Rectal cancer accounts for about one-third of all colorectal cancers. While most rectal cancers are treated with a combination of chemotherapy and radiation before surgery, a small subset - roughly 2-3% - has a molecular feature called microsatellite instability-high (MSI-H) or deficient mismatch repair (dMMR). These tumors have a defect in the cellular machinery that normally fixes DNA errors.
Importantly, dMMR/MSI-H status means these tumors respond poorly to standard neoadjuvant chemotherapy but exceptionally well to a different approach: neoadjuvant immunotherapy (NIT) - specifically drugs that block the PD-1 checkpoint and release the immune system to attack the tumor. Studies have shown complete response rates of up to 100% with this approach in MSI-H rectal cancer.
When a tumor disappears completely after treatment - confirmed by pathological examination of removed tissue - this is called a pathological complete response (pCR). Patients who achieve pCR may be candidates for a watch-and-wait (W&W) strategy that avoids surgery entirely, preserving the rectum and quality of life. But accurately identifying pCR without surgery is a major clinical challenge that this study addresses.
After traditional chemoradiation, doctors use MRI and endoscopy to check whether a tumor has responded. Criteria for a clinical complete response (cCR) - no visible tumor by imaging - have been carefully defined for this treatment. But these criteria were developed specifically for chemoradiation and may not apply after immunotherapy.
When the immune system attacks the tumor, it causes inflammation, immune cell infiltration, necrosis, and fibrosis - all changes that can appear on imaging as a persistent mass even when no live cancer cells remain. This phenomenon is sometimes called pseudoprogression or pseudoresidual tumor. In all three patients in this case series, imaging showed what looked like residual disease even though the pathology ultimately confirmed no viable tumor cells.
This means doctors face a difficult dilemma: recommend surgery (which may be unnecessary) or attempt to observe a patient who still appears to have tumor on imaging. Better imaging tools that can distinguish true residual cancer from treatment-related changes are urgently needed. This study is the first to systematically describe multimodal transrectal ultrasound (TRUS) in this specific clinical scenario.
The study presents three patients - two women (ages 65 and 51) and one man (age 68) - all diagnosed with moderately differentiated rectal adenocarcinoma confirmed as MSI-H. All three had tumors in the middle or lower rectum, which are surgically challenging locations where anal preservation is a priority for patients' quality of life.
Each patient underwent a comprehensive multimodal ultrasound assessment before and after NIT with sintilimab, a PD-1 inhibitor, given intravenously every 21 days. The ultrasound protocol combined four techniques: standard two-dimensional (2D) ultrasound for size and shape, color Doppler flow imaging (CDFI) for blood vessel activity, contrast-enhanced ultrasound (CEUS) for perfusion patterns, and shear wave elastography (SWE) for tissue stiffness.
The patients received 4-9 cycles of sintilimab. After treatment, all three showed persistent imaging abnormalities that did not meet the standard criteria for clinical complete response. Two patients proceeded to surgery - and both had pCR confirmed on pathology. The third patient had a biopsy that also showed no viable tumor cells, allowing her to choose the W&W strategy to preserve her anus.
The 65-year-old woman had a tumor measuring 43 mm that had invaded the tissue surrounding the rectum (mesorectum). Before treatment, CEUS showed the tumor was highly active - with strong, uniform blood flow enhancement throughout. SWE showed the tumor was much stiffer than the normal rectal wall, with maximum stiffness (Emax) of 73 kPa compared to 11 kPa for normal tissue.
After five cycles of immunotherapy, 2D ultrasound showed the tumor had shrunk to 19 mm - a major size reduction. CDFI still showed some blood flow near the intestinal lining, but no flow in the deeper layers. On CEUS, the deeper portions of the tumor showed heterogeneous hypoenhancement (patchy, reduced blood flow patterns), and the peak intensity was lower than the surrounding normal wall.
Most notably, the SWE stiffness values after treatment had normalized to match the adjacent normal rectal wall. This softening is significant because it suggests the tumor tissue had been replaced by non-viable material (scar, necrosis, or mucin). Pathology after surgery confirmed pCR with complete tumor regression grade 0 - no surviving cancer cells anywhere in the specimen.
