Yolk sac tumor

* Pure yolk sac tumor is the most common malignant gonadal tumor of infants and toddlers. However, the majority of extragonadal germ cell tumors in the midline are either seminomas (germinomas) or teratomas, and pure yolk sac tumors account for only a small fraction of these lesions. To date, only I primary urachal pure yolk sac tumor has been reported in the literature. We describe another case, occurring in a 7-month-old male infant who presented with a rapidly enlarging intra-abdominal tumor with marked engorgement of the superficial venous plexus around the umbilicus. With periodic follow-up for 3 years following surgical extirpation of the tumor and adjuvant chemotherapy, this patient is still alive without evidence of disease. Notably, the glandular elements predominating in the frozen sections resulted in the initial misdiagnosis of the tumor as a urachal adenocarcinoma, although the entirely resected specimen revealed typical histologic patterns and Schiller-Duval bodies. Immunohistochemistry showed that the tumor cells were diffusely reactive to alphafetoprotein, alpha^sub 1^,-antitrypsin, and cytokeratin. Tumor cells were negative for p53 protein, but revealed overexpression for MDM2 protein. Flow cytometry demonstrated a diploid DNA content with S-phase being as high as 55.36%. This case emphasizes that pure yolk sac tumor can occur primarily in the remnant of the urachus in young children.

(Arch Pathol Lab Med. 2002;126:1106-1109)

The urachus is a 5- to 6-cm vestigial structure located between the dome of the urinary bladder and the umbilicus. Most urachal lesions are congenital anomalies related to urachal remnants; however, tumors may also derive from this site, the majority of which are of epithelial origin. The most common urachal tumor is adenocarcinoma. Other primary epithelial tumors of the urachus include villous adenoma, transitional cell carcinoma, and, rarely, squamous cell carcinoma. Primary extragonadal germ cell tumors of the urachus are exceedingly rare. A review of the English literature identified only 1 mature teratoma1 and 1 pure yolk sac tumor (YST) of the urachus2 on record. The overall prognosis for the extragonadal pure YST is worse than that for its testicular counterpart, for example, those originating from the mediastinum and pineal gland. Furthermore, the. ages at presentation also differed between testicular YSTs and those of various extragonadal sites.3-7 Given the rarity of primary pure YST of the urachus, we performed a flow cytometric study and related immunohistochemical stains, including p53 and MDM2 proteins, in an attempt to compare the implication of these parameters on the biological behavior and pathogenesis of this tumor with previous data based on pure YSTs at other sites.

REPORT OF A CASE

A male infant was born at 37 weeks' gestation to a 24-year-old mother. At birth, he was generally healthy. However, at 6 months he began to suffer from intractable vomiting when fed with infant formula. At age 7 months, his abdomen was rapidly enlarging, and his mother found a palpable mass. On physical examination, a sizable bulging mass centered in the midabdomen around the umbilicus was seen with accompanying caput medusa-like engorgement of umbilical venous plexus. The bilateral testes were unremarkable on physical examination.

Computed tomography demonstrated a large lower intra-abdominal mass, measuring 12.0 x 7.0 x 7.0 cm, with heterogeneous enhancement, multiple necrotic areas, and a stalklike extension to the peritoneum just beneath the umbilicus (Figure 1). There was no evidence of tumor in the liver, bilateral kidneys, both adrenal glands, and the retroperitoneum. The presumptive diagnosis was an embryonal rhabdomyosarcoma with focal invasion of the abdominal wall. Unfortunately, the preoperative serumn alpha-fetoprotein (AFP) level was not available because a urachal YST was not suspected. At surgical exploration, the tumor appeared to arise from the urachus beneath the parietal peritoneum. After drainage of 200 mL of fresh and old blood, multiple pea-sized implants of tumor nodules, originally obscured by hemoperitoneum, were discovered in the subhepatic space and visceral peritoneal surfaces of the small bowel. The biopsy specimen submitted for frozen section showed a complex glandular structure, which was initially misinterpreted as a urachal adenocarcinoma. Surgical extirpation of both the main mass and the implanted tumor nodules was performed. However, the serum level of AFP assayed 3 days after the operation was still high (up to 1250 ng/mL; normal,

PATHOLOGIC FINDINGS

The resected specimen consisted of more than 10 tissue fragments of varying sizes, which in aggregate measured 11.5 X 8.5 X 6.5 cm. The main mass, measuring 8.0 x 7.5 x 7.0 cm, was partially covered by glistening fibromembranous tissue. On sectioning, most of the tumoral fragments were soft, friable, mucoid, and pale yellow with foci of hemorrhage and necrosis.

