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Pluripotent stem cell (PSC)-based therapies are currently in clinical trials. However, one of the main safety concerns includes the potential for cancer formation of the PSC-derived products. Currently, the teratoma in vivo assay is accepted by regulatory agencies for identifying whether PSCs have the potential to become malignant. Yolk sac elements (YSE) are one of the elements that could arise from PSC. Whereas the other malignant element, embryonal carcinoma, is thoroughly studied, this is not the case for YSE. Therefore, more research is needed to assess the nature of YSE. We propose that it is imperative to include the formation of YSE in the safety assessment of PSC due to their close resemblance to the clinical entity of yolk sac tumor (YST), a human malignant germ cell tumor (hGCT). In this review, we extrapolate knowledge from YST to better understand YSE derived from PSC. We demonstrate that both share a similar morphology and that the same characteristic immunohistochemical markers can be used for their identification. We discuss the risk these tumors pose, thereby touching upon genetic abnormalities and gene expression that characterize them, as well as possible disease mechanisms. Integrating the molecular and immunohistochemical markers identified in this review into future research will help to better address the potential malignancy associated with PSC.

Background Testicular yolk sac tumor (YST) is a rare neoplasm with limited practical guidance for preoperative diagnostic assessment. This study aims to conduct a retrospective analysis of the value of clinical profiles and MRI parameters in accurately diagnosing pediatric testicular YST while exploring characteristic indicators for these patients. Methods This retrospective study analyzed eighty patients with a testicular mass who underwent surgical treatment and preoperative MRI. Clinical characters (age, preoperative serum alpha-fetoprotein (AFP) levels), and radiology features were recorded and compared. Subsequently, patients were categorized into YST and non-YST groups based on histology. Comparative statistical analyses were then used to compare factors between the two groups. The receiver operating characteristic curve (ROC) analysis was conducted to evaluate the diagnostic performance of the indicators for pediatric testicular YST. Results Forty patients (50%) were diagnosed with YST. In comparison to the non-YST group, patients with testicular YST were younger and had larger tumor sizes, accompanied by significantly elevated AFP levels. On MRI, most YST cases ( n  = 38) exhibited predominantly solid lesions, whereas non-YST tumors were more likely to contain cystic components. The bright dot sign and thickened spermatic cord might also be helpful in differentiating YST ( p  < 0.05). The optimal factor for diagnosing testicular YST was signal intensity, with an AUC value of 0.936 (95%CI: 0.877 ~ 0.995). Conclusions A predominantly solid testicular mass with a bright dot sign, thickened spermatic cord ipsilaterally, and elevated AFP levels should raise suspicion for YST.

Yolk‐sac tumours (YSTs), a germ cell tumour subtype, occur in newborns and infants as well as in young adults of age 14‐44 years. In clinics, adult patients with YSTs face a poor prognosis, as these tumours are often therapy‐resistant and count for many germ cell tumour related deaths. So far, the molecular and (epi)genetic mechanisms that control development of YST are far from being understood. We deciphered the molecular and (epi)genetic mechanisms regulating YST formation by meta‐analysing high‐throughput data of gene and microRNA expression, DNA methylation and mutational burden. We validated our findings by qRT‐PCR and immunohistochemical analyses of paediatric and adult YSTs. On a molecular level, paediatric and adult YSTs were nearly indistinguishable, but were considerably different from embryonal carcinomas, the stem cell precursor of YSTs. We identified FOXA2 as a putative key driver of YST formation, subsequently inducing AFP, GPC3, APOA1/APOB, ALB and GATA3/4/6 expression. In YSTs, WNT‐, BMP‐ and MAPK signalling‐related genes were up‐regulated, while pluripotency‐ and (primordial) germ cell‐associated genes were down‐regulated. Expression of FOXA2 and related key factors seems to be regulated by DNA methylation, histone methylation / acetylation and microRNAs. Additionally, our results highlight FOXA2 as a promising new biomarker for paediatric and adult YSTs.

In the post-chemotherapy setting, germ cell tumors of the testis (GCTT) that resemble non-specific sarcomas and co-express cytokeratins and glypican-3 (GPC3) are diagnosed as “sarcomatoid yolk sac tumor postpubertal-type (YSTpt)”. The diagnosis of sarcomatoid YSTpt is clinically relevant but challenging due to its rarity, non-specific histology, and negative α-fetoprotein (AFP) staining. Recently, FOXA2 has emerged as a key-gene in the reprogramming of GCTT (activating the transcription of several genes, among which GATA3), and immunohistochemical studies showed that GATA3 and FOXA2 have a higher sensitivity for non-sarcomatoid YSTpt than GPC3 and AFP. We found that sarcomatoid YSTpt did not express FOXA2 [0: 14/14 (100%)] and showed focal expression of GATA3 [0: 12/14 (85.7%), 1 + : 2/14 (14.3%)], thus suggesting that these markers are not useful in diagnosing this tumor. Furthermore, we proposed a potential mechanism of sarcomatoid transformation in the post-chemotherapy setting of GCTT, mediated by the downregulation of FOXA2 and GATA3.

