Prostate cancer (PCa) has recently become the most prevalent malignant disease and was categorized as the 6th highest cause of death in Japanese men [1]. In addition, radical prostatectomy is a first-line therapy, and a robot-assisted procedure has become a standard method for patients with localized PCa. Despite the widespread use of prostate-specific antigen (PSA) screening, a saturation biopsy protocol with multiple cores, and progress in diagnostic imaging, contemporary PCa patients still include a highly heterogeneous population of oncological outcomes. The established risk stratification system consisting of PSA, Gleason score (GS), and clinical T stage seem to insufficiently identify patients with unfavorable pathologic features preoperatively, leading to biochemical, local, and systemic recurrence [2]. We previously raised this issue from data that selected Japanese patients with low-risk PCa who still demonstrated advanced-stage (≥ pT3) disease at around 15% [3]. Another Japanese study group compared five established risk stratification systems in their cohort, and found that all stratification systems could not discriminate between low- and intermediate-risk groups in terms of biochemical recurrence-free rate [4]. In view of racial differences, criteria developed from a Western cohort analysis cannot always be applied to Japanese or Asian patients [5].

Detailed information obtained from prostate biopsy has been suggested to include predictors of oncological outcomes since the sextant biopsy era [6], and protocols of multiple core biopsy with greater than 12 cores has enhanced its predictive value [7]. Currently, many urologic surgeons refer to biopsy core details for further risk assessment and decision making, and to determine whether an extended resection and/or lymph node dissection should be performed during radical prostatectomy. Furthermore, pretreatment use of PSA density (PSAD) for further risk stratification has been emphasized, not only for patients with low-risk PCa, but also those with intermediate- and high-risk PCa [8].

This study focused on these preoperative parameters and compared predictive impact on oncological outcomes, including unfavorable pathologic features and biochemical recurrence in Japanese patients with intermediate- and high-risk PCa.

The established risk stratification system is convenient but insufficient to make decisions for patients who need definitive therapy and for a urologic surgeon to decide upon the surgical strategy. On the other hand, prediction nomograms can offer individualized risk of PCa, but several variations exist and an ambiguity remains in the interpretation of the calculated probabilities. Accordingly, we must continue to refine the risk stratification system adapted to the demands of contemporary PCa patients. The aim of this study is to identify potential factors associated with oncological outcomes in the entire cohort and in each risk group, and to clarify the performance characteristics of predictive ability. Finally, we propose an improved risk assessment from the results.

Among those unfavorable pathologic findings, advanced tumor stage ≥ pT3, increased index tumor volume >0.718 ml, and high GS ≥8 showed independent predictive value for biochemical recurrence, but surgical margin status did not show independent value in this study (see Additional file 1: Table S1). During a median follow-up of 24.5 months (interquartile range 14.0–36.0), it was discovered that 19 patients (12.3%) in the intermediate-risk group and 26 patients (27.4%) in the high-risk group developed biochemical recurrence.

Figure 1 shows the Kaplan–Meier event curves for biochemical recurrence-free survivals. The entire cohort was divided into 4 subgroups according to the NCCN risk, percentage of positive cores, and PSAD: subgroup 1 (n = 100): intermediate-risk with low percentage of positive cores, subgroup 2 (n = 55): intermediate-risk with high percentage of positive cores, subgroup 3 (n = 74): high-risk with low PSAD, and subgroup 4 (n = 21): high-risk with high PSAD. Pairwise comparisons among groups revealed that subgroup 2 and 3 showed almost the same recurrence curves, and the entire cohort could be stratified into three different risk groups. The frequency of immediate recurrence was significantly higher in subgroup 4 than in the other subgroups (33.3% versus 6.1%, p < 0.001).

Our study demonstrated that PSAD and biopsy core details had predictive value for unfavorable pathologic results and biochemical recurrence in addition to the NCCN risk classification, and there was a difference in performance characteristics for the prediction of oncologic outcomes in each NCCN risk groups. The most significant predictor for biochemical recurrence was the percentage of positive cores in the intermediate-risk group and PSAD in the high-risk PCa group. To our knowledge, this study was the first attempt to improve a stratification model of intermediate- to high-risk PCa by using the difference in performance characteristics of these preoperative factors, and the entire cohort could be stratified into three distinct risk groups. Moreover, PSAD predicted biochemical recurrence more efficiently than a NCCN sub-classification of very high-risk or high-risk, and immediate recurrence was remarkably frequent in high-risk PCa patients with high PSAD. These results suggest that both PSAD and biopsy core details are important factors for predicting the oncologic outcome of contemporary PCa patients, but those with intermediate- and high-risk PCa, which include a highly heterogeneous population, are not uniformly stratified by a single factor, and a preoperative risk classification system should consider differences in performance characteristics when incorporating these factors.

