Complete Pathological Response After Neoadjuvant Long-Course Chemoradiotherapy for Rectal Cancer and Its Relationship to the Degree of T3 Mesorectal Invasion.

BACKGROUND: Many studies have shown significantly improved outcomes (reduced local recurrence and improved overall survival) for patients achieving a complete pathological response from neoadjuvant chemoradiotherapy. OBJECTIVE: This study aimed to document the complete pathological response rate and outcomes in patients receiving preoperative long-course chemoradiotherapy stratified for the extent of T3 mesorectal invasion measured on preoperative imaging. DESIGN: This is a retrospective study of prospectively collected data, of patients with rectal cancer in the Cabrini Monash University Department of Surgery colorectal neoplasia database, incorporating data from Cabrini Hospital and The Alfred Hospital, identifying patients entered between January 2010 and June 2014. PATIENTS AND SETTINGS: One hundred eighteen patients with T3 rectal cancer met the selection criteria for the study; 26 achieved complete pathological response (22%). MAIN OUTCOME MEASURES: Outcomes in terms of complete pathological response and oncological outcomes such as disease-free and overall survival were analyzed. RESULTS: Patients with complete pathological response had significantly less preoperative invasion than those with no complete pathological response (p < 0.001). Depth of invasion was the only variable associated with complete pathological response (p < 0.002), and the likelihood of complete pathological response decreased by 35% for every millimeter of invasion. Complete pathological response was associated with increased disease-free survival (p = 0.018) and a lower risk of cancer progression (p = 0.046). Depth of invasion was associated with an increased risk of death after surgery; HR increased by 1.07 (95% CI, 1.00-1.15) for each 1-mm increase in invasion. LIMITATIONS: This was a retrospective study with the usual limitations, although these were minimized through the use of a clinician-driven prospective database. CONCLUSIONS: The smaller the degree of T3 invasion, the higher the chance of achieving complete pathological response (up to 35%), which is associated with improved disease-free and overall survival. A higher complete pathological response rate is observed in early T3 disease in comparison with more extensive T3 invasion.