesion categorization [18]. All anatomical changes, except tumor size, do not contribute to differentiating PI-RADS 4 and 5 peripheral lesions, although the frequency of significant cancers is significantly different [18,19]. This is one of the limitations of the PI-RADS version 2.1 decision rules.
TRANSITION CANCER: SIGNIFICANT VS. INSIGNIFICANT
Generally, the charcoal-eraser sign is a well-known T2WI feature [20]; however, it requires time and experience to apply it in daily practice. Therefore, the current PI-RADS version 2.1 describes the charcoal-eraser sign as a lenticular or non-circumscribed homogeneous mass with moderately hypointense signal intensity on T2WI [1,21]. However, it does not state which MRI features are suggestive of significant cancer except for increasing tumor size (Table 1). In contrast, transition cancers become more heterogeneous and hyperechoic, in addition to increasing tumor size as the GS increases (Table 1, Figs. 4, 5) [12,22]. Additionally, the contour of a transition cancer becomes irregular and infiltrative as the GS increases [22]. Similar to peripheral cancer, all anatomical changes except tumor size do not contribute to differentiating between PI-RADS 4 and 5 transition lesions, although the frequency of significant cancers is significantly different [18,19]. This is one of the limitations of the PI-RADS version 2.1 decision rules.
LOCAL AGGRESSIVE FEATURES
Extracapsular extension (ECE) and seminal vesicle invasion(SVI) are typical locally aggressive features of prostate cancer. MRI is a good imaging modality for depicting ECE [23-25]. However, TRUS can provide additional findings to determine ECE when MRI is inconclusive [13,18]. A thick and irregular surface of a prostate capsule is a good finding suggesting ECE on TRUS (Fig. 3). Color Doppler TRUS helps to enhance the chances of identifying ECE if it shows hypervascular blood flow crossing the capsule from the tumor to the extracapsular fat. It is not difficult to detect ECE using TRUS if radiologists or urologists know how to achieve good image quality. MRI has been reported to be useful [26-28]; however, the role of TRUS in depicting SVI is controversial. It can show thick hypervascular walls in the seminal vesicle. Further investigations are necessary to demonstrate how TRUS detects SVI.
CURRENT LIMITATIONS OF USING TRUS
Many radiologists and urologists perform TRUS procedures; however, almost none of them can obtain a good image quality or perform biopsy techniques. They must use fundamental scanning instead of harmonic scanning, which is the default setting for current TRUS scanners. The former offers better tissue contrast than the latter to improve tumor depiction [12,13]. Another scanning technique for increasing tissue contrast is to use a lower dynamic range of < 50 [12,13]. These scanning techniques decrease image resolution, but increase tissue contrast to help differentiate prostate cancer from normal tissues (Figs. 2-5). Therefore, although fundamental scanning and low dynamic range reduce image resolution of TRUS, these protocols contribute to tumor detection and characterization. As a result, it is easy to identify a tumor that is more contrasted with normal tissue and to assess tumor size, echogenicity, texture, and contour.
When performing TRUS, radiologists and urologists should minimize prostate compression prior to tumor detection (Fig. 2). Frequently, small peripheral cancers are difficult to detect because they tend to be embedded due to transducer compression. Additionally, this compression deforms the prostate contour from a triangular to a banana shape (Fig. 4) [13]. This contour deformity also makes MRI-TRUS image fusion more difficult to register [12,13]. Therefore, a transducer should be placed in the posterior prostate capsule until a focal lesion is detected to avoid prostate deformation. The transducer is then pushed to the target tumor if the lesion contour is not too deformed. The contour deformity increases as the tumor approaches the posterior capsule. Therefore, transducer compression does not influence tumor contour so much, as the tumor approaches the anterior capsule (Figs. 4, 5).
CONCLUSION
TRUS can show various imaging findings that are not defined to characterize a focal lesion on PI-RADS version 2.1 decision rule. Therefore, when combined with MRI, TRUS contributes to the characterization of prostate cancer. If radiologists or urologists are familiar with TRUS techniques and imaging features, they can significantly improve cancer detection and preoperative staging work-up more precisely, and then make more appropriate treatment plans.
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