Residency · Residency · Radiation Oncology

Endometrial Cancer: Molecular Classification and Its Impact on Adjuvant Therapy

Introduction

The traditional approach to classifying endometrial cancer, which relies on histology and stage, has notable limitations in accurately predicting patient outcomes. The Cancer Genome Atlas (TCGA) project identified four distinct molecular subgroups of endometrial cancer that provide a more refined prognosis and increasingly influence decisions regarding adjuvant therapy. Translational studies such as PORTEC-3 and TransPORTEC have demonstrated that molecular classification can reassign patients to different risk categories, which may significantly alter treatment recommendations.

TCGA Molecular Subgroups

The first molecular subgroup identified by TCGA is the POLE ultramutated category. These tumors harbor pathogenic mutations in the exonuclease domain of DNA polymerase epsilon (POLE), resulting in an extremely high mutation burden exceeding 100 mutations per megabase. Despite often presenting as high-grade tumors with prominent tumor-infiltrating lymphocytes, POLE ultramutated cancers have an excellent prognosis regardless of grade or stage, with recurrence rates below 3% even in the absence of adjuvant therapy.

The second subgroup is characterized by microsatellite instability (MSI) or mismatch repair deficiency (MMRd), which involves loss of mismatch repair proteins such as MLH1, MSH2, MSH6, and PMS2. This deficiency may arise sporadically, often due to MLH1 promoter hypermethylation, or be inherited as part of Lynch syndrome. Tumors in this group have an intermediate prognosis with heterogeneous outcomes. They also exhibit a high mutation burden and may be sensitive to immune checkpoint inhibitors.

The third subgroup, termed copy number low or no specific molecular profile (NSMP), lacks POLE mutations, is microsatellite stable, and does not harbor TP53 mutations. These tumors predominantly display low-grade endometrioid histology and have an intermediate prognosis. Their outcomes are largely influenced by traditional clinicopathologic risk factors.

Finally, the copy number high subgroup, also known as p53 abnormal (p53abn), is defined by TP53 mutations accompanied by genomic instability and frequent copy number alterations. This group includes most serous carcinomas and some high-grade endometrioid tumors. It carries the worst prognosis among all molecular subgroups, with a five-year recurrence-free survival rate of approximately 50%.

Surrogate Immunohistochemical Markers

To translate these molecular classifications into clinical practice, the ProMisE (Proactive Molecular Risk Classifier for Endometrial Cancer) algorithm offers a practical approach. This method begins with immunohistochemistry for mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) to identify MMR deficiency. Next, sequencing of hotspot mutations in the exonuclease domain of POLE is performed. Following this, p53 immunohistochemistry is conducted, where aberrant patterns are defined as either complete absence (null) or overexpression. If none of these abnormalities are detected, the tumor is classified as NSMP. The order of testing is important: MMR deficiency is assessed first, followed by POLE mutations, and finally p53 status. This algorithm achieves over 90% concordance with the TCGA genomic classification.

Impact on Adjuvant Therapy Decisions

For POLE ultramutated tumors, outcomes remain excellent even when high-grade histology or deep myometrial invasion is present. Consequently, de-escalation of adjuvant therapy is under investigation. The PORTEC-4a trial is currently randomizing patients with POLE-mutant high-intermediate risk (HIR) tumors to either observation or vaginal cuff brachytherapy (VCB). Many centers currently consider observation alone appropriate for these patients.

In MMR deficient tumors, which have an intermediate prognosis, adjuvant therapy decisions continue to be guided by traditional staging criteria. Importantly, all MMRd tumors require mandatory screening for Lynch syndrome. Immune checkpoint inhibitors such as pembrolizumab and dostarlimab have shown promise in advanced or recurrent disease, with dostarlimab demonstrating remarkable complete response rates in MMRd solid tumors.

For NSMP tumors, risk stratification relies on established clinicopathologic factors including grade, stage, and lymphovascular space invasion (LVSI). Adjuvant therapy follows traditional PORTEC and GOG guidelines, with vaginal cuff brachytherapy recommended for high-intermediate risk patients and pelvic external beam radiation therapy (EBRT) with or without chemotherapy for those at high risk.

Tumors with p53 abnormalities require escalation of therapy regardless of apparent early stage. Even stage IA p53abn tumors carry a significant risk of recurrence. Combined chemoradiation, as recommended by the PORTEC-3 trial, is the standard approach. In cases where nodal involvement is suspected, inclusion of para-aortic fields in radiation planning should be considered.

TransPORTEC Studies

The TransPORTEC studies have provided key insights into the benefits of adjuvant therapy across molecular subgroups. Among patients enrolled in PORTEC-3, those with p53abn tumors derived the greatest benefit from combined chemoradiation. POLE-mutant tumors exhibited excellent outcomes regardless of whether they received EBRT alone or chemoradiation. The MMRd and NSMP groups showed intermediate benefit from combined therapy. Notably, molecular classification reclassified approximately 12% of patients into different prognostic groups, underscoring its clinical relevance.

These findings have important clinical implications. For example, a high-grade endometrioid tumor harboring a POLE mutation may be safely managed with observation alone, while a low-grade endometrioid tumor exhibiting a p53 aberrant pattern warrants aggressive treatment. Molecular classification can supersede traditional histopathologic risk factors in guiding therapy.

Emerging Directions

Future directions include the PORTEC-4a trial, which prospectively tests molecular-integrated risk profiles to guide adjuvant therapy decisions. Additionally, circulating tumor DNA (ctDNA) monitoring is being integrated for detecting minimal residual disease. Immune checkpoint inhibitors are increasingly incorporated into frontline therapy for MMRd tumors, while combinations such as lenvatinib plus pembrolizumab are being explored for advanced disease lacking MMR deficiency.

Key Clinical Pearls

Molecular classification using the ProMisE algorithm or an equivalent method should be performed for every case of endometrial cancer. Tumors with POLE mutations have an excellent prognosis and may not require adjuvant radiation, even when high-grade. Conversely, p53-abnormal tumors demand aggressive multimodality therapy regardless of low-stage presentation. Identification of MMR deficiency mandates a Lynch syndrome workup and opens therapeutic options with immunotherapy in recurrent disease. Importantly, molecular classification should be interpreted alongside traditional clinicopathologic factors rather than replacing them.

References

1. Kandoth C, Schultz N, Cherniack AD, et al. Integrated genomic characterization of endometrial carcinoma. Nature. 2013;497(7447):67-73.
2. Stelloo E, Nout RA, Osse EM, et al. Improved risk assessment by integrating molecular and clinicopathological factors in early-stage endometrial cancer: combined analysis of the PORTEC cohorts. Clin Cancer Res. 2016;22(16):4215-4224.
3. Leon-Castillo A, de Boer SM, Powell ME, et al. Molecular classification of the PORTEC-3 trial for high-risk endometrial cancer: impact on prognosis and benefit from adjuvant therapy. J Clin Oncol. 2020;38(29):3388-3397.
4. Kommoss S, McConechy MK, Kommoss F, et al. Final validation of the ProMisE molecular classifier for endometrial carcinoma in a large population-based case series. Ann Oncol. 2018;29(5):1180-1188.

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