At lower magnification, the resected tumor was found to arise beneath a loose fibromyxoid membrane formed by the urachus and underlying peritoneum, with focal tumoral necrosis and hemorrhage. The viable portion of the tumor exhibited a spectrum in architectural arrangement, ranging from the reticular microcystic/macrocystic pattern focally intermingled with myxomatous stroma through the tubulopapillary structure to focal solid growth within complex glandular areas (Figure 2). In the papillary areas, small branching papillae with delicate fibrovascular stroma lined by somewhat hobnailed nuclei were seen projecting into the cystic spaces. Clusters of exfoliated cells were present in the cysts (Figure 3). However, the alteration among varying histologic patterns was not abrupt; transitional forms or admixtures of patterns were easily observed in places. For example, in the present case, there were stellate to spindle primitive mesenchyme-like cells in the stroma, which often merged imperceptibly with the columnar or polyhedral cells of the adjacent reticular structure. On medium-power examination, many typical endodermal sinuslike structures (Schiller-Duval bodies) were found, which contained central small vessels and were ringed by a layer of malignant-appearing, cuboidal to columnar cells (Figure 4).

Immunohistochemistry

Representative paraffin-embedded, formalin-fixed blocks were cut using the streptavidin-biotin method with primary antibodies against AFP (Dako Corporation, Carpinteria, Calif; 1:300), alpha^sub 1^-antitrypsin (Novocastra Laboratories Ltd, Newcastle Upon Tyne, United Kingdom; 1:750), cytokeratin (high molecular weight and low molecular weight, Dako, 1:50), epithelial membrane antigen (Dako, 1:50), carcinoembryonic antigen (Dako, 1:200), placental alkaline phosphatase (8A9, Dako, 1:75), vimentin (Dako, 1:50), CD30 (MP-1, Novocastra, 1:30), CD34 (QBEnd10, Dako, 1:200), p53 (DO-7, Dako, 1:50), and MDM2 (SMP14, Dako, 1:25). The tumor cells showed diffuse strong reactions to AFP (Figure 4) and alpha^sub 1^,-antitrypsin. The staining with cytokeratin was intensely positive for low-molecularweight cytokeratin but weakly positive for high-molecular-weight cytokeratin. Reactivities to epithelial membrane antigen, placental alkaline phosphatase, and carcinoembryonic antigen were all focally weak, and staining was entirely negative for vimentin, CD30, and CD34. In addition, p53 protein could not be detected, whereas the majority of tumor cells showed strong staining for MDM2 protein.

Flow Cytometry

Eight 50-(mu)m-thick sections were cut from representative paraffin-embedded tissue blocks containing only viable portions of tumor cells. The thick sections were processed routinely and then subjected to a FACScan flow cytometer (Becton Dickinson, Mountain View, Calif). The flow cytometric analysis demonstrated a diploid DNA value with a high S-phase fraction up to 55.36%. The half peak coefficient of variation was 3.88%.

COMMENT

Regardless of histologic classification, germ cell tumors in children generally demonstrate a bimodal age distribution, with the first peak occurring prior to 3 years of age and involving extragonadal and testicular tumors, and the second peak emerging near puberty, consisting mainly of gonadal neoplasms.4,7 Conversely, although YST is one of the most frequently encountered types of germ cell tumor affecting children, it is relatively rare in extragonadal sites as compared with its testicular counterparts.4 The majority of extragonadal YSTs are centered in the midline of the body, such as the suprasellar region, pineal gland, sacrococcygeal area, and mediastinum.3,5,6 Germ cell tumors arising from the urachus are exceptionally rare. To the best of our knowledge, only one case of mature teratoma of the urachus1 and one case of primary urachal YST2 have been reported in the English literature to date. Little information on differential diagnosis and pathogenesis of the urachal YST described by D'Alessio et al2 could be gleaned from their report.

