Background: Colorectal cancer is the leading cause of cancer-related deaths in Taiwan, with rectal cancer accounting for approximately 27% of all cases. Total neoadjuvant therapy (TNT), which consists of chemotherapy and radiation therapy delivered before surgery, has become the standard of care for locally advanced rectal cancer. However, there is currently no reliable method for predicting the response to TNT or the occurrence of radiation proctitis, a common side effect of treatment.

Objective: This study aims to evaluate the metabolomic profiles of individuals with locally advanced rectal cancer undergoing TNT and to identify a panel of metabolites that can predict treatment response and toxicities.

Methods: A prospective cohort study will be conducted to enrol patients with locally advanced rectal cancer who are scheduled to receive TNT. Blood, urine, tissue, and faecal samples will be collected at baseline, during, and after chemoradiotherapy. Metabolomic profiling of the samples will be performed using liquid-chromatography mass spectrometry (LC-MS). Treatment response will be assessed based on clinical downstaging (defined as a decrease in tumour size and/or T and N stage after TNT) and pathological response, such as pathological complete response (pCR). Radiation proctitis will be assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0. PCA and PLSDA will be used to identify metabolites that are associated with treatment response and radiation proctitis. Receiver operating characteristic (ROC) curves will be used to assess the predictive performance of the identified metabolites. Univariate and multivariate logistic regression will be used to build models to predict treatment response and radiation proctitis.