While neoadjuvant immunotherapy shows astounding success in MSI-high rectal cancer, the primary difficulty for clinicians lies in accurately assessing complete response via endoscopy and MRI, and managing unique complications like mucin pools or stenosis, rather than simply administering the treatment.

Unlike rectal cancer where MRI aids response assessment, MSI-high colon cancer lacks a reliable imaging modality to confirm a pathologic complete response after neoadjuvant immunotherapy. This makes a "watch and wait" approach far more challenging and not currently recommended outside of a clinical trial.

Following positive Phase III data for adjuvant atezolizumab plus chemotherapy in Stage III MSI-high colon cancer, clinicians are extrapolating this approach to high-risk Stage II patients. For some, they favor using immunotherapy alone, omitting chemotherapy due to its perceived limited additional benefit in the Stage II setting.

While immunotherapy is largely ineffective in metastatic microsatellite stable (MSS) colorectal cancer, emerging data suggests it may have surprising efficacy in the early-stage (neoadjuvant) setting. This differential response is likely due to a more favorable tumor microenvironment in earlier disease, suggesting a new therapeutic window.

Standard cancer surgery often removes lymph nodes—the factories producing immune cells. Administering immunotherapy *before* this destructive process is critical. It arms the immune system while it is still intact and capable of mounting a powerful, targeted response against the tumor.

Retrospective data suggests patients with MSI-high rectal cancer might not just respond poorly to standard neoadjuvant chemoradiation (TNT), but their disease could actually progress. This makes immunotherapy a potentially safer and more effective first-line neoadjuvant choice, not just an alternative.

For MSI-high patients responding to immunotherapy, a lingering mass on a CT scan may not be active cancer. A negative ctDNA test can help confirm that the visible lesion is likely just scar tissue, potentially averting unnecessary surgery.

The COMET study found combining chemotherapy with atezolizumab did not improve overall survival versus atezolizumab alone. However, it nearly eliminated early progressive disease (2.8% vs. 32.4%), suggesting a critical role for patients with high tumor burden who cannot risk initial progression on monotherapy.

Mirroring success in rectal cancer, a new trial is exploring neoadjuvant immunotherapy for localized, MSI-high endometrial cancer. This strategy could potentially allow patients to avoid surgery and radiation, which is a particularly compelling option for those who wish to preserve their fertility.