The first entity to be distinguished from urachal YST is adenocarcinoma, which accounts for more than 90% of all urachal malignancies. Furthermore, primary urachal adenocarcinomas with production of AFP have been reported,8 and a complex glandular pattern may.focally predominate in YST in limited biopsy specimens. These characteristics tend to hamper arriving at the correct diagnosis of urachal YST. However, the urachal adenocarcinoma, whether AFP-producing or not, mainly affects patients older than 50 years and usually adopts an aggressive clinical course.8 Conversely, the pure urachal YSTs reported here and by D'Alessio et al2 involved children younger than 2 years and had favorable outcomes. The hepatoblastoma, especially the fetal-type variant, may share overlapping features with the urachal YST in clinical presentation, tumor marker, and histologic pattern. Both manifest as AFP-producing intra-abdominal masses with potential impedance of umbilical venous return. Although the urachal YSTs in both the first reported case and in the present case demonstrate conventional histologic patterns, a hepatoid variant of YST is known to occur in the gonads and in extragonadal sites, for example, the mediastinum,6 which bears a great resemblance to hepatocellular carcinoma, hepatoid adenocarcinoma, and the fetal variant of hepatoblastoma, especially when examining biopsy specimens. Caution should be exercised in the interpretation of AFP staining, since in rare instances adenocarcinoma can focally express this marker either with or without presence of hepatoid differentiation. Polyclonal carcinoembryonic antigen usually yields a bile canalicular staining in neoplasms of hepatic lineage, whereas the YSTs, including the hepatoid variant, fail to exhibit this reactivity. In focal areas, the tumor cells of YST may assume spindleshaped, mesenchyme-like features; moreover, cases admixed with scattered rhabdomyoblasts or characterized by predominant growth of a spindle sarcoma-like component have been reported in various gonadal and extragonadal sites.5,9 Given these facts, YST is prone to be mistaken as various sarcomatous neoplasms when based on a limited biopsy specimen. Furthermore, the radiologic findings of this case cannot be distinguished from an embryonal rhabdomyosarcoma, which is the most frequently encountered sarcoma in childhood. In this aspect, immunohistochemical study with AFP and sarcomeric actin is considered the best way to identify the origins of tumor cells.

There are 3 plausible hypotheses that can explain the existence of YSTs in extragonadal locations. First, the primordial germ cells may be arrested along the migratory route from the hindgut yolk sac region into the embryonic genital ridge, which accounts for many extragonadal YSTs arising in the midline,3,5,6 perhaps including the urachus. Second, YST of the urachus may derive from retrograde differentiation of a minor subset of multipotential stem cells harbored by the urachal remnant,9 based on the fact that primary urachal neoplasms of diverse cellular origin, including varying kinds of epithelium, mesoderm, and germ cells, have been described in the literature. Another theory is that a YST-like component in a few carcinomas of the alimentary tract arises from the aberrant evolution of the same clonal precursor as the adjacent mutated somatic epithelial cells10; nevertheless, this situation almost always occurs in adult cases and is comparable to AFP-- producing adenocarcinomas of the urachus in adults.8 Therefore, we do not consider the last hypothesis suitable to explain the pure YST of the urachus, because the case described by D'Alessio et alz and the case described here both affected young children.

There is no significant correlation between histologic features and the clinical behavior of YSTs. However, tumors arising at various locations seem to follow different clinical courses. For instance, the primary mediastinal YST is an aggressive lethal neoplasm mainly involving young males,5,6 but its testicular counterpart tends to affect young children and has a surprisingly favorable prognosis.4,7 Primary YST of the urachus appears to be comparable to those tumors occurring in the infantile gonads in clinical behavior, as exemplified by the first case described and the present case, both of which affected male children aged less than 2 years, measured larger than 10 cm in greatest dimension, but achieved complete remission with surgical extirpation and cisplatin-based multiagent chemotherapy. Flow cytometry performed in this case revealed a diploid DNA content with strikingly high S-- phase fraction (55.36%). Flow cytometric investigations on infantile testicular YSTs demonstrated that DNA contents of these tumors can be either diploid or near tetraploid.7 Conversely, the aneuploid stemlines in many adult testicular germ cell tumors were in the peritriploid or hypertriploid range, implying a basic difference in tumorigenesis between infantile and adult germ cell tumors.7

Overexpression of p53 protein is found frequently in adult testicular germ cell tumors, including seminomas and nonseminomas. However, testicular YSTs in infants usually fail to stain with p53 protein. In a series of germ cell tumors of infantile testes described by Mostert et al,4 both p53 protein and gene alterations of the p53 gene could not be detected in all 4 YSTs by immunohistochemistry and polymerase chain reaction single-strand conformation polymorphism analysis, respectively. On the other hand, MDM2 proteins were strongly expressed in all 4 YSTs, and 1 revealed high-level amplification of 12q13-q14 by comparative genomic hybridization, mapping to the encoding gene of MDM2 protein.4 On the contrary, an investigation by Feng et al3 on intracranial germ cell tumors demonstrated frequent missense mutations of the p53 gene in YSTs, which may give rise to functional alterations of the p53 protein. However, patients with intracranial YSTs in the series by Feng et al3 ranged from 8 to 19 years of age (median, 13 years) and most had poor outcomes. Therefore, the contribution of the p53 gene and its protein product to tumorigenesis of YST appears to be closely dependent on age at presentation and is perhaps related to tumor location. In our case, we also noticed strong expression of MDM2 protein but not p53 protein. Although we did not analyze the mutational status of the p53 gene or perform differential polymerase chain reaction to detect amplification of the MDM2 gene, the expression and interaction of p53 and MDM2 in the urachal YST is presumably analogous to that in the infantile testicular counterpart, in light of their similar age at presentation, clinical evolution, and favorable prognosis. Moreover, amplification of the MDM2 gene coupled with ubiquitin-dependent degradation of p53 protein is perhaps implicated in the histogenesis of both testicular and urachal YSTs in infants, instead of dysfunctional genetic alterations in p53 gene or down-regulation of MDM2 gene transcription by mutant p53 protein.

