## Type I vs Type II Endometrial Cancer: Key Discriminators **Key Point:** Type I and Type II endometrial cancers represent two distinct pathogenic pathways with different precursors, molecular profiles, and clinical behaviours. ### Comparison Table | Feature | Type I (Endometrioid) | Type II (Non-endometrioid) | |---------|----------------------|---------------------------| | **Precursor** | Atypical endometrial hyperplasia | Atrophic endometrium (de novo) | | **Estrogen dependence** | Yes (estrogen-driven) | No (estrogen-independent) | | **Histology** | Endometrioid, mucinous, squamous | Serous, clear cell, carcinosarcoma | | **Molecular hallmarks** | PTEN loss, MSI-H, KRAS mutations | TP53 mutations, HER2 amplification | | **Grade** | Usually low-grade (G1–G2) | Usually high-grade (G3) | | **Stage at presentation** | Early (I–II) | Advanced (III–IV) | | **Myometrial invasion** | Superficial to intermediate | Deep | | **Prognosis** | Better (5-yr OS ~85%) | Poor (5-yr OS ~30–40%) | | **Age of presentation** | 55–65 years | 65–75 years | **High-Yield:** Type I cancers arise from a background of chronic estrogen stimulation → atypical hyperplasia → carcinoma. Type II cancers arise *de novo* from an atrophic endometrium without a hyperplastic precursor. ### The Discriminating Feature The **precursor lesion and estrogen dependence** is the best discriminator: - **Type I:** Precursor is atypical endometrial hyperplasia; pathogenesis is estrogen-driven - **Type II:** No hyperplastic precursor; arises de novo; estrogen-independent **Clinical Pearl:** A postmenopausal woman with abnormal bleeding, Grade 1 endometrioid cancer, and MSI-H is classic Type I. The presence of a hyperplastic pathway and PTEN/KRAS mutations (not TP53) confirms this. **Mnemonic:** **PTEN** = **P**recursor (hyperplasia) present; **TP53** = **T**ype II (de novo, no precursor). [cite:Robbins 10e Ch 22]
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