ctDNA Monitoring to Predict the Efficacy of TNT for Rectal Cancer

This is a prospective observational study with three primary objectives: Objective 1: Evaluate the detection rate and changes in circulating tumor DNA (ctDNA) levels in blood samples from colorectal cancer patients before, during, and after total neoadjuvant therapy (TNT). Determine the detection rate and change of CtDNA in blood samples of cancer patients before, during, and after TNT then assess changes in ctDNA expression within the study population during treatment. * Determine the ctDNA positivity rate before treatment. * Determine the ctDNA positivity rate during TNT. * Determine the ctDNA positivity rate after TNT and assess ctDNA level changes during treatment. Objective 2: Investigate the relationship between ctDNA expression and MRI/CT scan imaging with the pathological complete response (pCR) to neoadjuvant therapy : * Correlation between ctDNA detection and pCR/TRG/NAR score. Calculate the Positive Prediction Value - PPV, Negative Prediction Value - NPV of ctDNA, * Correlation between MRI/CT scan imaging and pCR. Calculate the PPV and NPV of MRI/CT scan * Combination of ctDNA detection and MRI/CT scan imaging to predict pCR. Calculate the PPV and NPV of the combined markers. Objective 3: Evaluate the relationship between post-TNT ctDNA expression and disease-free survival in colorectal cancer patients.