Ovarian yolk sac tumors (YSTs) are rare neoplasms. No radiological study has been done to compare the imaging findings between this type of tumor and other ovarian tumors. Here we analyzed the CT findings of 11 pathologically proven ovarian YSTs and compared their imaging findings with 18 other types of ovarian tumors in the same age range. Patient age, tumor size, tumor shape, ascites and metastasis of two groups did not differ significantly ( P  > 0.05). A mixed solid-cystic nature, intratumoral hemorrhage, marked enhancement and dilated intratumoral vessel of two groups differed significantly ( P  < 0.05). The area under the ROC curve of four significant CT features was 0.679, 0.707, 0.705 and 1.000, respectively. Multivariate logistic regression analysis identified two independent signs of YST: intratumoral hemorrhage and marked enhancement. Our results show that certain suggestive CT signs that may be valuable for improving the accuracy of imaging diagnosis of YST and may be helpful in distinguishing YST from other ovarian tumors.

Hepatocyte Nuclear Factor 1 beta (HNF1β) is a transcription factor which plays an important role during early organogenesis, especially of the pancreato-biliary and urogenital tract. Furthermore, HNF1β is an established marker in the differential diagnosis of ovarian cancer and shows a distinct nuclear expression in the clear cell carcinoma subtype. Recently, it has been described in yolk sac tumor, which represents a common component in many non-seminomatous germ cell tumors. Due to its broad histologic diversity, the diagnosis may be challenging and additional tools are very helpful in the workup of germ cell tumors. Immunohistochemistry was used to study HNF1β expression in a tissue microarray (TMA) of 601 testicular germ cell tumors including seminoma, embryonal carcinoma, yolk sac tumor, choriocarcinoma, teratoma, germ cell neoplasia in situ (GCNIS), and normal tissue. The expression pattern was compared to glypican 3 (GPC3) and α-fetoprotein (AFP), two markers currently in use for the detection of yolk sac tumor. HNF1β showed a distinct nuclear staining in comparison to the cytoplasmic pattern of GPC3 and AFP. The sensitivity and specificity of HNF1β were 85.4% and 96.5%, of GPC3 83.3% and 90.7%, of AFP 62.5% and 97.7%. We conclude that HNF1β allows a reliable distinction of yolk sac tumor from other germ cell tumor components. Therefore, we propose HNF1β as a novel and robust marker in the immunohistochemical workup of testicular germ cell tumors.

Objective Testicular tumor (TT) is a uncommon disease posing serious health problem. There are differences in some aspects between adult and pediatric TT. The study was to compare their differences of clinical and histological characteristics through the analysis of the long-term experiences in TT patients from two institutions. Materials and methods The clinical data of hospitalized patients was collected and analyzed retrospectively from January 2014 to January 2024 at a pediatric and an adult institution, respectively. The data included composition, gender, age, initial presentation, tumor size, tumor markers, pathological diagnosis. Results A total of 195 hospitalized patients were included. There were 135 children and 60 adult with TT, respectively. Of these children, patients were aged from 1 month to 14 years, with a mean age of 2.32 years. More cases (37.04%) were diagnosed at age younger than 1 years. 69 cases were left-sided, 65 cases were right-side and only 1 case was bilateral. Pediatric TTs mainly included 82 prepubertal teratomas, 37 had prepubertal yolk sac tumors and 3 mixed malignant germ tumors. Testicular surgeries included testicular-sparing surgery (TSS) ( n  = 73), radical orchiectomy ( n  = 60), and testicular biopsy ( n  = 2). There were 24 patients receiving postoperative chemotherapy. Adult TTs mainly contained 17 seminomas, 10 prepubertal teratomas,7 postpubertal teratomas, 6 stromal tumors and 3 embryonal carcinomas. The average age was 34.08 years. There were 29 right-sided, 27 left-sided and 4 bilateral tumors. TSS ( n  = 26), radical orchiectomy ( n  = 33), and testicular biopsy ( n  = 1) were performed in these TT patients. Only 6 patients received postoperative chemotherapy. The most common symptom was a painless scrotal mass at initial diagnosis in both groups. In addition, we found that significant differences were explored between histological type and age, tumor size ( P  < 0.05). Yolk sac tumor and seminoma were the most common malignant TT in pediatric and adult population, respectively. After two year follow-up, two children with yolk sac tumor and 4 adults with seminoma died of their diseases. Conclusions The majority of pediatric cases were benign compared to adult. The most common type was prepubertal teratoma and yolk sac tumor. Pediatric TTs often occurred under the age of 1 year. Seminomas and prepubertal teratomas were commonly found in adult TTs, especially for young adult. We found that pediatric tumor type was associated with age and tumor size. TSS should be considered for benign TTs based on frozen biopsy findings in children.

