Molecular Classification and Management of Endometrial Cancer

Integrated Molecular Subgroups

• The integrated molecular system divides endometrial cancers into four prognostic groups—POLE‑ultramutated, mismatch‑repair deficient (dMMR/MSI), no specific molecular profile (NSMP), and p53‑abnormal—replacing the historic type I/type II histopathological scheme. 1
• The Cancer Genome Atlas (TCGA) classification, based on mutational burden and copy‑number alterations, provides superior prognostic stratification compared with histology alone. [1][2]

POLE‑Ultramutated (POLEmut)

• Represents approximately 5–15 % of endometrial cancers; tumors harbor >100 mutations per megabase, exhibit very low copy‑number alterations, and are microsatellite stable. 1
• Typical histology is high‑grade endometrioid with abundant tumor‑infiltrating lymphocytes and tertiary lymphoid structures; patients often have lower body‑mass index and present at early stage (IA–IB). 1
• Prognosis is excellent despite high‑grade morphology, supporting de‑escalation of adjuvant therapy in many cases (e.g., ongoing PORTEC‑4a trial). 1

Mismatch‑Repair Deficient (dMMR/MSI)

• Accounts for 25–30 % of cases; characterized by 10–100 mutations per megabase, low copy‑number alterations, and microsatellite instability. 1
• Frequently endometrioid with high‑grade features, prominent lymphovascular space invasion, and abundant tumor‑infiltrating lymphocytes; associated with higher body‑mass index and Lynch‑syndrome–related risk. 1
• Prognosis is intermediate; tumors are potential candidates for immunotherapy. 1

No Specific Molecular Profile (NSMP)

• The most common subgroup (30–40 %); shows <10 mutations per megabase, low copy‑number alterations, and microsatellite stability. 1
• Usually low‑grade endometrioid histology with squamous differentiation, diffuse estrogen‑ and progesterone‑receptor expression, and minimal lymphocytic infiltrate; patients often have higher body‑mass index. 1
• Prognosis is intermediate; treatment follows standard risk‑adapted protocols based on stage and grade. 1

p53‑Abnormal (p53‑abn/Copy‑Number High)

• Observed in 5–15 % of cases; features <10 mutations per megabase, high copy‑number alterations, and microsatellite stability. 1
• Encompasses all histologic subtypes (including serous and carcinosarcoma) but is predominantly high‑grade with marked cytonuclear atypia and low lymphocytic infiltrate; patients tend to have lower body‑mass index, present at advanced stage, and have later onset. 1
• Prognosis is poor, and adjuvant chemotherapy is almost universally indicated. 1

Diagnostic Algorithm (ESMO Guideline)

• The European Society for Medical Oncology (ESMO) recommends a hierarchical testing sequence: first assess POLE hotspot mutations (e.g., P286R, V411L, S297F, A456P, S459F); if negative, perform mismatch‑repair immunohistochemistry (MLH1, PMS2, MSH2, MSH6); if still negative, evaluate p53 immunohistochemistry. Tumors are classified hierarchically—POLEmut supersedes all other categories, followed by dMMR, then p53‑abn, with NSMP as the default when all tests are normal. 1

Clinical Implications of Molecular Subgroups

• POLEmut tumors: May forgo adjuvant therapy even when high‑grade or deeply invasive, reflecting their excellent outcomes. 1
• p53‑abn tumors: Derive the greatest benefit from adjuvant chemotherapy irrespective of stage, encompassing most serous carcinomas and a substantial portion of other histologies. 1
• dMMR tumors: Are eligible for checkpoint‑inhibitor immunotherapy. 1
• NSMP tumors: Receive standard risk‑adapted treatment based on conventional clinicopathologic factors. 1

Transition‑Period Nomenclature

• When molecular testing is unavailable, tumors should be labeled “endometrial cancer not‑otherwise‑specified (EC‑NOS)”; for example, “endometrioid‑type EC (EEC‑NOS)” clarifies the classification framework used. 1

Critical Pitfalls to Avoid

• Do not rely solely on histology for prognosis; a high‑grade endometrioid tumor may be POLEmut (excellent prognosis) or p53‑abn (poor prognosis). 1
• Do not omit molecular testing in early‑stage disease, as p53‑abn tumors require aggressive therapy regardless of stage. 1
• Recognize that not all serous carcinomas are p53‑abn—approximately 30 % are not—underscoring the need for molecular confirmation. 1
• Perform hierarchical testing in the prescribed order (POLE → MMR → p53) to prevent misclassification. 1

Endometrial Cancer Histological and Molecular Characteristics

Introduction to Endometrial Cancer Subtypes

• The American Cancer Society recommends that endometrioid carcinoma, which comprises approximately 80% of endometrial cancers, is characterized by glandular architecture with varying degrees of solid growth, while serous carcinoma exhibits papillary architecture with marked nuclear atypia and p53 abnormalities, and mucinous carcinoma shows mucin-producing glands with intracellular mucin 3, 4, 5

Endometrioid Carcinoma Characteristics

• The National Comprehensive Cancer Network states that endometrioid tumors display glandular patterns resembling normal endometrial glands, with grading based on solid growth pattern percentage, and are classified into three grades: Grade 1 (G1): ≤5% non-squamous, non-morular solid growth pattern, Grade 2 (G2): 5-50% solid growth pattern, and Grade 3 (G3): >50% solid growth pattern 3, 4
• The Society of Gynecologic Oncology notes that notable nuclear atypia inappropriate for architectural grade raises the tumor grade by one, and the majority (65%) of endometrioid carcinomas are copy number-low/microsatellite stable (MSS) tumors with frequent PTEN, PIK3CA, CTNNB1, and KRAS mutations 3, 4, 5
• The American College of Obstetricians and Gynecologists recommends that increased progesterone receptor expression in copy number-low tumors suggests potential responsiveness to progestin therapy 5

Serous Carcinoma Characteristics

• The International Federation of Gynecology and Obstetrics states that serous carcinomas exhibit complex papillary architecture with cellular budding, marked nuclear pleomorphism, and prominent nucleoli, and 70% of serous carcinomas have already spread outside the uterus at presentation, with frequent intraperitoneal disease even without myometrial invasion 5
• The European Society for Medical Oncology notes that serous carcinomas are predominantly copy number-high tumors with TP53 mutations in the vast majority of cases, and exhibit low mutational frequency but extensive chromosomal instability 5
• The National Cancer Institute recommends that omentectomy is specifically recommended for serous carcinomas during surgical staging 3, 4, 6

Mucinous Carcinoma Characteristics

• The World Health Organization states that mucinous carcinomas demonstrate glands lined by cells with intracellular mucin, resembling endocervical or intestinal epithelium, and this is a rare established subtype accounting for a small percentage of endometrial cancers 3, 4, 6

Prognostic Implications

• The American Joint Committee on Cancer notes that histological type is an established independent prognostic factor alongside FIGO stage, grade, depth of myometrial invasion, age, and lymphovascular space invasion, and stage IA/IB grade 1-2 endometrioid histology defines low-risk disease 3, 4, 6, 7
• The Society of Gynecologic Oncology recommends that any stage with serous, clear cell, or undifferentiated histology is classified as high-risk regardless of other factors 7, 8
• The National Comprehensive Cancer Network states that POLE-mutated tumors have the best progression-free survival despite frequently presenting as high-grade endometrioid histology, and copy number-high/p53-abnormal tumors (predominantly serous) have the worst prognosis and almost always require chemotherapy 5, 9