*2.4. Small Tubules and Nephrogenic Adenoma-Like Architecture*

Very occasionally, UC is composed of low-grade cells arranged in small tubules resembling *cystitis glandularis* or nephrogenic adenoma (Figure 1f) [27]. The bland cytologic features of this histologic subtype contrast with its frank infiltrative nature, even reaching the muscularis propria in some cases. Since nephrogenic adenoma may display also a concerning pseudo-infiltrative growth [28], an immunohistochemical study with PAX-8, CK7, p63, and napsin A [29] may be useful to make the differential diagnosis in problematic cases. The clinical significance of this histologic change is not established so far.

#### *2.5. Microcystic Architecture*

The microcystic histology has been rarely reported in the literature at UC. Aside from a handful of single case reports, the largest series published to date analyzes 20 cases [30]. The limited examples reported up to now show a bland histologic appearance, with round to oval cysts which often contain eosinophilic intraluminal secretion covered by low columnar or flattened urothelial cells (Figure 1g). Despite its deceptive bland histology, microcystic UC displays the same aggressiveness of conventional UC at the same stage. The main differential diagnosis is nephrogenic adenoma and adenocarcinoma of the bladder. In this sense, a basic immunohistochemical panel including p63 positivity and CK7/20 co-expression coupled with napsin A and PAX-8 negativities will resolve the eventual diagnostic troubles.

#### *2.6. Verrucous Architecture*

Genuine verrucous carcinoma is a rare tumor subtype in the urinary tract [31], however, conventional well-differentiated squamous cell carcinoma with "verrucous" architectural features is a much more common event. Since the difference between them has prognostic implications their

correct identification by the pathologist matters. Verrucous carcinoma may recur but never metastasize. Some cases are related to HPV infection, others to schistosomiasis, but there are also cases unrelated to any known specific etiology [32].

The diagnosis of a verrucous carcinoma in the urinary tract, as elsewhere, is subjected to very strict histological criteria. Only low-grade keratinizing squamous cell carcinomas with superficial verrucous architecture should be considered (Figure 1h). Verrucous carcinomas may display a pushing border of growth into the lamina propria, but a true invasion is lacking. Noteworthy, any high-grade area across the tumor or frank stromal infiltration makes the diagnosis of verrucous carcinoma unsuitable.

#### *2.7. Di*ff*use Architecture with Lymphoepithelioma-Like Changes*

Lymphoepithelioma is the classical histological term referring to an undifferentiated carcinoma first described in the nasopharyngeal region of Asian patients [33]. Some of them are related to Epstein-Barr virus infection. Since then, analog histology has been described in many carcinomas widely distributed in the body. Aside from multiple case reports, several series of this tumor subtype in the bladder [33–36] and the upper urinary tract [37] have been published in the literature. Remarkably, the theoretical relationship of lymphoepithelioma-like UC with Epstein–Barr virus infection is no longer sustainable in cases arising in the urinary tract after the results obtained with FISH analyses in the largest series [35–37].

The tumor shows a diffuse growth of ill-defined islands of poorly-differentiated cells with badly defined cytoplasmic borders, large nuclei, and patent nucleoli. The stroma is heavily infiltrated by lymphocytes occasionally showing lymphoepithelial lesion (Figure 1i). By immunohistochemistry, GATA3, cytokeratins 34βE12, AE1-AE3, and CK7, p53 and p63 are positive in a variable number of cases, whereas TTF-1, CD30, and CK20 are negative [36,37]. The prognosis does not differ from conventional UC at the same stage.

#### **3. Cytological Changes**

#### *3.1. Plasmacytoid Cells*

Plasmacytoid UC is an aggressive tumor. This cytologic variant of UC can present as pure tumors or mixed with conventional UC and/or with other non-conventional UC. For example, mixed micropapillary and plasmacytoid UC cases have been occasionally reported [38]. Histological similarities with multiple myeloma were noticed since the first report by Sahin et al. in 1991 [39]. Since this original description, several large series have been published so far all of them confirming its dismal prognosis [40].

In its typical presentation, the tumor appears as flat, non-papillary, highly cellular masses growing diffusely in the urinary tract wall with infiltrative edges and frequent vascular invasion images. Neoplastic cells are non-cohesively arranged and show lateralized cytoplasm, nuclear atypia, and high mitotic count (Figure 2a). In doubtful cases, or patients with a previous history of plasma cell dyscrasia, immunohistochemistry is of help revealing its epithelial, non-plasmacytic, nature. Briefly, GATA-3 and CK7 are positive and CD 38 is negative. Positive immunostaining with CD 138 may be observed in this neoplasm, but this finding does not preclude the diagnosis of plasmacytoid UC [41].

*HER2* overexpression has been observed by FISH in plasmacytoid UC [42]. Contrary to what happens in most UC, plasmacytoid variants do not seem to harbor *TP53* gene mutations in a sequencing analysis [41]. On the other hand, *TERT* gene promoter mutations have been detected [43]. A study using whole-exome sequencing has detected somatic alterations in the *CDH1* gene of 84% of plasmacytoid UC, a finding of clinical aggressiveness that seems to be specific to this tumor variant [44].

**Figure 2.** Cytological changes in bladder cancer. (**a**) Plasmacytoid cells (×250), (**b**) basaloid and squamous cells (×250), (**c**) syncytiotrophoblastic cells (×250), (**d**) trophoblastic cells (×250), (**e**) lipoblastic-like cells (×400), (**f**) pleomorphic giant cells (×250), (**g**) clear cells (×100), (**h**) eosinophilic (oncocytoid) cells (×400), and (**i**) sarcomatoid cells (×250).

## *3.2. Signet-Ring Cells*

Since signet-ring cell features are very rare in UC, and their identification in transurethral resection specimens can raise the possibility of a metastatic seed from a neoplasm originating in the digestive tract. A careful search of the classical urothelial features (nests of transitional cells, papillae, in situ carcinoma in the surface epithelium, etc.) in the biopsy, if present, may be of help in the differential diagnosis. Otherwise, the clinical context of the patient and a basic immunohistochemical panel, for example, CK7/20, GATA-3, CDX-2, and p63, should resolve the dilemma. The analysis of the national Surveillance, Epidemiology, and End Results (SEER) database of 318 such cases confirms the worse prognosis of this histologic variant compared with conventional UC [45].
