Yolk Sac Tumor (Endodermal Sinus Tumor)

Overview

Classification by Age and Site

Pathogenesis

• Derived from malignant germ cells differentiating along the extraembryonic yolk sac lineage
• Postpubertal tumors arise from germ cell neoplasia in situ (GCNIS) - the precursor lesion for most adult GCTs; prepubertal tumors do NOT have this association
• RUNX3 gene hypermethylation and GATA-4 overexpression are implicated in the pathogenesis of ovarian YST
• Postpubertal tumors frequently have isochromosome 12p [i(12p)], shared with other adult GCTs

Histological Features (The Core of Diagnosis)

Growth Patterns (multiple, variable):

1. Reticular (lace-like) pattern - most common; loose network of medium-sized cuboidal, flattened, or spindled cells with less cytologic atypia than embryonal carcinoma
2. Papillary pattern
3. Solid cords
4. Microcystic / glandular pattern
5. Embryoid bodies (resemble 1-2 week embryos, surrounded by syncytio- and cytotrophoblasts)

Pathognomonic Structures:

• A glomerulus-like structure: central blood vessel (capillary) enveloped by tumor cells within a space that is also lined by tumor cells
• Resembles a primitive glomerulus or endodermal sinus
• Composed of a mesodermal core with a visceral and parietal layer of cells

• Intracellular and extracellular, eosinophilic, hyaline-like globules
• Present in up to 84% of cases
• Contain AFP and alpha-1-antitrypsin; stain positive by immunoperoxidase

Here is the classic histology showing Schiller-Duval bodies:

Schiller-Duval bodies in a yolk sac carcinoma - note the central capillary surrounded by tumor cells in a glomeruloid arrangement (Robbins Pathology)

Immunohistochemistry / Markers

Key rule: YST always produces AFP but NOT HCG. This is diagnostically critical.

Clinical Presentation

Testicular YST:

• Prepubertal: painless testicular mass in infant/toddler (< 3 years); serum AFP elevated (but note AFP is physiologically high in neonates, interpret with age-adjusted norms)
• Postpubertal/Adult: testicular mass, often part of mixed GCT; markedly elevated serum AFP

Ovarian YST:

• Children and young women
• Abdominal pain and rapidly growing pelvic mass involving a single ovary
• Elevated serum AFP

Extragonadal YST:

• Mediastinum (most common extragonadal site in adults), retroperitoneum, pineal gland
• Elevated AFP

Staging and Prognosis

• Stage I-II: >95% 5-year survival
• Stage III: ~70% 5-year survival
• Stage IV: ~50% 5-year survival

Comparison: Prepubertal vs. Postpubertal YST

Differential Diagnosis

Treatment

1. Radical orchiectomy / salpingo-oophorectomy - primary surgery
2. Chemotherapy: BEP (Bleomycin, Etoposide, Cisplatin) - backbone of treatment for advanced disease
3. Surveillance vs. adjuvant chemotherapy based on stage for prepubertal testicular YST
4. AFP monitoring - used for treatment response and relapse detection

High-Yield Summary for Exams

• Also called: Endodermal sinus tumor
• Most common testicular tumor in children < 3 years
• Second most common malignant ovarian GCT
• Key histology: Schiller-Duval bodies + eosinophilic hyaline globules
• Key marker: AFP positive, HCG negative
• IHC: AFP, glypican-3, keratin, SALL4 positive; OCT3/4 negative
• Prepubertal = no GCNIS, no i(12p), excellent prognosis
• Postpubertal = associated with GCNIS, usually mixed, AFP used in IGCCCG staging
• Treatment: BEP chemotherapy for advanced stages

• Robbins & Cotran Pathologic Basis of Disease - Chapters on Testicular and Ovarian GCTs
• Campbell-Walsh-Wein Urology, Germ Cell Tumors of the Testis
• Smith and Tanagho's General Urology, 19th Edition
• Recent evidence: Hepatic YST systematic review (2025) and mediastinal YST meta-analysis (2024) confirm no major changes to existing clinical framework; BEP-based chemotherapy remains standard.