Although most papillary urothelial carcinomas are characterized by an exophytically growing tumor; some exhibit an EGP and subepithelial growth pattern. A study by Amin et al. reported detailed morphological descriptions of 18 cases of UC with EGP and reviewed the problems associated with assessment of tumor invasion [3]. At present, there is little data on the frequency and prognostic significance of EGP [2,3]. Von Brunn's nests are clusters of urothelial cells within the lamina propria that have become detached from the overlying epithelium [4]. These nests have been commonly identified in 80-90% of normal bladders in autopsy studies [4,5]. A retrospective study demonstrated that VBNI lesions occurred in 73/371 (19.1%) patients with NMIBC, and its presence was not a risk factor for disease progression nor an absolute indication for radical cystectomy [5]. However, since the first report describing VBNI by Seemayer et al. [6], there have been very few reports on its clinical significance [2,5].

In bladder pathology, molecular mimicry between the subepithelial growing tumor and the tumor invading the lamina propria exists. Both of subepithelial growing patterns and appearance of invasion to the lamina propria are morphologically pushing into the stromal tissue. Thus, we aimed to investigate the association between EGP or VBNI and the tumor growth pattern at the invasion front (hereafter referred to as "growth pattern"). According to the classification by Jiminez et al. [7], 3 growth patterns were detected: "nodular" (composed mostly of well delineated, round nests of tumor cells); "trabecular" (composed of broad trabeculae, which usually anastomosed with each other); and "infiltrative" (composed of infiltrating narrow cords or single cells). Growth pattern has been reported to be a poor prognostic factor, both in MIBC and NMIBC [7-10].

A higher rate of EGP has been reported in high-grade tumors as compared to low-grade tumors [2]. In our cohort, there was no significant correlation between EGP and tumor grade. Moreover, we found a higher incidence of EGP as compared to that reported by Gofrit et al. (30.8% vs. 11.9%). Several factors can account for this discrepancy. Artifact from difference in pathologic interpretation cannot be ruled out and further analysis on confirmatory review by a secondary pathologist would be required. In addition, various environmental factors are associated with carcinogenesis of bladder cancer and the degree of exposure to these environmental carcinogens may vary in different countries and cultures [15]. Moreover, susceptibility to carcinogens also depends on genetic background and such differences could contribute to carcinogenesis of bladder cancer and the subsequent discrepancy in the incidence of EGP. Additionally, in contrast to the previous reports [2], EGP was not identified as a significant indicator of tumor recurrence or progression. However, it is interesting to note that EGP was associated with the tumor growth pattern at the invasion front in pT1 tumors (Table ​(Table2).2). Jimenez et al. were the first to report that the infiltrative growth pattern of MIBC was associated with poor survival [7]. Recently, Denzinger et al. reported that the growth pattern in a series of pT1G3 tumors was a strong predictor of cancer-specific survival and could thus be important in planning therapeutic strategies [10]. Similarly, our data revealed that the infiltrative growth pattern was related to worse progression-free survival as compared to the trabecular growth pattern (Figure ​(Figure2C).2C). Although there was no statistical significance in the recurrence- and progression-free survival between tumors with and without EGP, it was a notable finding that none of the tumors with EGP underwent disease progression (Figure ​(Figure2D).2D). Further studies are warranted to clarify the potential prognostic power of EGP in pT1 tumors.

The infiltrative growth pattern is characterized by single cell infiltration of the invasive front of tumors, possibly due to a loss of cell cohesion. In other malignant diseases, a loss of adhesional markers was seen in the epithelial mesenchymal transition (EMT) phenomenon, associated with poor prognosis [16]. Initial studies on EMT in bladder cancer suggested that low levels of the adhesion molecules plakoglobin and β-catenin were related to worse prognosis [17]. Thus, we suggest that tumors with EGP may have a tendency to retain the expression and function of the adhesion molecules.

A high prevalence of von Brunn's nests (89%) was shown in the normal urothelium. However, VBNI was present in only 3.9% of our subjects: in 1/83 (1.2%) LG tumors, 4/25 (12.1%) HG tumors, and none of the 13 PUNLMP cases (p = 0.006, Table ​Table1).1). VBNI was significantly correlated with high tumor grade. There are conflicting reports on the prognostic value of VBNI in disease progression [2,5]. We could not confirm any of the previous findings due to the small number of subjects with VBNI. In our study, of the 5 cases with VBNI, 1 received no adjuvant treatment and experienced intravesical tumor recurrence a year after the initial TURBT. Of the 4 patients that received intravesical BCG treatment, 3 showed neither recurrence nor progression during the follow-up duration. However, 1 patient experienced disease progression, wherein the tumor extended into the prostatic duct 4 years after the initial TURBT. For this case, an early radical cystectomy would be the optimal treatment. However, the clinical relevance of VBNI as a prognostic marker and therapeutic guide remains unclear.