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.

For colorectal cancer patients in surveillance, serial ctDNA testing offers profound reassurance. Data shows that after achieving one year of consistently negative results, the probability of a future recurrence drops to just 0.9%, providing a level of confidence previously unattainable with other methods.

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.

A leading hypothesis for why adding immunotherapy to chemoradiation failed is that radiation, particularly for central tumors, destroys the very lymphocytes immunotherapy aims to activate. This biological mechanism suggests the radiation essentially canceled out the drug's intended effect.

The next frontier in CSCC isn't just about new drugs, but about optimizing existing ones. A key research area is determining the minimum number of immunotherapy doses required for an optimal response—potentially just one or two—to limit toxicity, reduce treatment burden, and personalize care for high-risk patients.

Current bladder cancer trials often fail to differentiate between patients with primary resistance (never responded) versus acquired resistance (responded, then progressed). Adopting this distinction, common in lung cancer research, could help identify patient subgroups more likely to benefit from immunotherapy re-challenge and refine trial eligibility criteria.

After immunotherapy, many colorectal cancer patients have residual nodules on scans that appear to be partial responses. However, ctDNA testing can confirm these are often just scar tissue, not active disease. This provides the confidence to stop therapy at the two-year mark and avoid unnecessary surgeries for what are effectively complete responses.

For patients who previously received immunotherapy (IO), a recurrence more than 12 months after completing treatment makes re-challenging with an IO agent a reasonable option. The likelihood of benefit is lower if the recurrence is within 6-12 months and minimal if under 6 months.

A potential complication of successful neoadjuvant immunotherapy in rectal cancer is the development of stenosing scar tissue. This can block the lumen, making endoscopic surveillance impossible and necessitating surgery for a patient who may have otherwise achieved a complete clinical response.