Abstract Objectives The concept of rete hyperplasia with hyaline globules simulating testicular yolk sac tumor was first reported in a mostly retrospective review over 30 years ago. Nonetheless, we continue to encounter examples where this scenario resulted in misdiagnosis. Herein, we sought to investigate the incidence of rete hyperplasia/hyaline globules in germ cell tumors and their associated subtypes and hypothesize an etiology. Methods A consecutive series of 348 germ cell tumor orchiectomies was evaluated for the presence of rete hyperplasia and hyaline globules, with clinicopathologic features recorded. Results The incidence of rete hyperplasia and/or hyaline globules in our cohort was 30%, with 56% of specimens with rete hyperplasia containing concomitant hyaline globules. Hyaline globules were more often identified in specimens with nonfocal rete hyperplasia (78%) vs focal rete hyperplasia (22%). Absence of a yolk sac tumor component was seen in over half (61%) of orchiectomies with concurrent rete hyperplasia/hyaline globules (n = 105), inclusive of tumors with “pure” subtypes (ie, pure seminoma, pure teratoma, or pure embryonal carcinoma). Of these 105 specimens, rete invasion was seen in only 48%; notably, Paneth cell–like metaplasia was identified in efferent ductules/epididymis in 13%. Conclusions Rete hyperplasia and hyaline globules are not uncommon findings in the setting of germ cell tumors (including occurrences in various pure/mixed germ cell tumors) and can show striking overlap with yolk sac tumor. We hypothesize that these histologic pitfalls evolve secondary to testicular obstruction by the tumor mass. Recognition of and distinguishing this morphologic mimicry is fundamental to guide appropriate clinical management.

Uterine yolk sac tumors have gained increased recognition in recent years. The current study is a multi-faceted examination of yolk sac tumor-like phenotypes in endometrial tumors, based on an analysis of 3 groups of uterine tumors: Group 1: 9 endometrial tumors that had been classified as yolk sac tumor, or as having a yolk sac tumor component, were assessed with a 35-marker immunohistochemical panel, with the goal of defining their immunophenotypic spectrum; Group 2, comprised of 70 endometrial carcinomas of various histotypes, were analyzed for their expression of SALL4, Glypican-3, and AFP, to assess the specificity of these markers for yolk sac tumors relative to endometrial carcinomas; Group 3, comprised of 626 archived cases of endometrial carcinoma/carcinosarcoma, reviewed to define the frequency of yolk sac tumor-like morphology therein. Yolk sac tumor areas in the Group 1 cases were consistently immunoreactive for SALL4 and Glypican-3; variably positive for AFP (89%), Villin (89%), PLAP (78%), 34βE12 (67%), CAM 5.2 (62.5%), EMA (56%), CD117 (50%), p16 (50%), CDX2 (44%), p53 (44% aberrant), MOC31 (37.5%), CK7 (33%), GATA3 (33%), CK5 (25%), and PAX8 (11%); and were negative for CD30, Napsin A, OCT4, estrogen, androgen, and progesterone receptors. 29 (41%) of the 70 group-2 cases expressed at least one of the 3 markers, and 96% of the positive cases was a high-grade histotype. Glypican-3, SALL4, and AFP were positive in 30, 20, and 2.8% of group-2 cases respectively; however, co-expression of any 2, or all 3 markers was uncommon (<9 and 1.4% of cases respectively). Potential yolk sac tumor-like morphology was identified in 5 (0.8%) of 626 group-3 cases, and three were ultimately deemed to be true yolk sac tumor phenotypes based on their morphologic and immunophenotypic similarity to the group 1 cases. These findings highlight the broad immunophenotypic spectrum of uterine yolk sac tumors, the potential pitfalls associated with using immunophenotypes alone to define yolk sac tumor differentiation in endometrial carcinoma, and the utility and limitations of morphologic assessment to identify yolk sac tumors at this site.

Purpose To evaluate the association between serum alpha-fetoprotein (AFP) half-life (HL) and prognosis in prepubertal children with elevated AFP values 3 to 4 weeks after surgery for testicular yolk sac tumors (YST). Methods Prepubertal patients with testicular YST treated with radical orchiectomy between January 2016 and December 2022 were retrospectively reviewed. Negative outcomes were defined as relapse, metastasis or death. Univariate and multivariate logistic regression analyses were conducted to select risk factors for negative outcomes. Results A total of 42 patients were eventually enrolled into the study. Patients were divided into non-negative and negative outcomes groups, consisting of 35 and 7 patients, respectively. Thirty-five patients were stage I, two cases were stage II, and five cases were stage IV, according to the Children’s Oncology Group staging system. The overall survival (OS) rate was 100%. Average AFP values significantly decreased after resection ( P  < 0.001). A significant positive correlation was shown between pre- and postoperative AFP values ( r  = 0.60, P  < 0.001). Long AFP HL was considered as an independent risk factor for negative outcomes in YST patients underwent radical orchiectomy ( P  = 0.04). The cut-off value for AFP HL was 5.78 days, regardless of age division. Conclusion Testicular YST is a relatively rare disease in children with an OS of 100%, and salvage chemotherapy is effective even in grade IV patients. The postoperative AFP HL was significantly associated with prognosis in prepubertal patients with testicular YST. The cut-off value for AFP HL is 5.78 days regardless of the effect of physiological AFP elevation.