Following the Keynote B21 trial, clinicians have become more selective with adjuvant immunotherapy in endometrial cancer. The study showed minimal benefit for pMMR patients and less impressive gains overall in its lower-risk population compared to advanced disease trials. This has led to a practice of reserving adjuvant chemo-IO primarily for stage 3 dMMR patients.

The GOG-B21 trial found that while adding pembrolizumab to chemotherapy benefits the dMMR subgroup, it paradoxically leads to worse outcomes in the pMMR subgroup. This highlights the critical need for molecular testing to avoid potential harm.

The four TCGA molecular profiles (e.g., POLE-mutated, p53-abnormal) have evolved beyond predicting outcomes to actively guiding treatment, such as de-escalating therapy for low-risk groups and escalating for high-risk ones.

The KEYNOTE-B21 adjuvant trial revealed a crucial paradox: adding pembrolizumab may worsen outcomes for mismatch repair proficient (pMMR) patients (HR 1.2), while being highly beneficial for dMMR patients. This highlights that metastatic and adjuvant settings are not equivalent.

When considering escalating therapy for a patient with a high-risk p53 mutation, clinicians are adopting a key checkpoint: confirming the absence of a concurrent POLE mutation. The presence of a POLE mutation is thought to mitigate the aggressiveness of p53-mutated tumors, potentially making treatment escalation unnecessary.

While immunotherapy is transformational for DMMR endometrial cancer, its benefit is much smaller for the PMMR majority (two-thirds of patients). This reality requires more nuanced patient counseling and selective use in this population.

The RUBY trial surprisingly revealed that patients with p53-mutated tumors, a subset of the generally less responsive pMMR group, derive significant benefit from adding immunotherapy to chemotherapy, challenging previous assumptions about this molecular subtype.

Clinical trials show a sustained overall survival benefit for upfront chemo-immunotherapy in dMMR patients, even with over 90% of the placebo group receiving immunotherapy upon progression. This demonstrates that delaying immunotherapy fails to rescue outcomes, making upfront use critical.

For dMMR endometrial cancer patients on combination therapy, clinicians are quicker to reduce or stop chemotherapy (like taxanes) when side effects arise. This practice reflects a growing confidence that immunotherapy is the primary driver of efficacy in this subgroup, allowing for a reduction in chemotherapy-related toxicity.