Case Report: Ultrasound features of pathological complete response in middle and low rectal cancer with high microsatellite instability after neoadjuvant immunotherapy: a case series.

Accurate assessment of complete clinical response following neoadjuvant chemoradiotherapy remains challenging, and radiological evaluation criteria established for this treatment modality may not be applicable to neoadjuvant immunotherapy (NIT). Currently, there are no published studies describing the ultrasound features of pathological complete response (pCR) in patients with microsatellite instability-high (MSI-H) rectal cancer following NIT. In this case series, we comparatively analyzed multimodal transrectal ultrasound (TRUS) features before and after NIT in three patients with MSI-H mid- to low-rectal cancer. Despite imaging findings suggestive of residual lesions, two patients achieved pCR as confirmed by postoperative pathology, and one patient had no definite atypical components identified on ultrasound-guided biopsy. Our preliminary observations suggest that multimodal TRUS features, including reduced lesion size, diminished vascularity, heterogeneous hypoenhancement with non-enhancing areas on contrast-enhanced ultrasound, and reduced stiffness on shear wave elastography, may serve as potential imaging indicators of pCR in patients with MSI-H mid- to low-rectal cancer. Ultrasound-guided biopsy may provide complementary diagnostic information for confirming pCR in cases with persistent imaging abnormalities.