Photoacoustic-ultrasound endoscopy for assessment of rectal cancer treatment response: A prospective study with T2-weighted MRI radiomics comparison

Accurate identification of pathological complete response (pCR) following neoadjuvant chemoradiotherapy for locally advanced rectal cancer (LARC) is critical for non-operative "watch-and-wait" strategies. In this study, we evaluated whether co-registered acoustic resolution photoacoustic microscopy and ultrasound (ARPAM-US) endoscopy with deep learning could better identify pCR than T2-weighted MRI radiomics models. A pretrained ResNet50 model utilizing ARPAM-US B-scans was developed on a prospective cohort (n = 25) to predict pCR, based on surgical histopathological assessment, using 5-fold cross-validation. MRI radiomics models were trained on a retrospective cohort (n = 119). Performances between ARPAM-US ResNet50 and MRI radiomics models were compared. The ARPAM-US model achieved promising diagnostic performance (AUC 0.956, 95% CI 0.912-1.000), distinguishing normalized vascular architecture in complete responders. MRI radiomics models demonstrated degraded prospective generalization. While limited responders precluded definitive head-to-head comparison, deep learning-enhanced ARPAM-US endoscopy demonstrates promising diagnostic accuracy for assessing rectal cancer treatment response. Larger prospective studies are needed to validate these findings.