A significant challenge in assessing complete response after neoadjuvant immunotherapy for rectal cancer is the presence of mucin pools. These imaging abnormalities can persist for up to two years, mimicking residual tumor and complicating decisions about non-operative management.

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.

Even with negative biopsies, post-immunotherapy scans and scopes can show residual masses or mucin pools that are mistaken for active cancer. This makes determining a true complete clinical response difficult and can lead to unnecessary surgeries where no cancer is found, as these changes can take years to resolve.

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.

With 72% response rates to neoadjuvant immunotherapy, surgeons are shifting from immediate, aggressive surgery to a "wait-and-see" approach. Shrinking the tumor first can turn a morbid, disfiguring operation into a much simpler procedure, fundamentally changing the initial surgical evaluation for cutaneous squamous cell carcinoma (CSCC).

In a pivotal neoadjuvant trial of cemiplimab for CSCC, none of the 40 patients who achieved a pathologic complete response (path CR) had relapsed at long-term follow-up. This suggests that path CR can be used as a powerful early indicator of long-term disease control and potential cure.

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.

Data from trials like CheckMate 816 shows that achieving a Pathologic Complete Response (PCR) after neoadjuvant chemo-immunotherapy is a powerful early surrogate endpoint. Patients with PCR demonstrate markedly improved overall and event-free survival.

Clinical Complete Response (cCR), assessed by imaging and biopsy, is the primary endpoint for avoiding surgery in new trials. However, these tools are known to be unreliable, potentially missing up to 25% of residual post-mucosal tumors and leading to undertreatment.