In conclusion, we describe a 7-month-old infant with a rare primary YST of the urachus, which followed an indolent course, similar to the testicular counterpart with comparable age at presentation. This patient had a satisfactory outcome following surgical excision and cisplatin-- based chemotherapy. This case also emphasizes that this rare entity can occur as a primary tumor in the urachus. Otherwise, although the entirely resected specimen in this case finally revealed a YST predominantly of conventional pattern, it can mimic adenocarcinoma with or without AFP production on limited biopsy specimen. Furthermore, primary urachal YSTs with either predominantly hepatoid or sarcomatoid growth or with an admixture of rhabdomyoblasts can be reasonably expected to emerge in the future, which may pose a diagnostic challenge in distinguishing this entity from metastatic hepatoblastoma and other sarcomatous neoplasms, particularly the embryonal rhabdomyosarcoma. We emphasize that correct a diagnosis can only be reached by careful integration of clinical manifestation, imaging findings, thorough sampling, and application of immunohistochemistry.

References

1. Defabiani N, Iselin CE, Khan HG, Pache JC, Rohner S. Benign teratoma of the urachus. Br Urol. 1998;81:760-761.

2. D'Alessio A, Verdelli G, Bernardi M, et al. Endodermal sinus (yolk sac) tumor of the urachus. Eurj Pediatr Surg. 1994;4:180-181.

3. Feng X, Zhang S, Ichikawa T, et al. Intracranial germ cell tumors: detection of p53 gene mutations by single-strand conformation polymorphism analysis.jpn J Cancer Res. 1995;86:555-561.

4. Mostert M, Rosenberg C, Stoop H, et al. Comparative genomic and in situ hybridization of germ cell tumors of the infantile testis. Lab Invest. 2000;80:10551064.

5. Moran CA, Suster S. Yolk sac tumors of the mediastinum with prominent spindle cell features: a clinicopathologic study of three cases. Am J Surg Pathol. 1997;21:1173-1177.

6. Moran CA, Suster S. Hepatoid yolk sac tumors of the mediastinum: a clinicopathologic and immunohistochemical study of four cases [see comments]. Am J Surg Pathol. 1997;21:1210-1214.

7. Oosterhuis JW, Castedo SM, de Jong B, et al. Ploidy of primary germ cell tumors of the testis: pathogenetic and clinical relevance. Lab Invest. 1989;60:1421.

8. Lertprasertsuke N, Tsutsumi Y. Alpha-fetoprotein-producing urachal adenocarcinoma. Acta Pathol Jpn. 1991;41:318-326.

9. Michael H, Ulbright TM, Brodhecker CA. The pluripotential nature of the mesenchyme-like component of yolk sac tumor. Arch Pathol Lab Med. 1989;113: 1115-1119.

10. Puglisi F, Damante G, Pizzolitto S, et al. Combined yolk sac tumor and adenocarcinoma in a gastric stump: molecular evidence of clonality. Cancer. 1999;85:1910-1916.

Hsuan-Ying Huang, MD; Sheung-Fat Ko, MD; Jiin-Haur Chuang, MD; Yung-Ming Jeng, MD; Ming-Tse Sung, MD; Wei-Jen Chen, MD

Accepted for publication November 7, 2001.

From the Departments of Pathology (Drs Huang, Sung, and Chen), Radiology (Dr Ko), and Pediatric Surgery (Dr Chuang), Chang Gung Memorial Hospital, Kaohsiung Medical Center, Chang Gung University, Kaohsiung, Taiwan; and the Department of Pathology, National Taiwan University, Taipei, Taiwan (Dr Jeng).

Reprints: Wei-Jen Chen, MD, Department of Pathology, Chang Gung Memorial Hospital, Kaohsiung Medical Center, 123, Ta-Pei Rd, NiaoSung, Kaohsiung Hsien, Taiwan, Republic of China (e-mail: al 20600310@yahoo.com).

Copyright College of American Pathologists Sep 2002
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