The percentage of positive cores and percentage of cancer extent in each positive core are representative factors among biopsy core details to assess cancer involvement in the prostate, and these predictive values of oncologic outcome have been investigated since the sextant biopsy era [6]. Egawa, et al. reported that these two factors in conjunction with the three known variables (PSA, clinical stage, and biopsy GS) improved the predictability of non-organ-confined PCa [15]. Meanwhile, Freeland et al. reported that a combination of PSA, biopsy GS, and percentage of cancer extent defined a preoperative model for predicting PSA recurrence [16], and almost simultaneously, they reported that the percentage of positive cores from the dominant side was a slightly better predictor of PSA recurrence than the total percentage of positive cores [17]. Briganti et al. performed a comprehensive analysis of the importance of the information contained within the variable that codes either the number or the percentage of positive cores; the percentage of cores improved stage predictions, and the number of cores improved mostly biochemical recurrence predictions [18]. The protocols of multiple core biopsy would contribute to enhance the predictive value of the biopsy core details. Briganti et al. later updated their analyses by increasing the number of cores taken from biopsy, and they repeatedly emphasized that the inclusion of the percentage of positive cores should be mandatory in the prediction model for lymph node invasion of PCa [7, 19]. Hinev, et al. demonstrated that Briganti’s nomograms showed a higher predictive accuracy for lymph node invasion as compared with the Memorial Sloan-Kettering Cancer Center nomogram, which provide predictions without information on biopsy cores [20]. Despite the positive correlation between the percentage of positive cores and the percentage of cancer extent in each positive core, each of the two factors also shows an independent predictive value for the advanced tumor stage and an increased index tumor volume in our entire cohort. Statistical superiority or inferiority between these factors might have arisen from the difference in the composition of patient data set or the saturation degree of biopsy. The relatively large number of biopsy cores might support universality as a prediction tool of these factors, but an advantage of the percentage of positive cores from the dominant side was only observed in prediction of biochemical recurrence in the entire cohort. Currently, many urologic surgeons refer to biopsy core details through prediction nomograms to assess individual risk for lymph node invasion and to make decisions regarding whether an extended resection and/or lymph node dissection should be performed during surgery.

PSAD is a convenient tool to offset the impact of prostate size that contributes to an elevated PSA and intensifies the potential value of PSA. Therefore, PSAD was initially proposed as a means of distinguishing benign hyperplasia from cancer [21]. PSAD as a predictor for adverse pathologic features or biochemical recurrence has also been debated since the middle 1990s [22]. Some studies demonstrated that PSAD provided greater advantages in predicting oncologic outcome than PSA alone in the entire cohort that underwent definitive therapy [8, 23], but conflicting results were also reported [24, 25]. Because PSAD has been incorporated into the major criteria of clinically insignificant cancer or the protocol of active surveillance for lower-risk cancer, such as Epstein or Prostate Cancer Research International: Active Surveillance (PRIAS) criteria with strict thresholds, clinical interest and discussion of PSAD have shifted toward lower risk cancers [26, 27]. Against these backgrounds, the predictive value of PSAD in risk groups of those who require definitive therapy is not more widely recognized than that of biopsy core details. Similar to our study, Koie et al. analyzed subjects limited to high-risk PCa and revealed that PSAD had independent predictive value for an adverse pathologic stage and biochemical recurrence [28]. More interestingly, Hamada et al. demonstrated that both PSAD and the percentage of cores positive from the dominant side had independent predictive value for biochemical recurrence in high-risk PCa, and they stratified patients into three groups using a statistical formula [29]. Although our study did not show the predictive value of PSAD with any threshold in intermediate-risk PCa, Narita et al. and Kang et al. demonstrated a predictive value, and Kang et al. further proposed incorporating PSAD to identify patients for whom active surveillance would be appropriate [30, 31]. The predictive value of PSAD might become conspicuous when patients are stratified into each risk group, implicitly suggesting that it is important to recognize the difference in the performance characteristics of predictive factors. Because variations in the methods for prostate size measurement may influence PSAD, such that it becomes an unstable factor, our method of estimating prostate size from MRI measurements can be recommended to minimize the fluctuation between examiners [3, 10].

Pretreatment risk stratification or nomograms must be sophisticated to make decisions for patients who need definitive therapy and for a urologic surgeon to decide upon the treatment strategy. Beyond the clinical and pathological factors, non-invasive biomarkers from urine or peripheral blood and genetic panels consisting of multiple gene profiling would help distinguish aggressive cancer from an indolent case and forecast a clinical course of PCa in a pre- and post-treatment setting [32–34]. Thus, clinical and translational research are urgently required to realize future individualized management of PCa patients. The current study has some limitations; it is a retrospective study based on a relatively small population, the median follow-up time was also short to fully determine oncological outcomes, and the results might not apply to patients of other races. The cut-off values of PSAD or biopsy core details used in this study were tentative data used to find the potential predictive impacts, and therefore, we did not intend to draw a conclusion regarding distinct thresholds for these factors. Furthermore, we did not perform lymph node dissection in all patients and the degree of dissection varies, thus we could not address the association between factors and LVI. Simultaneously, this would to a certain extent also affect the interpretation of the results of biochemical recurrence. Nevertheless, both PSAD and biopsy core details are important preoperative predictors in addition to the NCCN classification, and the findings of this study should be validated using a larger, independent dataset.

Contemporary PCa patients who require definitive therapy include a highly heterogeneous population of oncological outcomes, and the established risk stratification system is insufficient to offer individualized management of PCa. PSAD and biopsy core details are important preoperative factors to predict oncologic outcomes, and should be incorporated into risk assessment for intermediate- and high-risk PCa patients. In addition, we must recognize the difference in the performance characteristics of these factors when PCa patients are stratified into each of the NCCN risk groups; where the percentage of positive cores and PSAD are independent predictors for biochemical recurrence in the intermediate- and high-risk groups, respectively.