The 68-year-old man had a larger, deeper tumor (48 mm, invading 12 mm into the mesorectum). After four cycles of sintilimab, the tumor showed only minimal size reduction - but CEUS revealed a characteristic pattern: heterogeneous hypoenhancement with large non-enhancing areas inside the tumor. These dark, avascular regions corresponded to pools of mucin (a gel-like substance) seen on pathology - a sign of tumor death and replacement by non-viable tissue. Pathology after surgery confirmed pCR.
The 51-year-old woman had a tumor near the anorectal ring - a critically sensitive location where surgery would have permanently damaged sphincter function. After nine cycles, her tumor showed similar CEUS features: heterogeneous hypoenhancement with large non-enhancing areas, closely mirroring Case 2's pattern. A multidisciplinary team compared the imaging to Case 2 and judged that pCR was probable.
Rather than surgery, the team performed an ultrasound-guided transperineal biopsy - specifically targeting the areas that still showed contrast enhancement on CEUS to sample only the potentially viable portions. The biopsy confirmed no atypical cells, supporting pCR. The patient chose the W&W strategy, preserving her anus and avoiding a permanent colostomy.
Drawing from all three cases, the authors identify several multimodal TRUS features that may indicate pCR after neoadjuvant immunotherapy. The strongest signals appear to be: reduced lesion size, decreased or absent intratumoral vascularity on color Doppler, and heterogeneous hypoenhancement with large non-enhancing areas on contrast-enhanced ultrasound (CEUS).
The non-enhancing areas on CEUS are particularly significant. In Cases 2 and 3, these dark zones corresponded directly to mucinous lakes on pathological examination - areas where immune attack had killed the tumor cells, leaving pools of mucin. Recognizing this pattern could help sonographers distinguish pCR from residual disease, even when overall tumor size hasn't dramatically changed.
Shear wave elastography (SWE) may also contribute: in Case 1, stiffness values normalized after treatment, and prior research supports tumor Emean as an independent predictor of complete response. However, the absence of pre-treatment SWE data in Cases 2 and 3 limits conclusions. Standard 2D imaging and Doppler alone were insufficient to confirm pCR in all cases - reinforcing the need for the full multimodal approach.
An important contribution of this study is demonstrating the value of ultrasound-guided targeted core biopsy when imaging findings are ambiguous after immunotherapy. Conventional endoscopic biopsy - which samples tissue from the intestinal surface - misses the deeper layers where residual cancer cells tend to hide after neoadjuvant treatment.
By using real-time CEUS guidance, the ultrasound physician in Case 3 could precisely target the contrast-enhancing (potentially viable) areas of the residual lesion while avoiding the large avascular regions that represented only necrotic tissue or mucin pools. This selective sampling dramatically improves the diagnostic yield compared to random biopsies.
The ultimate goal of accurate pCR detection is enabling the watch-and-wait strategy for appropriate patients - avoiding major surgery with its associated risks of complications, incontinence, and permanent colostomy. For mid-to-low rectal cancer patients like Case 3, where surgery would destroy sphincter function, this is not merely a preference but a life-changing choice. Multimodal TRUS may serve as a practical, real-time guide to making this decision safely.
This case series is the first published report describing the ultrasound features of pCR in MSI-H rectal cancer after neoadjuvant PD-1 immunotherapy. While the sample size is just three patients, the consistency of findings across all three cases - and the clear pathological confirmation - provides a meaningful foundation for future research.
The key clinical message is that standard imaging criteria for complete response, developed for chemoradiation, should not be automatically applied to immunotherapy patients. MSI-H rectal cancers treated with PD-1 inhibitors may have persistent imaging abnormalities that represent treatment effects, not viable tumor. Multimodal TRUS offers a practical way to characterize these changes in real time, at the bedside.
Larger prospective studies are needed to validate these observations and develop standardized ultrasound criteria for pCR assessment after NIT. If validated, multimodal TRUS could become a routine complement to MRI in the post-treatment workup of MSI-H rectal cancer patients, helping more patients safely pursue organ preservation and avoid unnecessary surgery.