A 16-year-old girl presents with abdominal distension and elevated serum alpha-fetoprotein (AFP). Imaging reveals a large ovarian mass. Histology shows primitive germ cells admixed with endodermal and mesodermal elements in a disorganized pattern, with areas of yolk sac differentiation. Which single feature best distinguishes this tumor from a dysgerminoma?

Explanation

## Distinguishing Immature Teratoma from Dysgerminoma ### Key Discriminating Feature **Key Point:** Immature teratoma (malignant teratoma) is distinguished from dysgerminoma by the presence of **yolk sac (endodermal sinus) differentiation and markedly elevated serum AFP**. Dysgerminoma is composed purely of primitive germ cells with no somatic or extragonadal differentiation, and typically elevates hCG, not AFP. ### Comparative Table | Feature | Immature Teratoma | Dysgerminoma | | --- | --- | --- | | **Composition** | Primitive germ cells + somatic elements (endo-, meso-, ectoderm) | Primitive germ cells only | | **Yolk sac differentiation** | Present (endodermal sinus pattern) | Absent | | **Serum AFP** | Markedly elevated (>100 ng/mL common) | Normal | | **Serum hCG** | Normal or mildly elevated | Elevated | | **Age** | Adolescents and young adults (10s–20s) | Adolescents and young women (10s–20s) | | **Grade** | I–III (based on immature element proportion) | Uniform malignancy | | **Prognosis** | Grade I: excellent; Grade III: guarded | Good with chemotherapy (90% 5-yr survival) | | **Chemotherapy response** | Excellent (BEP: bleomycin, etoposide, cisplatin) | Excellent (BEP) | ### Pathological Hallmark **High-Yield:** Immature teratoma = **primitive germ cells + somatic differentiation + yolk sac elements**. The yolk sac component (endodermal sinus pattern with papillary or microcystic architecture) is the pathognomonic feature that distinguishes it from dysgerminoma, which lacks any extragonadal differentiation. ### Clinical Pearl **Clinical Pearl:** Immature teratomas are graded 1–3 based on the proportion of immature (embryonic) elements. Grade 1 tumors have an excellent prognosis (>95% 5-year survival) with chemotherapy. The presence of yolk sac differentiation with elevated AFP is the key diagnostic clue that separates immature teratoma from dysgerminoma, which would have normal AFP. ### Mnemonic **Mnemonic:** **YEAST** — **Y**olk sac, **E**levated **A**FP, **S**omatic elements, **T**eratoma (immature). This distinguishes immature teratoma from dysgerminoma. [cite:Robbins 10e Ch 22]