The presence of visible lesions on prebiopsy MRI increases the likelihood of PCa diagnosis. For classifying visible lesions identified on prebiopsy MRI, the PI-RADS was developed for predicting csPC and interpreting MRI results3,10. In order to increase the efficacy of using prebiopsy MRI results for the prediction of PCa and csPS11–14, new methods for the classification of lesions are needed. We hypothesized that image analysis of hypoechoic lesions on TRUS matched with target lesions on MRI in MRI/TRUS fusion-targeted biopsies could be used effectively for predicting PCa and csPC. Using grayscale values in addition to conventional variables was helpful for increasing the probability of correctly detecting csPC. Especially, with respect to PI-RADS 3 lesions, grayscale values were identified as the only discriminating factor for benign and cancerous lesions.
Until now, several studies have reported that the presence of hypoechoic lesions with definite visibility in TRUS is related to the diagnosis of csPC with or without visible lesions on MRI2,15. However, in several guidelines, hypoechoic lesions have not been included as indicators for prostate biopsy1,16,17. The reasons were considered as lack of reproducibility and representativeness associated with subjective measurements of hypoechoicity for selecting hypoechoic lesions and the fact that increasing cancer aggressiveness has an effect on fibrotic change and echogenicity, which show apparent differences on ultrasound images of PCa18. To overcome limitations regarding reproducibility and representativeness, in our previous study, we reported the usefulness of grayscale values for the quantitation and stratification of hypoechoic lesions on cancer aggressiveness9. In the present study, through quantification with grayscale values, we showed that the characteristics of visible lesions observed through TRUS reaffirmed the results of a previous study2.
To the best of our knowledge, no studies involving the use of numerical analyses, such as the analysis of grayscale values for explaining the relationship between increasing cancer aggressiveness and hypoechoicity, have been conducted. Therefore, we collected and analyzed the images obtained during MRI/TRUS fusion-targeted biopsies. Regarding the measurement of grayscale values for target lesions, no difference was found between the median grayscale values for benign and cancerous lesions. However, the grayscale value interquartile range for benign lesions was larger than that for cancerous lesions. Therefore, we identified the optimal grayscale range for predicting PCa and csPC. Finally, we confirmed that the grayscale range was a significant predictive factor of PCa and csPC. Combined evaluation of clinical variables and images increases the accuracy of predicting csPC, which is consistent with the results of our previous study9.
For PI-RADS 3 lesions, there was a relatively low predicting ability for PCa diagnosis19–21. Therefore, several researchers investigated the discrimination of PI-RADS 3 lesions using clinical parameters and biomarkers22,23. Sheridan et al. reported that when PI-RADS 3 lesions are present, csPC detection rates can be increased by evaluating factors such as clinical parameters (age ≥ 70 years, size ≤ 36 cm3, positive digital rectal examination)7. Another study found that evaluation of the Prostate Health Index in addition to MRI results improved the predictive performance for overall cancer (AUC 0.71 vs. 0.60) and csPC detection (AUC 0.75 vs. 0.64)5. Our findings may improve the rate of accurate overall cancer diagnosis (AUC 0.70 vs. 0.66) among individuals with PI-RADS 3 lesions. For predicting csPC in PI-RAD 3, the grayscale characteristics of target lesions was the only factor in this study.
Certain meaningful findings were made in this study. First, we presented the grayscale range for predicting csPC through a quantitative image analysis of target lesions matched in MRI/TRUS fusion-targeted biopsies. This use of grayscale values is a novel method to stratify suspicious cancer lesions on MRI. Second, previous studies on PCa diagnosis in hypoechoic lesions estimated with low accuracy due to several reasons. Through this study, we present objective findings that explain correlations between echogenicity and histologic grade, which were not made in previous studies18. Furthermore, we qualitatively confirmed this change pattern for the first time through the grayscale trend according to the Gleason grade group. Measurement of the grayscale values requires minimal expenditure because they are measured using ultrasound, which is used for TRUS, and PACS can be used to store TRUS results, instead of a new equipment. A limitation of our study is that it is a single-institution study. For external validation, a multicenter study must be performed in future. Moreover, it is necessary to study whether different results would be obtained if ultrasonic equipment produced by different companies is used.
The efficacy of prebiopsy MRI has been proven; however, with respect to detecting PCa, results associated with PI-RADS 3 lesions have been equivocal. We confirmed the distribution of grayscale values according to the PI-RADS scores and found that the grayscale values of hypoechoic lesions on TRUS are an important predictor of csPC. Our results showed that for PI-RADS 3 lesions, grayscale values were useful to predict csPC, and the cut-off range was 42.9–71.9.