The initial histology findings obtained from transurethral resection showed both solid and papillary tumor components, invasion between the detrusor bundles and a focal glandular component that showed GPC3, SALL4, and AFP positivity. The presence of papillary tumor changes imitating urothelial tumor and YST structures led us to a working diagnosis of urothelial carcinoma with YST differentiation. The definitive histological findings obtained after surgery caused us to correct the diagnosis to EGYST, and the tissue surrounding the urothelium and the urothelium itself were free of tumor changes. Atypical changes suggestive of the diagnosis of urothelial carcinoma were later interpreted as reactive.

The histological variability of YST and its ability to mimic somatic tumors may cause diagnostic difficulties [11]. Compared with other germ cell tumors, YST presents very varied and diverse histological findings. In particular, its occurrence in extragonadal locations and outside the typical age range cause diagnostic challenges. YST can occur anywhere along the midline. From a urogenital pathology perspective, involvement of the renal pelvis [6], prostate [12], seminal vesicle [13], bladder [14, 15], and retroperitoneum [5] is known. YSTs are highly aggressive tumors that can metastasize lymphatically and hematogenously. EGYSTs occur at various ages with a tendency for a worse prognosis with increasing age. Typically, EGYSTs manifest in childhood and in young women [11]. The patient presented in our case report was 37 years of age. Only one case of urachal YST reported thus far involved a patient aged 44 years [9]. In the rest of the reported cases, the patients were children under 2 years of age [7, 10].

YST differentiation of somatic tumors is relatively rare, and reported cases of YST differentiation of urothelial tumors are even less common [6, 16, 17]. Histologically, tumors can contain both YST and somatic tumor structures. The YST component corresponds to the germ cell tumor, and it is not uncommon for the YST component to grow larger than the somatic tumor component due to its higher proliferative activity [18]. Somatic malignancies with YST differentiation are characterized by the presence of different patterns, the most typical being the glandular pattern [1]. In our case, solid and papillary patterns were the main patterns found in the biopsy. The glandular pattern that is usually described as typical for YST differentiation was observed only focally (Fig. 2). There are available data on AFP-producing urothelial tumors [19,20,21]. Most of these tumors present hepatoid tissue areas in the context of adenocarcinoma or urothelial carcinoma and show AFP positivity. In our case, no area with a hepatoid pattern similar to that in the YST were observed. Samaratunga et al. published a case report of urothelial carcinoma of the renal pelvis with focal hepatoid adenocarcinoma differentiation, and the tumor showed strong AFP immunoreactivity and serum AFP positivity [16]. There are also documented cases of bladder adenocarcinomas that have shown both immunoreactivity for AFP and elevated serum AFP levels [16]. Recently, a case of urothelial tumor with YST differentiation was presented by Espejo-Herrera et al. The patient was a 76-year-old male with a history of recurrent urothelial carcinoma of the bladder [17].

In addition to the abovementioned microscopic features, immunohistochemical methods are essential for accurate diagnosis. Extragonadal tumors usually show the same immunoreactivity as their gonadal counterparts [11]. AFP and GPC3 are characteristic immunohistochemical markers of YST, which may correlate with their serum levels [1, 22]. Some stem and pluripotent cell antigens (SALL4, Lin28 or IMP3) may also be helpful in diagnosis.

AFP remains the standard marker for YST, even though it can also be produced by several non-germ cell tumors, especially those of the female genital tract, and tumors of endodermal origin with a frequent hepatoid component [1]. AFP staining shows strong granular cytoplasmic positivity, and AFP may also be expressed in hyaline globules, although this is not always present [1]. Determination of serum AFP is useful in the diagnosis and follow-up of patients with YST or EGYST.

Most YSTs show positivity for AFP, GPC3 and SALL4 [23,24,25]. In contrast, most differentiated epithelial markers, such as EMA and KRT17 shows negative reactions. Inconsistent results were shown for the markers HepPar1, which may be positive in hepatoid pattern regions, and CDX2, which may be positive in glandular pattern regions of the tumor. There has also been a report of pure urothelial carcinoma with SALL4 positivity [26]. Most published case reports of urachal YSTs, including our case report, reported positivity for at least AFP and keratins [8, 9].

Genetic alterations are common in germ cell tumors, including isochromosome 12p, p53 alteration, and other changes [27, 28]. We did not find a gain of 12p when we tested for numerical changes in the 12p region by FISH. Extragonadal germ cell tumors do not appear to share a common genetic basis with their gonadal counterparts.

Prognostically, extragonadal forms of germ cell tumors are significantly worse than those in the gonads [29]. Tumors with a YST component are diagnosed at a younger age and may be more likely to be pure EGYSTs [6]. The determination of whether a tumor is EGYST or a somatic tumor with YST differentiation also has a major impact on therapy. While most gonadal YSTs respond relatively well to chemotherapy, a benefit of chemotherapy for EGYST is more likely to be seen in younger patients. The benefit of chemotherapy for somatic tumors with YST differentiation is unclear [6]. In the case of advanced or metastatic germ cell cancer, cisplatin-based chemotherapy is the mainstay. Five-year survival rates range from 40 to 90%, with a more favorable prognosis for seminomas or retroperitoneal tumors than for nonseminomas or mediastinal tumors [29].

This case includes a rare presentation of urachal EGYST. Chemotherapy was not associated with the achievement of a curative response. Remission was achieved only after radical surgery. The finding of glandular and hepatoid structures in the tumor is suspicious for EGYST. Serum AFP marker positivity may be an advantage. Suspicion of EGYST should be verified by immunohistochemistry.

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